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JAXB Users Guide

Abstract

This document explains various interesting/complex/tricky aspects of JAXB, based on questions posted on the JAXB users forum and answers I provided. This is an ongoing work-in-progress. Any feedback appreciated.

1. Compiling XML Schema

1.1. Dealing with errors

1.1.1. Schema errors

Because XML Schema is so complicated, and because there are a lot of tools out there do not implement the spec correctly, it is often the case that a schema you are trying to compile has some real errors in it. When this is the case, you'll see XJC reporting somewhat cryptic errors such as rcase-RecurseLax.2: There is not a complete functional mapping between the particles.

The JAXB RI uses the schema correctness checker from the underlying JAXP implementation, which is the JAXP RI in a typical setup. The JAXP RI is one of the most conformant schema validators, and therefore most likely correct. So the first course of action usually is to fix problems in the schema.

However, in some situations, you might not have an authority to make changes to the schema. If that is the case and you really need to compile the schema, you can bypass the correctness check by using the -nv option in XJC. When you do this, keep in mind that you are possibly feeding "garbage" in, so you may see XJC choke with some random exception.

1.1.2. Property 'fooBarZot' is already defined

One of the typical errors you'll see when compiling a complex schema is:

Example 1. Multiple property definitions error

parsing a schema...
[ERROR] Property "MiOrMoOrMn" is already defined.
  line 132 of
file:/C:/kohsuke/Sun/JAXB/jaxb-unit/schemas/individual/MathML2/presentation/scripts.xsd

[ERROR] The following location is relevant to the above error
  line 138 of
file:/C:/kohsuke/Sun/JAXB/jaxb-unit/schemas/individual/MathML2/presentation/scripts.xsd

This is an actual example of the offending part of a schema, taken from MathML. If you go to line 132 of scripts.xsd, you'll see that it has a somewhat complicated content model definition:

Example 2. Multiple property definitions in MathML

<xs:group name="mmultiscripts.content">
    <xs:sequence>
        <xs:group ref="Presentation-expr.class"/>
        <xs:sequence minOccurs="0" maxOccurs="unbounded">      <!-- line 132 -->
            <xs:group ref="Presentation-expr-or-none.class"/>
            <xs:group ref="Presentation-expr-or-none.class"/>
        </xs:sequence>
        <xs:sequence minOccurs="0">
            <xs:element ref="mprescripts"/>
            <xs:sequence maxOccurs="unbounded">                 <!-- line 138 -->
                <xs:group ref="Presentation-expr-or-none.class"/>
                <xs:group ref="Presentation-expr-or-none.class"/>
            </xs:sequence>
        </xs:sequence>
    </xs:sequence>
</xs:group>

This is a standard technique in designing a schema. When you want to say "in this element, B can occur arbitrary times, but C can occur only up to once", you write this as B*,(C,B*)?. This, however, confuses JAXB, because it tries to bind the first B to its own property, then C to its own property, then the second B to its own property, and so we end up having a collision again.

In this particular case, B isn't a single element but it's a choice of large number of elements abstracted away in <xs:group>s, so they are hard to see. But if you see the same content model referring to the same element/group twice in a different place, you can suspect this.

In this case, you'd probably want the whole thing to map to a single list so that you can retain the order those elements show up in the document. You can do this by putting the same <jaxb:property> customization on the whole "mmultiscripts.content" model group, like this (or you can do it externally with XPath):

Example 3. How to fix the problem?

<xs:groupname="mmultiscripts.content">
<xs:annotation>
    <xs:appinfo>
        <jaxb:propertyname="content"/>
    </xs:appinfo>
</xs:annotation>
<xs:sequence>
<xs:groupref="Presentation-expr.class"/>

Another way to fix this problem is to use the simpler and better binding mode in XJC, which is a JAXB RI vendor extension.

1.1.3. Two declarations cause a collision in the ObjectFactory class

When schemas contain similar looking element/type names, they can result in "Two declarations cause a collision in the ObjectFactory class" errors. To be more precise, for each of all types and many elements (exactly what elements get a factory and what doesn't is bit tricky to explain), XJC produces one method on the ObjectFactory class in the same package. The ObjectFactory class is created for each package that XJC generates some files into. The name of the method is derived from XML element/type names, and the error is reported if two elements/types try to generate the same method name.

There are two approaches to fix this problem. If the collision is coming from two different schemas with different target namespaces, then you can easily avoid the collision by compiling them into different Java packages. To do this, use <schemabindings> customization on two schemas and specify the package name.

Another way to fix this problem is to use <factoryMethod> customization on two conflicting elements/types to specify different factory method names. This can be used in all cases, but if you have a large number of conflicts, you'll have to specify this customization one by one.

Notice that <class> customization doesn't affect the ObjectFactory method name by itself.

1.1.4. Customization errors

1.1.4.1. XPath evaluation of ... results in empty target node

External JAXB customizations are specified by using XPath (or using SCD.) This works by writing an XPath expression that matches a particular element in the schema document. For example, given the following schema and binding file:

Example 4. Schema and external binding file

test.xsd

<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
    <xs:complexTypename="foo"/>
</xs:schema>

test.xjb

<bindings version="2.0" xmlns="http://java.sun.com/xml/ns/jaxb" xmlns:xs="http://www.w3.org/2001/XMLSchema">
    <bindings schemaLocation="test.xsd">
        <bindings node="//xs:complexType[@name='foo']">
            <classname="Bar"/>
        </bindings>
    </bindings>
</bindings>

will be interpreted as if the class customization is attached to the complex type 'foo'.

For this to work, the XPath expression needs to match one and only one element in the schema document. When the XPath expression is incorrect and it didn't match anything, you get this "XPath evaluation of ... results in empty target node" problem.

Common causes of this problem include typos, incorrect namespace URI declarations, and misunderstanding of XPath.

1.2. Fixing broken references in schema

Sometimes a schema may refer to another schema document without indicating where the schema file can be found, like this:

Example 5. Schema reference without location

<xs:import namespace="http://www.w3.org/1999/xlink" />

In other cases, a schema may refer to another schema on the network, which often slows down your compilation process and makes it unreliable. Yet in some other cases, a schema may reference another schema in relative path, and that may not match your directory structure.

XJC bundles a catalog resolver so that you can work around these situations without changing the schema documents. The main idea behind the catalog is "redirection" --- when XJC is about to fetch resources, it will consult the catalog resolver to see if it can find the resource elsewhere (which is usually your local resources.)

1.2.1. Catalog format

The catalog resolver supports many different formats, but the easiest one is a line based *.cat format. Other than comments and empty lines, the file mainly consists of two kinds of declarations, SYSTEM, and PUBLIC.

Example 6. sample-catalog.cat

--
  sample catalog file.

  double hyphens are used to begin and end a comment section.
--

SYSTEM "http://www.w3.org/2001/xml.xsd" "xml.xsd"

PUBLIC "-//W3C//DTD XMLSCHEMA 200102//EN" "s4s/XMLSchema.dtd"

1.2.2. Resolve by system ID

The SYSTEM entry has the format of "SYSTEM REFERENCE ACTUAL-LOCATION", which defines a simple redirection. Every time XJC loads any resource (be it schemas, DTDs, any entities referenced within), it will first resolve relative paths to absolute paths, then looks for a matching REFERENCE line. If it is found, the specified actual location is read instead. Otherwise XJC will attempt to resolve the absolutepath.

ACTUAL-LOCATION above accepts relative paths, and those are resolved against the catalog file itself (so in the above example, xml.xsd is assumed to be in the same directory with sample-catalog.cat.

What you need to be careful is the fact that the REFERENCE portion must be absolute, and when XJC finds a reference in schema, it will first convert that to the absolute path before checking the catalog. So what this means is that if your schema is written like this:

Example 7. Schema reference by relative path

<xs:import namespace="http://www.w3.org/1999/xlink" schemaLocation="xlink.xsd" />

Then your catalog entry would have to look like this:

Example 8. xlink.cat

-- this doesn't work because xlink.xsd will be turned into absolute path --
SYSTEM "xlink.xsd" "http://www.w3.org/2001/xlink.xsd"

-- this will work, assuming that the above schema is in /path/to/my/test.xsd --
SYSTEM "/path/to/my/xlink.xsd" "http://www.w3.org/2001/xlink.xsd"

1.2.3. Resolve by public ID / namespace URI

Another kind of entry has the format of "PUBLIC PUBLICID ACTUAL-LOCATION" or "PUBLIC NAMESPACEURI ACTUAL-LOCATION".

The "PUBLICID" version is used to resolve DTDs and entities in DTDs. But this type of entry is also used to resolve <xs:import> statements. XJC will match the value of the namespace attribute and see if there's any matching entry. So given a schema like this:

Example 9. Schema import

<xs:import namespace="http://www.w3.org/1999/xlink" schemaLocation="xlink.xsd" />
<xs:import namespace="http://www.w3.org/1998/Math/MathML" />

The following catalog entries will match them.

Example 10. by-publicid.cat

PUBLIC "http://www.w3.org/1999/xlink" "http://www.w3.org/2001/xlink.xsd"
PUBLIC "http://www.w3.org/1998/Math/MathML" "/path/to/my/mathml.xsd"

As you can see, XJC will check the PUBLIC entries regardless of whether <xs:import> has the schemaLocation attribute or not. As with the case with the SYSTEM entry, the ACTUAL-LOCATION part can be relative to the location of the catalog file.

1.2.4. Specifying the catalog file

Once you write a catalog file, you'd need to specify that when you invoke XJC.

CLI

To do this from the CLI, use the -catalog option. See xjc -help for more details.

Ant

Use the catalog attribute on the <xjc> task. See XJC ant task documentation for more details.

Maven

For the Maven plugin, use the <catalog> element in the configuration:

<plugin>
    <groupId>org.jvnet.jaxb2.maven2</groupId>
    <artifactId>maven-jaxb2-plugin</artifactId>
    <configuration>
        <!-- relative to the POM file -->
        <catalog>mycatalog.cat</catalog>
    </copnfiguration>
</plugin>

1.2.5. Debugging catalog file

If you are trying to write a catalog file and banging your head against a wall because it's not working, you should enable the verbose option of the catalog resolver. How you do this depends on what interface you use:

CLI

Specify export XJC_OPTS="-Dxml.catalog.verbosity=999" then run XJC.

Ant/Maven

Add -Dxml.catalog.verbosity=999 as a command line option to Ant/Maven.

If you are otherwise invoking XJC programmatically, you can set the above system property before invoking XJC.

1.3. Mapping of <xs:any />

XJC binds <xs:any /> in the following ways:

1.3.1. processContents="skip"

<xs:any /> with processContents=skip means any well-formed XML elements can be placed. Therefore, XJC binds this to DOM Element interface.

Example 11. Any/Skip schema

<xs:element name="person">
  <xs:complexType>
    <xs:sequence>
      <xs:element name="name" type="xs:string" />
      <xs:any processContents="skip" maxOccurs="unbounded" minOccurs="0" />
    </xs:sequence>
  </xs:complexType>
</xs:element>

Example 12. Any/Skip binding

import org.w3c.dom.Element;

@XmlRootElement
class Person {
  public String getName();
  public void setName(String);

  @XmlAnyElement
  public List<Element> getAny();
}

1.3.2. processContents="strict"

<xs:any /> with processContents=strict (or <xs:any /> without any processContents attribute, since it defaults to "strict") means any XML elements placed here must have corresponding schema definitions. This mode is not what people typically expect as "wildcard", but this is the default. The following shows this binding. (lax=true is unintuitive, but it's not an error in this document):

Example 13. Any/Strict schema

<xs:element name="person">
  <xs:complexType>
    <xs:sequence>
      <xs:element name="name" type="xs:string" />
      <xs:any maxOccurs="unbounded" minOccurs="0" />
    </xs:sequence>
  </xs:complexType>
</xs:element>

Example 14. Any/Strict binding

@XmlRootElement
class Person {
  public String getName();
  public void setName(String);

  @XmlAnyElement(lax=true)
  public List<Object> getAny();
}

JAXB binds any such element to an Object, and during unmarshalling, all elements encountered are unmarshalled into corresponding JAXB objects (including JAXBElements if necessary) and placed in this field. If it encounters elements that cannot be unmarshalled, DOM elements are produced instead.

At runtime, you can place either DOM elements or some JAXB objects that map to elements. A typical mistake is to put a String that contains XML fragment, but this won't work; you'd have to first read that into a DOM.

1.3.3. processContents="lax"

<xs:any /> with processContents=lax means any XML elements can be placed here, but if their element names match those defined in the schema, they have to be valid. XJC actually handles this exactly like processContents='strict', since the strict binding allows unknown elements anyway.

1.4. Mapping of <xs:element /> to JAXBElement

Sometimes XJC binds an element declaration to JAXBElement. Sometimes XJC binds an element declaration to a Java class. What makes this difference?

TODO: Copy from http://weblogs.java.net/blog/kohsuke/archive/2006/03/why_does_jaxb_p.html

1.5. How modularization of schema interacts with XJC

Over time schema authors have developed several techniques to modularize large schemas. Some of those techniques have some noteworthy interactions with XJC.

1.5.1. Chameleon schema

Chameleon schema" (read more, in particular this) is a technique used to define multiple almost-identical sets of definitions into multiple namespaces from a single schema document.

For example, with this technique, you can write just one "foo" complex type and define it into namespace X and Y. In this case, one tends to hope that XJC will only give you one Foo class for this, but unfortunately because it's actually defined in two namespaces, JAXB needs two Java classes to distinguish X:foo and Y:foo, so you'll get multiple copies.

If you find this to be problematic, there are a few ways to work around the problem.

  1. If you are in control of the schema, see if you can rewrite the schema to avoid using this technique. In some cases, the schema doesn't actually exploit the additional power of this technique, so this translation can be done without affecting XML instance documents. In some other cases, the chameleon schema can be argued as a bad schema design, as it duplicates definitions in many places.

  2. If you are not in control of the schema, see if you can rewrite the schema nevertheless. This will only work if your transformation doesn't affect XML instance documents.

  3. Perhaps there can be a plugin that eases the pain of this, such as by defining common interfaces among copies.

1.6. Adding behaviors

Adding behaviors to the generated code is one area that still needs improvement. Your feedback is appreciated.

Suppose if JAXB generated the following classes.

Example 15. Simple JAXB Generated Code

package org.acme.foo;

@XmlRootElement
class Person {
  private String name;

  public String getName() { return name; }
  public void setName(String) { this.name=name; }
}

@XmlRegistry
class ObjectFactory {
  Person createPerson() { ... }
}

To add a behavior, first write a class that extends from Person. You also need to extend ObjectFactory to return this new class. Notice that neither classes have any JAXB annotation, and I put them in a separate package. This is because we'd like PersonEx class to be used in place of Person, and we don't want PersonEx to be bound to its own XML type.

Example 16. Extended Person class

package org.acme.foo.impl;

class PersonEx extends Person {
  @Override
  public void setName(String name) {
    if(name.length()<3) throw new IllegalArgumentException();
    super.setName(name);
  }
}

@XmlRegistry
class ObjectFactoryEx extends ObjectFactory {
  @Override
  Person createPerson() {
    return new PersonEx();
  }
}

At runtime, you can create JAXBContext normally, like this.

Example 17. Creating JAXBContext

JAXBContext context = JAXBContext.newInstance(ObjectFactory.class);
// or JAXBContext.newInstance("org.acme.foo");

PersonEx can be marshalled out just like Person:

Example 18. Marshalling

Person p = new PersonEx();
context.createMarshaller().marshal(p,System.out);
// this will produce <person />

To unmarshal XML documents into PersonEx, you'll need to configure the unmarshaller to use your ObjectFactoryEx as the factory, like this:

Example 19. Unmarshalling

Unmarshaller u = context.createUnmarshaller();
u.setProperty("com.sun.xml.bind.ObjectFactory",new ObjectFactoryEx());
PersonEx p = (PersonEx)u.unmarshal(new StringReader("<person />"));

If you have multiple packages and thus multiple ObjectFactorys, you can pass in an array of them (new Object[]{new OFEx1(),new OFEx2(),...}.)

1.6.1. Inserting your class in the middle

If you have a type hierarchy and would like to insert your class in the middle, you can use the combination of XmlTransient and @implClass of <class> customization. See the following example:

Example 20. Hierarchy of types and <jaxb:class implClass>

<xs:schema ...>
  <xs:complexType name="vehicle">
    <xs:annotation><xs:appinfo>
      <jaxb:class implClass="MyVehicle" />
    </xs:appinfo></xs:annotation>
  </xs:complexType>

  <xs:complexType name="car">
    <xs:complexContent>
      <xs:extension base="vehicle" />
    </xs:complexContent>
  </xs:complexType>

  <xs:complexType name="bicycle">
    <xs:complexContent>
      <xs:extension base="vehicle" />
    </xs:complexContent>
  </xs:complexType>
</xs:schema>

Example 21. This creates a class hierarchy like the following (among the generated Java code):

            Vehicle
               ^
               |
            MyVehicle
               ^
          _____|______
         |            |
        Car          Bicycle

You'll then manually write MyVehicle class that extends from Vehicle. Annotate this class with XmlTransient to achieve the desired effect.

1.7. Avoid strong databinding

Under some limited circumstances, a weaker databinding is preferable for various reasons. JAXB does offer a few ways for you to achieve this.

1.7.1. Avoid mapping to enum

The following customization will stop binding a simple type to a type-safe enum. This can be convenient when number of constants is too large to be an useful enum (by default, the JAXB spec won't generate enum with more than 256 constants, but even 100 might be too large for you.)

Example 22. Avoid mapping one simple type

<xs:simpleType name="foo">
  <xs:annotation><xs:appinfo>
    <jaxb:typesafeEnumClass map="false" />
  </xs:appinfo></xs:annotation>
  <xs:restriction base="xs:string">
    <xs:enumeration value="x" />
    <xs:enumeration value="y" />
    <xs:enumeration value="z" />
  </xs:restriction>
</xs:simpleType>

To disable such type-safe enum binding altogether for the entire schema, use a global binding setting like this (this is actually telling XJC not to generate enums if a simple type has more than 0 constants --- the net effect is no enum generation):

Example 23. Avoid generating enums at all

<xs:schema ...>
  <xs:annotation><xs:appinfo>
    <jaxb:globalBindings typesafeEnumMaxMembers="0" />
  </xs:appinfo></xs:annotation>
  ...
</xs:schema>

1.7.2. Mapping to DOM

The <jaxb:dom>customization allows you to map a certain part of the schema into a DOM tree. This customization can be attached to the following schema components:

  • Wildcards (<xs:any>)

  • Type definitions (<xs:complexType> and <xs:simpleType>)

  • Model groups (<xs:choice>,<xs:all>,<xs:sequence>)

  • Model group declarations (<xs:group>)

  • Particles

  • Element declarations (<xs:element>)

In the following example, a wildcard is mapped to a DOM node. Each element that matches to the wildcard will be turned into a DOM tree.

Example 24. Dom Customization example

<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
               xmlns:jaxb="http://java.sun.com/xml/ns/jaxb"
               jaxb:version="2.0">

        <xs:element>
           <xs:complexType>
              <xs:sequence>
                 <xs:any maxOccurs="unbounded" processContents="skip">
                    <xs:annotation><xs:appinfo>
                      <jaxb:dom/>
                    </xs:appinfo></xs:annotation>
                 </xs:any>
              </xs:sequence>
           </xs:complexType>
        </xs:element>
    .
    .
    .
    </xs:schema>

This extension can be used to access wildcard content or can be used to process a part of a document by using other technologies that require "raw" XML. By default, JAXB generates a getContent() method for accessing wildcard content, but it only supports "lax" handling which means that unknown content is discarded. You may find more information in 7.12 chapter of JAXB 2 specification.

1.8. Working with generated code in memory

1.8.1. Cloning

The generated beans (and in particular the JAXBElement class) do not support the clone operation. There was a suggestion by another user that beanlib has been used successfully to clone JAXB objects.

2. Customization of Schema Compilation

2.1. Customizing Java packages

The JAXB specification provides a <jaxb:schemaBindings> customization so that you can control which namespace goes to which package. See the example below:

Example 25. package customization

    <jaxb:schemaBindings>
      <jaxb:package name="org.acme.foo"/>
    </jaxb:schemaBindings>

You can do this as an internal customization (in which case you put this in <xs:annotation><xs:appinfo> under place it right under the <xs:schema> element), or do this as an external customization, like this:

Example 26. External package customization

<bindings xmlns="http://java.sun.com/xml/ns/jaxb" version="2.1">
  <bindings schemaLocation="../path/to/my.xsd">
    <schemaBindings>
      <package name="org.acme.foo"/>
    </schemaBindings>
  </bindings>
</bindings>

Note that this customization is per namespace. That is, even if your schema is split into multiple schema documents, you cannot put them into different packages if they are all in the same namespace.

2.1.1. Tip: get rid of the org.w3._2001.xmlschema package

Under some rare circumstances, XJC will generate some Java classes into a package called org.w3._2001.xmlschema. This happens when XJC decides that it needs some Java artifacts for the XML Schema built-in namespace of http://www.w3.org/2001/XMLSchema.

Since this package name is most often problematic, you can rename this by simply saving the following text in an .xsd file and submitting it to XJC along with the other schemas you have:

Example 27. Schemalet to get rid of org.w3._2001.xmlschema

<schema xmlns="http://www.w3.org/2001/XMLSchema"
  targetNamespace="http://www.w3.org/2001/XMLSchema"
  xmlns:jaxb="http://java.sun.com/xml/ns/jaxb"
  jaxb:version="2.0">
  <annotation><appinfo>
    <jaxb:schemaBindings>
      <jaxb:package name="org.acme.foo"/>
    </jaxb:schemaBindings>
  </appinfo></annotation>
</schema>

This is bit tricky, but the idea is that since you can define a schema for one namespace in multiple schema documents, this makes XJC think that this schema is a part of the built-in "XML Schema for XML Schema".

2.2. Using SCD for customizations

When using an external customization file, the JAXB spec requires that you use XPath as a means to specify what your customization is attached to. For example, if you want to change the class name generated from a complex type, you'd write something like:

Example 28. External customization example

<bindings xmlns="http://java.sun.com/xml/ns/jaxb" version="2.0" xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <bindings schemaLocation="../path/to/my.xsd" node="/xs:schema/xs:complexType[@name='foo']">
    <class name="FooType"/>
  </bindings>
</bindings>

While the above process works, the problem with this is that the XPath+ schemaLocation combo tends to be verbose and error prone. It's verbose, because often a trivial target schema component like this "global complex type foo" takes up a lot of characters. The xs namespace declaration also takes up some space, although in this case we managed to avoid declaring the "tns" namespace (that represents the namespace that the schema defines.)

It's also error prone, because it relies on the way schema documents are laid out, because the schemaLocation attribute needs to point to the right schema document file. When a schema is split into multiple files for modularity (happens especially often with large schemas), then you'd have to find which schema file it is. Even though you can use relative paths, this hard-coding of path information makes it hard to pass around the binding file to other people.

JAXB RI 2.1 and onward offers a better way to do this as a vendor extension.

The key technology to solve this problem is a "schema component designator" (SCD.) This is a path language just like XPath, but whereas XPath is designed to refer to XML infoset items like elements and attributes, SCD is designed to refer to schema components like element declarations or complex types.

With SCD, the above binding can be written more concisely as follows:

Example 29. External customization by SCD

<bindings xmlns="http://java.sun.com/xml/ns/jaxb" version="2.1" xmlns:tns="http://my.namespace/">
  <bindings scd="/type::tns:foo">
    <class name="FooType"/>
  </bindings>
</bindings>

/type::tns:foo can be written more concisely as /~tns:foo, too. If you are interested in more about the syntax of SCDs, read the example part of the spec, and maybe EBNF. If you know XPath, I think you'll find this fairly easy to learn.

Another benefit of an SCD is that tools will have easier time generating SCDs than XPath, as XPaths are often vulnerable to small changes in the schema document, while SCDs are much more robust. The downside of using SCD is as of JAXB 2.1, this feature is a vendor extension and not defined in the spec.

2.3. Using different datatypes

JAXB has a built-in table that determines what Java classes are used to represent what XML Schema built-in types, but this can be customized.

One of the common use cases for customization is to replace the XMLGregorianCalendar with the friendlier Calendar or Date. XMLGregorianCalendar is designed to be 100% compatible with XML Schema's date/time system, such as providing infinite precision in sub-seconds and years, but often the ease of use of those familiar Java classes win over the precise compatibility.

One very easy way to do this is to simply use your IDE (or even "sed") to replace all the references to XMLGregorianCalendar by Calendar. This is of course not a very attractive option if your build process runs XJC as a part of it.

Alternatively, the following customization file can be used to do this. When using external customization file, the JAXB spec requires you to use XPath as a means to specify what your customization is attached to. For example, if you want to change the class name generated from a complex type, you'd use the following customization:

Example 30. Customization to use Calendar for xs:date

<bindings xmlns="http://java.sun.com/xml/ns/jaxb" version="2.0" xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <globalBindings>
    <javaType name="java.util.Calendar" xmlType="xs:date"
      parseMethod="javax.xml.bind.DatatypeConverter.parseDate"
      printMethod="javax.xml.bind.DatatypeConverter.printDate"
    />
  </globalBindings>
</bindings>

Save this in a file and specify this to JAXB with the "-b" option.

To use the Date class, you'll need to do a bit more work. First, put the following class into your source tree:

Example 31. Adapter for Date

package org.acme.foo;
public class DateAdapter {
  public static Date parseDate(String s) {
    return DatatypeConverter.parseDate(s).getTime();
  }
  public static String printDate(Date dt) {
    Calendar cal = new GregorianCalendar();
    cal.setTime(dt);
    return DatatypeConverter.printDate(cal);
  }
}

... then your binding file will be the following:

Example 32. Customization to use Date for xs:date

<bindings xmlns="http://java.sun.com/xml/ns/jaxb" version="2.0" xmlns:xs="http://www.w3.org/2001/XMLSchema">
  <globalBindings>
    <javaType name="java.util.Date" xmlType="xs:date"
      parseMethod="org.acme.foo.DateAadpter.parseDate"
      printMethod="org.acme.foo.DateAdapter.printDate"
    />
  </globalBindings>
</bindings>

If you are using JAXB 2.0, and not 2.1, see this blog entry for how to do this for 2.0.

3. Annotating Your Classes

3.1. Mapping your favorite class

3.1.1. ResultSet

JAXB (or any other databinding engine, for that matter) is for binding strongly-typed POJO-like objects to XML, such as AddressBook class, PurchaseOrder class, and so on, where you have fields and methods that shape a class.

There are other kinds of classes that are more close to reflection. Those classes don't have methods like getAddress, and instead you'd do get("Address"). JDBC ResultSet is one of those classes. It's one class that represents million different data structures, be it a customer table or a product table. Generally speaking, these classes does not allow JAXB to statically determine what the XML representation should look like. Instead, you almost always need to look at an instance to determine the shape of XML.

These classes are not really suitable for binding in JAXB. If this is the only object that you'd want to write out, then you'd be better off using XMLStreamWriter or some such XML infoset writing API. There are a few online articles that cover this topic. Also, many modern database offers a native ability to export a query into XML, which is supposed to work a lot faster than you'd do in Java (and saves your time of writing code.)

If you are using ResultSet as a part of your object tree that you want to marshal to JAXB, then you can use XmlJavaTypeAdapter.

3.1.2. HashMap

JAXB spec defines a special handling for Map when it's used as a propety of a bean. For example, the following bean would produce XMLs like the following:

Example 33. Bean with Map

@XmlRootElement
class Foo {
  public HashMap<String,Integer> map;
}

Example 34. XML representation

<foo>
  <map>
    <entry>
      <key>a</key>
      <value>1</value>
    </entry>
    <entry>
      <key>b</key>
      <value>2</value>
    </entry>
  </map>
</foo>

Unfortunately, as of 2.1, this processing is only defined for bean properties and not when you marshal HashMap as a top-level object (such as a value in JAXBElement.) In such case, HashMap will be treated as a Java bean, and when you look at HashMap as a bean it defines no getter/setter property pair, so the following code would produce the following XML:

Example 35. Bean with Map

m = new HashMap();
m.put("abc",1);
marshaller.marshal(new JAXBElement(new QName("root"),HashMap.class,m),System.out);

Example 36. XML representation

<root />

This issue has been recorded as #223 and the fix needs to happen in later versions of the JAXB spec.

In the mean time, such top-level objects have to be first adapted to a bean that JAXB can process. This has added benefit of being able to control XML representation better. The following code illustrates how to do this:

Example 37. Adapting HashMap

public class MyHashMapType {
    public List<MyHashMapEntryType> entry = new ArrayList<MyHashMapEntryType>();
    public MyHashMapType(Map<String,Integer> map) {
        for( Map.Entry<String,Integer> e : map.entrySet() )
            entry.add(new MyHashMapEntryType(e));
    }
    public MyHashMapType() {}
}

public class MyHashMapEntryType {
    @XmlAttribute // @XmlElement and @XmlValue are also fine
    public String key;

    @XmlAttribute // @XmlElement and @XmlValue are also fine
    public int value;

    public MyHashMapEntryType() {}
    public MyHashMapEntryType(Map.Entry<String,Integer> e) {
       key = e.getKey();
       value = e.getValue();
    }
}

marshaller.marshal(new JAXBElement(new QName("root"),MyHashMapType.class,new MyHashMapType(m)),System.out);

If you have a lot of difference kinds of Map, you can instead use Object as the key and the value type. In that way, you'll be able to use maps with different type parameters, at the expense of seeing xsi:type attribute on the instance document.

3.2. Mapping interfaces

Because of the difference between the XML type system induced by W3C XML Schema and the Java type system, JAXB cannot bind interfaces out of the box, but there are a few things you can do.

3.2.1. Use @XmlRootElement

When your interface is implemented by a large number of sub-classes, consider using XmlRootElement annotation like this:

Example 38. XmlRootElement for open-ended interfaces

@XmlRootElement
class Zoo {
  @XmlAnyElement
  public List<Animal> animals;
}

interface Animal {
  void sleep();
  void eat();
  ...
}

@XmlRootElement
class Dog implements Animal { ... }

@XmlRootElement
class Lion implements Animal { ... }

This will produce XML documents like this:

Example 39. XML for XmlRootElement

<zoo>
    <lion> ... </lion>
    <dog> ... </dog>
</zoo>

The key characteristics of this approach is:

  1. Implementations are open-ended; anyone can implement those interfaces, even by different people from different modules, provided they are all given to the JAXBContext.newInstance method. There's no need to list all the implementation classes in anywhere.

  2. Each implementation of the interface needs to have an unique element name.

  3. Every reference to interface needs to have the XmlElementRef annotation. The type=Object.class portion tells JAXB that the greatest common base type of all implementations would be java.lang.Object.

@XmlElementWrapper is often useful with this, as it allows you need to group them. Such as:

Example 40. XmlRootElement for open-ended interfaces

@XmlRootElement
class Zoo {
  @XmlElementWrapper
  @XmlAnyElement
  public List<Animal> onExhibit;
  @XmlElementWrapper
  @XmlAnyElement
  public List<Animal> resting;
}

Example 41. Effect of XmlElementWrapper

<zoo>
    <onExhibit>
        <lion> ... </lion>
        <dog> ... </dog>
    </onExhibit>
    <resting>
        <lion> ... </lion>
        <dog> ... </dog>
    </resting>
</zoo>

3.2.2. Use @XmlJavaTypeAdapter

When you use interfaces just to hide your implementation classes from exposure, and when there's 1-to-1 (or close to 1-on-1) relationship between a class and an interface, XmlJavaTypeAdapter can be used like below.

Example 42. XmlJavaTypeAdapter for interfaces

@XmlJavaTypeAdapter(FooImpl.Adapter.class)
interface IFoo {
  ...
}
class FooImpl implements IFoo {
  @XmlAttribute
  private String name;
  @XmlElement
  private int x;

  ...

  static class Adapter extends XmlAdapter<FooImpl,IFoo> {
    IFoo unmarshal(FooImpl v) { return v; }
    FooImpl marshal(IFoo v) { return (FooImpl)v; }
  }
}

class Somewhere {
  public IFoo lhs;
  public IFoo rhs;
}

Example 43. XML of XmlJavaTypeAdapter

<somewhere>
  <lhs name="...">
    <x>5</x>
  </lhs>
  <rhs name="...">
    <x>5</x>
  </rhs>
</somewhere>

The key characteristics of this approach is:

  1. Interface and implementation will be tightly coupled through an adapter, although changing an adapter code will allow you to support multiple implementations.

  2. There's no need of any annotation in where interfaces are used.

A variation of this technique is when you have a few implementations for interface, not just one.

Example 44. XmlJavaTypeAdapter for interfaces with multiple implementations

@XmlJavaTypeAdapter(AbstractFooImpl.Adapter.class)
interface IFoo {
  ...
}
abstract class AbstractFooImpl implements IFoo {
  ...

  static class Adapter extends XmlAdapter<AbstractFooImpl,IFoo> {
    IFoo unmarshal(AbstractFooImpl v) { return v; }
    AbstractFooImpl marshal(IFoo v) { return (AbstractFooImpl)v; }
  }
}

class SomeFooImpl extends AbstractFooImpl {
  @XmlAttribute String name;
  ...
}

class AnotherFooImpl extends AbstractFooImpl {
  @XmlAttribute int id;
  ...
}

class Somewhere {
  public IFoo lhs;
  public IFoo rhs;
}

Example 45. XML of XmlJavaTypeAdapter with multiple implementations

<somewhere>
  <lhs xsi:type="someFooImpl" name="...">
  </lhs>
  <rhs xsi:type="anotherFooImpl" id="3" />
</somewhere>

Note that SomeFooImpl and AnotherFooImpl must be submitted to JAXBContext.newInstance one way or the other.

To take this example a bit further, you can use Object instead of AbstractFooImpl. The following example illustarates this:

Example 46. XmlJavaTypeAdapter for interfaces with multiple implementations

@XmlJavaTypeAdapter(AnyTypeAdapter.class)
interface IFoo {
  ...
}
public class AnyTypeAdapter extends XmlAdapter<Object,Object> {
  Object unmarshal(Object v) { return v; }
  Object marshal(Object v) { return v; }
}

class SomeFooImpl implements IFoo {
  @XmlAttribute String name;
  ...
}

class Somewhere {
  public IFoo lhs;
  public IFoo rhs;
}

Example 47. Corresponding schema

<xs:complexType name="somewhere">
  <xs:sequence>
    <xs:element name="lhs" type="xs:anyType" minOccurs="0"/>
    <xs:element name="rhs" type="xs:anyType" minOccurs="0"/>
  </xs:sequence>
</xs:complexType>

As you can see, the schema will generated to accept xs:anyType which is more relaxed than what the Java code actually demands. The instance will be the same as the above example. Starting from JAXB RI 2.1, we bundle the com.sun.xml.bind.AnyTypeAdapter class in the runtime that defines this adapter. So you won't have to write this adapter in your code.

3.2.3. Use @XmlElement

If the use of interface is very little and there's 1-to-1 (or close to) relationship between interfaces and implementations, then you might find XmlElement to be the least amount of work.

Example 48. XmlElement for interfaces

interface IFoo {
  ...
}
class FooImpl implements IFoo {
  ...
}

class Somewhere {
  @XmlElement(type=FooImpl.class)
  public IFoo lhs;
}

Example 49. XML of XmlElement

<somewhere>
  <lhs> ... </lhs>
</somewhere>

This effectively tells JAXB runtime that "even though the field is IFoo, it's really just FooImpl.

In this approach, a reference to an interface has to have knowledge of the actual implementation class. So while this requires the least amount of typing, it probably wouldn't work very well if this crosses module boundaries.

Like the XmlJavaTypeAdapter approach, this can be used even when there are multiple implementations, provided that they share the common ancestor.

The extreme of this case is to specify @XmlElement(type=Object.class).

3.2.4. Hand-write schema

Occasionally the above approaches cause the generated schema to become somewhat ugly, even though it does make the JAXB runtime work correctly. In such case you can choose not to use the generated schema and instead manually modify/author schemas tht better match your needs.

3.2.5. Do schema-to-java

With sufficient knowlege, one can also use <jaxb:class ref="..."/> annotation so that you can cause XJC to use the classes you already wrote. See this thread for an example. TODO: more details and perhaps an example.

3.2.6. DOESN'T WORK: Have JAXB generate interaces and swap different implementations

Some users attempted to use the "generateValueClass" customization and see if they can completely replace the generated implementations with other implementations. Unfortunately, this does not work.

Even with the interface/implementation mode, JAXB runtime still requires that the implementation classes have all the JAXB annotations. So just implementing interfaces is not sufficient. (This mode is mainly added to simplify the migration from JAXB 1.0 to JAXB 2.0, and that's a part of the reason why things are done this way.)

3.3. Evolving annotated classes

Here is the basic problem of evolution. You got your CoolApp v1, which contains class Foo that has some JAXB annotations. Now you are working towawrd CoolApp v2, and you want to make some changes to Foo. But you want to do so in such a way that v1 and v2 can still talk to each other.

The evolution compatibility has two different aspects. One is the schema compatibility, which is about the relationship between the v1 schema and the v2 schema. The other is about runtime compatibility, which is about reading/writing documents between two versions.

3.3.1. Runtime compatibility

There are two directions in the runtime compatibility. One is whether v1 can still read what v2 write (forward compatible), and the other is whether v2 can read what v1 wrote (backward compatible).

3.3.2. "Semi-compatible"

JAXB can read XML documents that don't exactly match what's expected. This is the default behavior of the JAXB unmarshaller, yet you can change it to a more draconian behavior (TODO: pointer to the unmarshalling section.)

When we are talking about evolving classes, it's often convenient to leave it in the default behavior, as that would allow JAXB to nicely ignore elements/attributes newly added in v2. So we call it backward semi-compatible if v2 can read what v1 wrote in this default unmarshalling mode, and similarly forward semi-compatible if v1 can read what v2 wrote in this default unmarshalling mode.

Technically, these are weaker than true backward/forward compatibility (since you can't do a draconian error detection), yet in practice it works just fine.

3.3.3. Adding/removing/changing non-annotated things

You can add, remove, or change any non-annotated fields, methods, inner/nested types, constructors, interfaces. Those changes are both backward and forward compatible, as they don't cause any change to the XML representation.

Adding super class is backward compatible and forward semi-compatible. Similarly, removing super class is forward compatible and backward semi-compatible.

3.3.4. Adding/removing/changing properties

Adding new annotated fields or methods is backward compatible and forward semi-compatible. Similarly, removing them is forward compatible and backward semi-compatible.

Changing a property is bit more tricky.

  1. If you change the property name from X to Y, that would be the equivalent of deleting X and adding Y, so it would be backward and forward semi-compatible. What JAXB really cares is properties' XML names and not Java names, so by using the name parameter of XmlElement , XmlAttribute et al, you can change Java property names without affecting XML, or change XML without affecting Java properties. These are backward and forward semi-compatible. See below:

  2. Example 50. Changing Java without affecting XML

    // BEFORE
    public class Foo {
        public String abc;
    }
    // AFTER: Java name changed, but XML remains the same
    public class Foo {
        @XmlElement(name="abc")
        public String def;
    }

    Example 51. Changing XML without affecting Java

    // BEFORE
    public class Foo {
        public String abc;
    }
    // AFTER: no Java change, but XML will look different
    public class Foo {
        @XmlElement(name="def")
        public String abc;
    }

  3. If you change a property type, generally speaking it will be not compatible at all. For example, you can't change from java.util.Calendar to int and expect it to work. To make it a somewhat compatible change, the old type and the new type has to be related. For example, String can represent all int values, so changing int to String would be a backward compatible and forward semi-compatible change. XmlJavaTypeAdapter allows you to make changes to Java without affecting XML (or vice versa.)

3.3.5. Changing class names

XmlType and XmlRootElement allows you to change a class name without affecting XML.

Example 52. Changing class name without affecting XML (1)

// BEFORE
@XmlRootElement
public class Foo { ... }

// AFTER: no XML change
@XmlRootElement(name="foo")
@XmlType(name="foo")
public class Bar { ... }

Example 53. Changing class name without affecting XML (2)

// BEFORE
public class Foo { ... }

// AFTER: no XML change
@XmlType(name="foo")
public class Bar { ... }

3.3.6. Schema Compatibility

TODO.

3.4. XML layout and in-memory data layout

Your program sometimes needs to have a different in-memory data structure from its XML representation. JAXB has a few different ways to achieve this.

3.4.1. XmlJavaTypeAdapter

XmlJavaTypeAdapter allows you to de-couple the in-memory representation and the XML representation by introducing an intermediate representation. The basic model is as follows:

In-memory objects  <===>  Intermediate objects   <===>
XML
                  adapter                         JAXB

Your adapter code will be responsible for converting in-memory objects to/from intermediate objects. Intermediate objects are then bound to XML by following the standard JAXB rules. See XmlAdapter for a general description of how adapters works.

Adapters extend from the XmlAdapter class and provide two methods "unmarshal" and "marshal" that converts values in both directions, and then the XmlJavaTypeAdapter annotation is used to tell JAXB where and what adapters kick in.

(TODO: more info about XmlJavaTypeAdapter needed)

  1. adapting a class

  2. adapting a property

  3. adapting an external class

  4. adapting a collection and its effect

  5. adapting and using interfaces

3.4.2. Using XmlJavaTypeAdapter for element/attribute values

One of the common use cases of XmlJavaTypeAdapter is to map a "value object" to a string in XML. The following example illustrates how to do this, by using java.awt.Color as an example.

Example 54. Mapping Color to #RRGGBB

@XmlRootElement
class Box {
  @XmlJavaTypeAdapter(ColorAdapter.class)
  @XmlElement
  Color fill;
}

class ColorAdapter extends XmlAdapter<String,Color> {
  public Color unmarshal(String s) {
    return Color.decode(s);
  }
  public String marshal(Color c) {
    return "#"+Integer.toHexString(c.getRGB());
  }
}

This maps to the following XML representation:

Example 55. Box instance

<box>
  <fill>#112233</fill>
</box>

Since XmlJavaTypeAdapter is on a field, this adapter only kicks in for this particular field. If you have many Color fields and would like them all to use the same adapter, you can move the annotation to a package:

Example 56. package-info.java

@XmlJavaTypeAdapter(type=Color.class,value=ColorAdapter.class)
package foo;

Example 57. Box.java

@XmlRootElement
class Box {
  @XmlElement Color fill;
  @XmlElement Color border;
}

This causes all the fields in the classes in the foo package to use the same specified adapter.

Also see the DatatypeConverter class that defines a series of basic conversion routines that you may find useful.

3.4.3. Pair property

Another useful technique is to define two properties, one for JAXB and the other for your application. See the following example:

Example 58. Pair property sample

@XmlRootElement
class Person {
  private int age;

  // This public property is for users
  @XmlTransient
  public int getAge() {
    return age;
  }
  public void setAge(int age) {
    this.age = age;
  }

  // This property is for JAXB
  @XmlAttribute(name="age")
  private String getAge_() {
    if(age==-1)  return "dead";
    else         return String.valueOf(age);
  }
  private void setAge_(String v) throws NumberFormatException {
    if(v.equals("dead"))   this.age=-1;
    else                   this.age=Integer.parseInt(age);
}

The main "age" property is public, but marked as XmlTransient , so it's exposed in your program, but JAXB will not map this to XML. There's another private "age_" property. Since this is marked with XmlAttribute , this is what JAXB is going to use to map to the attribute. The getter and setter methods on this property will handle the conversion between the in-memory representation and the XML representation.

3.5. Mapping cyclic references to XML

Object models designed in Java often have cycles, which prevent straight-forward conversion to XML by JAXB. In fact, when you try to marshal an object tree that contains a cycle, the JAXB marshaller reports an error, pointing out the objects that formed the cycle. This is because JAXB by itself cannot figure out how to cut cycles into a tree.

Thus it is your responsibility to annotate classes and use other means to "tell" JAXB how to handle a cycle. This chapter talks about various techniques to do this.

3.5.1. Parent pointers

One of the very common forms of cycle is a parent pointer. The following example illustrates a typical parent pointer, and how this can be turned into "natural" XML:

Example 59. Classes with parent pointer

@XmlRootElement
class Department {
  @XmlAttribute
  String name;
  @XmlElement(name="employee")
  List<Employee> employees;
}

class Employee {
  @XmlTransient
  Department department;  // parent pointer
  @XmlAttribute
  String name;

  public void afterUnmarshal(Unmarshaller u, Object parent) {
    this.department = (Department)parent;
  }
}

This will produce the following XML:

Example 60. XML view of department

<department name="accounting">
  <employee name="Joe Chin" />
  <employee name="Adam Smith" />
  ...
</department>

And reading this document back into Java objects will produce the expected tree with all the proper parent pointers set up correctly.

The first technique here is the use of XmlTransient on the parent pointer. This tells JAXB that you don't need this parent pointer to be represented explicitly in XML, because the fact that employee is always contained inside department implies the parent/child relationship. This causes the marshaller to produce the expected XML. However, when you unmarshal it, since this field is not bound, the Employee.department field will be left null.

That's where the second technique comes in, which is the use of the afterUnmarshal callback. This method is invoked by the JAXB implementation on each instance when the unmarshalling of a Employee object completes. Furthermore, the second parameter to the method is the parent object, which in this case is a Department object. So in this example, this sets up the parent pointer correctly.

This callback can be also used to perform other post-unmarshalling set up work.

3.5.2. Many-to-many relationship

TBD

3.5.3. @XmlID and @XmlIDREF

When a reference to another object is annotated with XmlIDREF , its corresponding XML it will be referenced by xs:IDREF, instead of containment. See below for an example:

Example of @XmlID and @XmlIDREF

@XmlRootElement
class Root {
  List<Foo> foos;
  List<Bar> bars;
}
class Foo {
  // you don't have to make it an attribute, but that's more common
  @XmlAttribute @XmlIDREF Bar bar;
}
class Bar {
  // you don't have to make it an attribute, but that's more common
  @XmlAttribute @XmlID String id;
}

Example 61. Schema for above

<xs:complexType name="foo">
  <xs:sequence/>
  <xs:attribute name="bar" type="xs:IDREF"/>
  </xs:sequence>
</xs:complexType>
<xs:complexType name="bar">
  <xs:sequence/>
  <xs:attribute name="id" type="xs:ID"/>
</xs:complexType>

Example 62. A sample instance

<root>
  <foo bar="x"/>
  <foo bar="y"/>
  <bar id="x"/>
  <bar id="y"/>
</root>

There are a few things to consider when you do this. First, the object to be referenced must have an ID that is unique within the whole document. You'd also need to ensure that the referenced objects are contained somewhere else (like in the Root class in this case), or else Bar objects will never be marshalled. This technique can be used to remove the cyclic references, but it's only possible when your object model has an easy cut point.

3.5.4. Use the CycleRecoverable interface

Starting 2.1 EA2, the JAXB RI exposes CycleRecoverable interface. Applications can choose to implement this interface in some of its objects. When a cyclic reference is detected during marshalling, and if the object that formed a cycle implements this interface, then the method on this interface is called to allow an application to nominate its replacement to be written to XML. In this way, the application can recover from a cycle gracefully.

See a posting from gadams00 for a complete illustration of this feature.

This technique allows you to cope with a situation where you cannot easily determine upfront as to where a cycle might happen. On the other hand, this feature is a JAXB RI feature. Another downside of this is that unless you nominate your replacement carefully, you can make the marshalling output invalid with respect to the schema, and thus you might hit another problem when you try to read it back later.

4. Unmarshalling

4.1. @XmlRootElement and unmarshalling

Classes with XmlRootElement can be unmarshalled from XML elements simply by invoking the unmarshal method that takes one parameter. This is the simplest mode of unmarshalling.

Unmarshalling with @XmlRootElement

@XmlRootElement
class Foo {
  @XmlAttribute
  String name;
  @XmlElement
  String content;
}

Unmarshaller u = ...;
Foo foo = (Foo)u.unmarshal(new File("foo.xml"));

Example 63. foo.xml

<foo name="something">
  <content>abc</content>
</foo>

However, sometimes you may need to unmarshal an instance of a type that does not have an XmlRootElement . For example, you might dynamically find out at the runtime that a certain element has a certain type. For example, the following document illustrates an XML instance where the content of <someOtherTagName> element is represented by the Foo class.

Example 64. foo2.xml

<someOtherTagName name="something">
  <content>abc</content>
</someOtherTagName>

To unmarshal this into a Foo class, use the version of the unmarshal method that takes the 'expectedType' argument, as follows:

Example 65. Unmarshalling into a known type

Unmarshaller u = ...;
JAXBElement<Foo> root = u.unmarshal(new StreamSource(new File("foo.xml")),Foo.class);
Foo foo = root.getValue();

To reduce the number of the unmarshal methods, this two-argument version is not defined for every single-argument version. So as in this example, you might need to perform additional wrapping of the input parameter.

This instructs JAXB that the caller is expecting to unmarshal Foo instance. JAXB returns a JAXBElement of Foo, and this JAXBElement captures the tag name of the root element.

4.2. Unmarshalling is not working! Help!

There are a few common causes for this problem. These causes often exhibit similar symptoms:

  1. Instance documents are invalid

  2. JAXBContext is not created correctly.

4.2.1. Make sure your instance document is valid

First, use an independent schema validator to check if your document is really valid with respect to the schema you compiled. When the root element of a document is invalid, then the unmarshaller will issue "unexpected element" errors. When a portion of a document is invalid, JAXB skips that portion, so the end result is that the unmarshalling returns normally, yet you notice that a part of the content tree is missing. This is often the desirable behavior, but it sometimes ends up masking a problem.

Also, try to install ValidationEventHandler on the unmarshaller. When a portion of a document is skipped, the unmarshaller notifies a ValidationEventHandler, so it allows you to see what's going on.

Example 66. Installing ValidationEventHandler

Unmarshaller u = ...;
// this implementation is a part of the API and convenient for trouble-shooting,
// as it prints out errors to System.out
u.setEventHandler(new javax.xml.bind.helpers.DefaultValidationEventHandler());

u.unmarshal(new File("foo.xml"));

Also consider installing a Schema object to the unmarshaller, so that the unmarshaller performs a schema validation while unmarshalling. Earlier I suggested that you try an independent schema validator, but for various reasons (not all tools are reliable, you might have made an error and used a different schema/instance), using validating unmarshalling is a better way to guarantee the validity of your instance document being unmarshalled. Please follow the JAXP tutorial for more about how to construct a Schema object from your schema.

If you are unmarshalling from XML parser APIs (such as DOM, SAX, StAX), then also make sure that the parser/DOM is configured with the namespace enabled.

4.2.2. Check if your JAXBContext is correct

(TODO: This also applies to the marshaller. Think about moving it.)

The other possibility is that JAXBContext is not set up correctly. JAXBContext "knows" a set of classes, and if it doesn't know a class that it's supposed to know, then the unmarshaller may fail to perform as you expected.

To verify that you created JAXBContext correctly, call JAXBContext.toString(). It will output the list of classes it knows. If a class is not in this list, the unmarshaller will never return an instance of that class. Make you see all the classes you expect to be returned from the unmarshaller in the list. When dealing with a large schema that spans across a large number of classes and packages, this is one possible cause of a problem.

If you noticed that a class is missing, explicitly specify that to JAXBContext.newInstance. If you are binding classes that are generated from XJC, then the easiest way to include all the classes is to specify the generated ObjectFactory class(es).

4.3. Element default values and unmarshalling

Because of the "strange" way that element default values in XML Schema work, people often get confused about their behavior. This section describes how this works.

When a class has an element property with the default value, and if the document you are reading is missing the element, then the unmarshaller does not fill the field with the default value. Instead, the unmarshaller fills in the field when the element is present but the content is missing. See below:

Example 67. XML instance 1

<foo />

Example 68. XML instance 2

<foo>
  <a/>  <!-- or <a></a> -->
</foo>

Example 69. XML instance 3

<foo>
  <a>abc</a>
</foo>

Example 70. Element defaults and XML

@XmlRootElement
class Foo {
  @XmlElement(defaultValue="value") public String a=null;
}

Foo foo = unmarshaller.unmarshal("instance1.xml");
System.out.println(foo.a);   // null

Foo foo = unmarshaller.unmarshal("instance2.xml");
System.out.println(foo.a);   // "value". The default kicked in.

Foo foo = unmarshaller.unmarshal("instance3.xml");
System.out.println(foo.a);   // "abc". Read from the instance.

This is consistent with the XML Schema spec, where it essentially states that the element defaults do not kick in when the element is absent, so unfortunately we can't change this behavior.

Depending on your expectation, using a field initializer may achieve what you are looking for. See below:

Example 71. Possible changes by using field initializer

@XmlRootElement
class Foo {
  @XmlElement public String a="value";
}

Foo foo = unmarshaller.unmarshal("instance1.xml");
System.out.println(foo.a);   // "value", because JAXB didn't overwrite the value

Foo foo = unmarshaller.unmarshal("instance2.xml");
System.out.println(foo.a);   // "", because <a> element had 0-length string in it

Foo foo = unmarshaller.unmarshal("instance3.xml");
System.out.println(foo.a);   // "abc". Read from the instance.

Alternatively, attribute default values work in a way that agrees with the typical expectation, so consider using that. Also, see Element default values and marshalling.

4.4. Dealing with large documents

JAXB API is designed to make it easy to read the whole XML document into a single tree of JAXB objects. This is the typical use case, but in some situations this is not desirable. Perhaps:

  1. A document is huge and therefore the whole may not fit the memory.

  2. A document is a live stream of XML (such as XMPP) and therefore you can't wait for the EOF.

  3. You only need to databind the portion of a document and would like to process the rest in other XML APIs.

This section discusses several advanced techniques to deal with these situations.

4.4.1. Processing a document by chunk

When a document is large, it's usually because there's repetitive parts in it. Perhaps it's a purchase order with a large list of line items, or perhaps it's an XML log file with large number of log entries.

This kind of XML is suitable for chunk-processing; the main idea is to use the StAX API, run a loop, and unmarshal individual chunks separately. Your program acts on a single chunk, and then throws it away. In this way, you'll be only keeping at most one chunk in memory, which allows you to process large documents.

See the streaming-unmarshalling example and the partial-unmarshalling example in the JAXB RI distribution for more about how to do this. The streaming-unmarshalling example has an advantage that it can handle chunks at arbitrary nest level, yet it requires you to deal with the push model --- JAXB unmarshaller will "push" new chunk to you and you'll need to process them right there.

In contrast, the partial-unmarshalling example works in a pull model (which usually makes the processing easier), but this approach has some limitations in databinding portions other than the repeated part.

4.4.2. Processing a live stream of XML

The techniques discussed above can be used to handle this case as well, since they let you unmarshal chunks one by one. See the xml-channel example in the JAXB RI distribution for more about how to do this.

4.4.3. Creating virtual infosets

For further advanced cases, one could always run a streaming infoset conversion outside JAXB API and basically curve just the portion of the infoset you want to data-bind, and feed it as a complete infoset into JAXB API. JAXB API accepts XML infoset in many different forms (DOM, SAX, StAX), so there's a fair amount of flexibility in choosing the right trade off between the development effort in doing this and the runtime performance.

For more about this, refer to the respective XML infoset API.

5. Marshalling

5.1. Changing prefixes

By default, a JAXB marshaller uses random namespace prefixes (such as ns1, ns2, ...) when it needs to declare new namespace URIs. While this is perfectly valid XML wrt the schema, for human readability, you might want to change them to something that makes more sense.

The JAXB RI defines NamespacePrefixMapper to allow you to do this. See the namespace-prefix sample in the distribution for more details.

5.2. Element default values and marshalling

Because of a "strange" way element default values in XML Schema work, people often get confused about its behavior. This section describes how this works.

When a class has an element property with the default value, and if a value is null, then the marshaller will not produce the corresponding element in XML:

Example 72. Element defaults and XML

@XmlRootElement
class Foo {
  @XmlElement(defaultValue="value") public String a=null;
}

marshaller.marshal(new Foo(),System.out);

Example 73. Marshalling output from above

<foo />

This is consistent with the XML Schema spec, where it essentially states that the element defaults do not kick in when the element is absent. Attribute default values do not have this problem, so if you can change the schema, changing it to an attribute is usually a better idea. Alternatively, depending on your expectation, setting the field to a default value in Java may achieve what you are looking for.

Example 74. Possible changes

@XmlRootElement
class Foo {
  @XmlElement public String a="value";
}
@XmlRootElement
class Bar {
  @XmlAttribute public String a;
}

marshaller.marshal(new Foo(),System.out);
marshaller.marshal(new Bar(),System.out);

Example 75. Marshalling output from above

<foo>
    <a>value</a>
</foo>

<bar/>

Also, see Element default values and unmarshalling.

5.3. Different ways of marshalling

5.3.1. Different output media

The most basic notion of the marshalling is to take a JAXB-bound object that has @XmlRootElement, and write it out as a whole XML document. So perhaps you have a class like this:

Example 76. JAXB POJO

class Point {
  @XmlElement
  public int x;
  @XmlElement
  public int y;
  Point(...) { ... }
}

Then you can do:

Example 77. Plain marshalling

marshaller.marshal( new Point(1,3), System.out );
marshaller.marshal( new Point(1,3), new File("out.xml") );

.. and so on. There're seven Marshaller.marshal methods that takes different output media as the second parameter. If you are writing to a file, a socket, or memory, then you should use the version that takes OutputStream. Unless you change the target encoding to something else (default is UTF-8), there's a special marshaller codepath for OutputStream, which makes it run really fast. You also don't have to use BufferedOutputStream, since the JAXB RI does the adequate buffering.

You can also write to Writer, but in this case you'll be responsible for encoding characters, so in general you need to be careful. If you want to marshal XML into an encoding other than UTF-8, it's best to use the JAXB_ENCODING property and then write to OutputStream, as it escapes characters to things like &#x1824; correctly.

The next medium we support is W3C DOM. This is bit unintuitive, but you'll do it like this:

Example 78. Marshal to DOM

DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
dbf.setNamespaceAware(true);
Document doc = dbf.newDocumentBuilder().newDocument();

marshaller.marshal( new Point(1,3), doc );

And after the method invocation you get a complete DOM tree that represents the marshalled document.

The other versions of the marshal methods are there to write XML documents in terms of other XML APIs, such as SAX and StAX. The version that takes ContentHandler is useful when you need a custom formatting needs (like you want each attribute to be in new line, etc), but otherwise they are not very interesting if you are writing a whole document.

5.3.2. Marshalling into a subtree

Another common use of JAXB is where you are writing a bigger document, and you use JAXB to generate part(s) of it. The JAX-WS RI is the prime example. It produces a SOAP message, and JAXB is only used to produce the body. When you are doing this, you first set JAXB_FRAGMENT property on the marshaller. This changes the behaviors of the marshaller so that it works better in this situation.

If you are writing to an OutputStream or Writer and generally sending it to someone else, you can do something like this:

Example 79. Marshalling into a subtree

System.out.println("<envelope>");
marshaller.marshal( object, System.out );
System.out.println("</envelope>");

Like I mentioned, this is probably the fastest, even though println isn't very pretty. JAXB_FRAGMENT prevents the marshaller from producing an XML declaration, so the above works just fine. The downside of this approach is that if the ancestor elements declare the namespaces, JAXB won't be able to take advantage of them.

You can also marshal an object as a subtree of an existing DOM tree. To do this, you pass the Element object as the second parameter, and the marshaller will marshal an object as a child of this node.

StAX is also very convenient for doing this sort of things. You can create XMLStreamWriter, write some stuff, and then pass that to the marshaller. JAXB_FRAGMENT prevents the marshaller from producing startDocument and endDocument token. When doing this sub-tree marshaling to DOM and StAX, JAXB can take advantage of available in-scope namespace bindings.

Finally, you can marshal an object as a subtree into ContentHandler, but it requires a fair amount of SAX programming experience, and it goes beyond the scope of this entry.

5.3.3. Marshalling a non-element

Another common use case is where you have an object that doesn't have @XmlRootElement on it. JAXB allows you to marshal it like this:

Example 80. Marshalling a non-element

marshaller.marshal( new JAXBElement(
  new QName("","rootTag"),Point.class,new Point(...)));

This puts the <rootTag> element as the root element, followed by the contents of the object, then </rootTag>. You can actually use it with a class that has @XmlRootElement, and that simply renames the root element name.

At the first glance the second Point.class parameter may look redundant, but it's actually necessary to determine if the marshaller will produce (infamous) @xsi:type. In this example, both the class and the instance are Point, so you won't see @xsi:type. But if they are different, you'll see it.

This can be also used to marshal a simple object, like String or an integer.

Marshalling a non-element with @xsi:type

marshaller.marshal( new JAXBElement(
  new QName("","rootTag"),String.class,"foo bar"));

But unfortunately it cannot be used to marshal objects like List or Map, as they aren't handled as the first-class citizen in the JAXB world.

5.3.4. Connecting to other XML APIs

Because of the Source and Result support, JAXB objects can be easily marshalled into other XML APIs that are not mentioned here. For example, dom4j has DocumentResult that extends Result, so you can do:

Example 81. Marshalling to dom4j

DocumentResult dr = new DocumentResult();
marshaller.marshal( object, dr );
o = dr.getDocument();

Similar mechanism is available for JDOM and XOM. This conversion is much more efficient than first marshalling to ByteArrayOutputStream and then read it back into these DOMs. The same mechanism can be used to marshal to FastInfoset or send the marshaled document to an XSLT engine (TransformerHandler.)

The other interesting connector is JAXBSource, which wraps a marshaller and allows a JAXB object to be used as a "source" of XML. Many XML APIs take Source as an input, and now JAXB object can be passed to them directly.

For example, you can marshal a JAXB object and unmarshal it into another JAXBContext like this:

Example 82. Loading into a different JAXBContext

JAXBContext context1 = ... ;
JAXBContext context2 = ... ;

context1.createUnmarshaller().unmarshal( new JAXBSource(context2,object) );

This amounts to looking at the same XML by using different schema, and again this is much more efficient than going through ByteArrayOutputStream.

5.4. Interaction between marshalling and DOM

Sometimes you may notice that JAXB is producing XML with seemingly unnecessary namespace declarations. In this section, we'll discuss the possible causes and how to resolve this.

5.4.1. Caused by DOM mapping

The #1 cause of extra namespace declarations is due to the DOM mapping. This mainly happens because of a schema construct that forces XJC to generate a property with DOM. This includes the use of wildcard <xs:any/> (see more about this Mapping of <xs:any />), as well as xs:anyType (which can also happen by omission, such as <xs:element name="foo"/>, which is interpreted as <xs:element name="foo" type="xs:anyType" />.

During unmarshalling, when a subtree of the input XML is converted into XML, JAXB copies all the in-scope namespace bindings active at that time to the root of the DOM element. So for example, given the following Java class and XML, the DOM tree that the child field will get will look like the following:

Example 83. Bean with wildcard

@XmlRootElement
class Foo {
  @XmlAnyElement
  public Element child;
}

Example 84. Instance with subtree matching wildcard

<foo xmlns:a="a" xmlns:b="b" xmlns:c="c">
  <subtree xmlns:c="cc">
    <data>a:xyz</data>
  </subtree>
</foo>

Example 85. DOM tree to be stored in Foo.child

<subtree xmlns:a="a" xmlns:b="b" xmlns:c="cc">
    <data>a:xyz</data>
  </subtree>

Note that the two namespace declarations are copied over, but c is not because it's overridden. Also not that JAXB is not touching the whitespace in document. This copying of namespace declarations is necessary to preserve the infoset in the input document. For example, if the <data> is a QName, its meaning would change if JAXB unmarshaller doesn't copy it.

Now, imagine what happens when you marshal this back to XML. Despite the fact that in this example neither b nor c prefixes are in use, JAXB cannot delete them, because it doesn't know if those attributes are significant to the application or not. Therefore, this could end up producing XML with "extra namespace declarations" like:

Example 86. DOM tree to be stored in Foo.child

<foo>
  <subtree xmlns:a="a" xmlns:b="b" xmlns:c="cc">
    <data>a:xyz</data>
  </subtree>
</foo>

Resolving this problem is not possible in the general case, but sometimes one of the following strategy works:

  1. Sometimes schema author incorrectly assumes that <xs:element name="foo"/> means <xs:element name="foo" type="xs:string"/>, because attribute declarations work somewhat like this. In such a case, adding explicit type attribute avoids the use of DOM, so things will work as expected.

  2. The wildcard processing mode " strict" would force a typed binding, and thereby eliminate any DOM mapping.

  3. You might be able to manulally go into the DOM tree and remove unnecessary namespace declarations, if your application knows what are necessary and what are not.

6. Schema Generation

6.1. Invoking schemagen programatically

Schemagen tools by default come in as CLI, ant task, and Maven plugin. These interfaces allow you to invoke schemagen functionality from your program.

6.1.1. At runtime

If the classes you'd like to generate schema from are already available as java.lang.Class objects (meaning they are already loaded and resolved in the current JVM), then the easiest way to generate a schema is to use the JAXB API:

Example 87. Generate schema at runtime

File baseDir = new File(".");

class MySchemaOutputResolver extends SchemaOutputResolver {
    public Result createOutput( String namespaceUri, String suggestedFileName ) throws IOException {
        return new StreamResult(new File(baseDir,suggestedFileName));
    }
}

JAXBContext context = JAXBContext.newInstance(Foo.class, Bar.class, ...);
context.generateSchema(new MySchemaOutputResolver());

6.1.2. CLI interface

The CLI interface (public static int com.sun.tools.jxc.SchemaGenerator.run(String[])) is the easiest API to access. You can pass in all the schemagen command-line arguments as a string array, and get the exit code as an int value. Messages are sent to System.err and System.out.

6.1.3. Ant interface

Ant task can be invoked very easily from a non-Ant program. The schemagen ant task is defined in the SchemaGenTask class,

For more about invoking an Ant task programatically, see the article "Keep the Ant, Hold the XML".

6.1.4. Native Java API

The above two interfaces are built on top of externally committed contracts, so they'll evolve only in a compatibile way. The downside is that the amount of control you can exercise over them would be limited.

So yet another approach to invoke schemagen is to use JAXB RI's internal interfaces. But be warned that those interfaces are subject to change in the future versions, despite our best effort to preserve them. This is the API that the JAX-WS RI uses to generate schema inside WSDL when they generate WSDL, so does some other web services toolkits that work with the JAXB RI.

Most of those interfaces are defined and well-documented in the com.sun.tools.xjc.api package. You can see how the schemagen tools are eventually calling into this API at the implementaion of SchemaGenerator class.

6.2. Generating Schema that you want

This section discusses how you can change the generated XML schema. For changes that also affect the infoset (such as changing elements to attributes, namespaces, etc.), refer to a different section " XML layout and in-memory data layout."

6.2.1. Adding facets to datatypes

As of JAXB 2.1.4, currently no support for this, although there has been several discussions in the users alias.

The JAXB project is currently lacking resources to attack this problem, and therefore looking for volunteers to work on this project. The basic idea would be to define enough annotations to cover the basic constraint facets (such as length, enumerations, pattern, etc.) The schema generator would have to be then extended to honor those annotations and generate schemas accordingly.

Some users pointed out relevance of this to JSR 303: Bean Validation. If you are interested in picking up this task, let us know!

7. Deployment

7.1. Migrating JAXB 2.0 applications to JavaSE 6

JavaSE 6 ships with its own JAXB 2.0 implementation. This implementation is based on the JAXB RI, where the only differences are:

  • No RI-specific vendor extensions are supported: This is so that portability across different JavaSE 6 implementations will be guaranteed.

  • Code in JavaSE 6 is hosted in its own packages to avoid conflicts: This allows applications to continue to use a specific version of the JAXB RI that they choose to use.

Therefore, if you develop an application that uses JAXB 2.0 for JavaSE 5 today, the easiest way to upgrade to JavaSE 6 is to do nothing. You should keep the JAXB RI in your development environment, keep bundling the JAXB RI runtime jars to your app, just like you do that today.

7.1.1. Reducing footprint

If you'd like to reduce the footprint of your application by taking advantage of a JAXB implementation in JavaSE 6, you can take the following steps:

  1. You will no longer have to ship jaxb-api.jar in your application. This doesn't require any change to your code.

  2. If your application does not use any of the vendor extension features of the JAXB RI runtime (such as unmarshaller/marshaller properties whose names start with "com.sun."), then you will no longer have to ship jaxb-impl.jar (nor jaxb1-impl.jar, jaxb-libs.jar.) When you do this, be sure to test your application since it's not very easy to find all such dependencies.

7.1.2. Using JAXB 2.1/2.2 with JavaSE 6

JavaSE 6 comes with JAXB 2.0 API in rt.jar, newer releases (since update 4) of JavaSE 6 contain JAXB 2.1 API. Therefore, using JAXB 2.1/2.2 with JavaSE 6 requires one to override a portion of rt.jar with the new API. There are several ways to do this:

  1. Place the 2.1/2.2 jaxb-api.jar into $JRE_HOME/lib/endorsed. This essentially makes your JRE to "JRE 6 + JAXB 2.x". This won't affect any other applications that use this JRE, and it's easy. On the other hand, in various scenarios you may not be able to alter the JRE.

  2. Use the system property java.endorsed.dirs when you launch your application, and have it point to the directory that contains the 2.1/2.2 jaxb-api.jar. This allows you use use JAXB 2.1/2.2 without modifying the JRE. Make sure not to include any other JAXB RI jar files (such as jsr173-api.jar or jaxb-impl.jar.)

  3. Implement a custom ClassLoader and block delegation to javax.xml.bind package, so that code running inside this class loader will load the JAXB API from elsewhere. This is a very advanced approach.

No matter which approach you take, make sure not to include jar files other than jaxb-api.jar. Doing so, for example including jaxb-xjc.jar, may result in classloading related errors such as "taskdef A class needed by class com.sun.tools.xjc.XJCTask cannot be found: org/apache/tools/ant/...."

See the endorsed directory mechanism for more details.

7.1.3. Where's the XJC ant task?

JavaSE has never shipped an Ant task implementation, so we are just following that tradition. There's an (process-wise) overhead of adding additional dependencies during the JavaSE build, and there would likely be some runtime dependency issues in having a class in tools.jar that would require the ant classes, due to class loader delegation.

We are thinking about perhaps releasing a small jar that only contains the ant task for JDK6.

Please also note that the syntax of <xjc> task is neither defined in the JAXB spec nor in the JavaSE spec. Therefore other JavaSE vendors may not implement that at all, or do so in a different class name, etc. Therefore, from a portability perspective, if you choose to depend on the <xjc> task you should bundle the JAXB RI.

7.2. Which JAXB RI is included in which JDK?

This table captures the history of the JAXB RI integration into JDK. This is for information purposes only. In particular, the JAXB RI in JDK is package renamed to avoid any conflict with the stand-alone JAXB RI distribution or with the jaxb2-reflection library. You can use any version of these with any version of JDK without worrying about implementation conflicts.

JDK6 first release - JDK6 u3

JAXB RI 2.0.3

JDK6 u4

JAXB RI 2.1.3

JDK6 u14

JAXB RI 2.1.10

JDK7 first release

JAXB RI 2.2.4-1

7.3. Running JAXB 1.0 and 2.x side by side

You can always run an application X that uses JAXB 1.0 and another application Y that uses JAXB 2.x side by side, in the same classloader. To do this, remove the runtime jars from JAXB 1.0 from X, and instead use jaxb-impl.jar and jaxb1-mpl.jar from JAXB 2.0.x. Those two jars are designed to be compatible with earlier 1.0 versions of the runtime.

When you have a runtime from 1.0 in your classpath before 2.0.x, a user reported that you'll get java.lang.NoSuchMethodError: com.sun.xml.bind.WhiteSpaceProcessor.isWhiteSpace(Ljava/lang/CharSequence;)Z. Starting with JAXB 2.1.6 the runtime will diagnose this error nicely, but if you are using earlier versions, check your classpath.

7.4. Migrating JAXB 1.0 applications to JAXB 2.x

While it's always possible to keep your JAXB 1.0 applications as-is and run them on the JAXB 2.x runtime, or have some parts of your application use JAXB 1.0 while others use 2.x, there are situations where you might prefer to migrate your existing applications from JAXB 1.0 to JAXB 2.x.

Common reasons why people would like to migrate includes:

  • Taking advantage of the flexible, robust unmarshalling in JAXB 2.0

  • Exposing the JAXB-bound objects through JAX-WS as web services

7.4.1. Typical migration work

This section outlines how the typical migration work proceeds:

  1. Replace the invocation of XJC 1.0.x with XJC 2.x. Both the CLI and ant task syntax are backward-compatible to make this easier.

  2. Re-compile your schema with XJC 2.x. This will generate a new and different set of classes and methods.

  3. Update your application code that deals with the generated code to use the newer classes and methods. This step is mostly straight-forward but may be time consuming, especially if your project is big.

  4. Remove the JAXB 1.0.x runtime from your application, and place in JAXB 2.x runtime instead. See the list for what jars were used in 1.0 and what are used in 2.0.

The classes generated from JAXB 2.x tends to be a lot more compact.

JAXB RI 2.x ships many of the same samples that JAXB RI 1.0.x shipped, except that they are updated to work with JAXB 2.0. So comparing those samples may help you better understand what this type of migration involves.

7.4.2. Other Miscellaneous Notes

  • JAXB 2.x requires JavaSE 5 or above, whereas JAXB 1.0 runs on JavaSE 1.3 or above.

  • The javax.xml.bind interfaces remain the same. So the part of the code that deals with those interfaces may remain the same.

  • JAXB 1.0 unmarshaller was a lot more draconian to the errors in the input document than 2.0 is. In a sense, you can think of the 1.0 unmarshaller as equivalent to the 2.0 unmarshaller + validator. If your application was relying on this behaivior, you may want to enable validation in 2.0 by using Unmarshaller.setSchema.

  • JAXB 2.x by default creates plain Java beans, whereas in 1.0 it created separated interfaces and implementations. Many people find beans to be easier to work with, but you can use the following customization to tell JAXB 2.x to generate interfaces and implementations in a similar fashion.

    Example 88. Interface/Implementation customization

    <jxb:bindings version="2.1"
                  xmlns:jxb="http://java.sun.com/xml/ns/jaxb">
      <jxb:globalBindings generateValueClass="false" />
    </jxb:bindings>


  • When creating instances of the generated classes, JAXB 1.0 required that you do so through ObjectFactory, like new ObjectFactory().createAbc(). With 2.0, you can simply do new Abc(). This no longer throws JAXBExceptions, either.

  • Binding customization syntax of 2.0 is backward compatible with 1.0. However, there are several customizations that were previously allowed in 1.0 but not in 2.0, such as using class customizations on model groups.

7.5. Runtime Errors

7.5.1. Illegal class modifiers for package-info: 0x1600

When you compile your sources files with JDK from Java6 and try to run it on Java5 JVMs, you may see a ClassFormatError at runtime like this:

main" java.lang.ClassFormatError: Illegal class modifiers in class
com/alu/parentalcontrol/jaxb/package-info: 0x1600
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClass(ClassLoader.java:620)
at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:124)
at java.net.URLClassLoader.defineClass(URLClassLoader.java:260)
at java.net.URLClassLoader.access$100(URLClassLoader.java:56)
at java.net.URLClassLoader$1.run(URLClassLoader.java:195)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:188)
at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:268)
at java.lang.ClassLoader.loadClass(ClassLoader.java:251)
at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:319)
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Class.java:242)
at java.lang.Package.getPackageInfo(Package.java:350)
at java.lang.Package.getAnnotation(Package.java:361)
at com.sun.xml.bind.v2.model.annotation.RuntimeInlineAnnotationReader.getPackageAnnotation(RuntimeInlineAnnotationReader.java:125)
at com.sun.xml.bind.v2.model.annotation.RuntimeInlineAnnotationReader.getPackageAnnotation(RuntimeInlineAnnotationReader.java:53)
at com.sun.xml.bind.v2.model.impl.TypeInfoImpl.parseElementName(TypeInfoImpl.java:122)
at com.sun.xml.bind.v2.model.impl.ClassInfoImpl.<init>(ClassInfoImpl.java:166)

This is due to a bug, and so far the only way to fix this is to compile your project with JavaSE 5.

Here is what's happening. Java5 added a new class file modifier attribute called ACC_SYNTHETIC, whose bit value is 0x1000. This attribute marks types and methods that are not present in the source file but generated by the compiler. When package-info.java is compiled into package-info.class, javac in Java5 apparently only put 0x0600 (= ACC_ABSTRACT|ACC_INTERFACE.) Some time during Java6 development, someone filed a bug, saying it should also include ACC_SYNTHETIC bit, since it was a synthesized class.

Later it is discovered that the corresponding VM change needs to be made to allow this 0x1600 combination, but apparently no one realized the real implication of this --- namely, 0x1600 will break all the past JVMs. Of course, this problem is eventually discovered, but as of this writing there's still no fix for this.

So as you see, this is why the only workaround is to use javac from Java5.

8. Other Miscellaneous Topics

8.1. Performance and thread-safety

The JAXBContext class is thread safe, but the Marshaller, Unmarshaller, and Validator classes are not thread safe.

For example, suppose you have a multi-thread server application that processes incoming XML documents by JAXB. In this case, for the best performance you should have just one instance of JAXBContext in your whole application like this:

Example 89. Singleton JAXBContext

class MyServlet extends HttpServlet {
    static final JAXBContext context = initContext();

    private static JAXBContext initContext() {
        return JAXBContext.newInstance(Foo.class,Bar.class);
    }
}

And each time you need to unmarshal/marshal/validate a document. Just create a new Unmarshaller/Marshaller/Validator from this context, like this:

Example 90. Thread local Unmarshaller

    public void doGet( HttpServletRequest req, HttpServletResponse ) {
        Unmarshaller u = context.createUnmarshaller();
        u.unmarshal(...);
    }

This is the simplest safe way to use the JAXB RI from multi-threaded applications.

If you really care about the performance, and/or your application is going to read a lot of small documents, then creating Unmarshaller could be relatively an expensive operation. In that case, consider pooling Unmarshaller objects. Different threads may reuse one Unmarshaller instance, as long as you don't use one instance from two threads at the same time.

8.2. Compiling DTD

The JAXB RI is shipped with an "experimental" DTD support, which let's you compile XML DTDs. It is marked "experimental" not because the feature is unstable nor unreliable, but rather because it's not a part of the JAXB specification and therefore the level of commitment to compatibility is lower.

Example 91. To compile a DTD, run the XJC binding compiler as follows:

$ xjc.sh -dtd test.dtd

All the other command-line options of the XJC binding compiler can be applied. Similarly, the XJC ant task supports DTD. The generated code will be no different from what is generated from W3C XML Schema. You'll use the same JAXB API to access the generated code, and it is portable in the sense that it will run on any JAXB 2.0 implementation.

DTD long predates XML namespace, although people since then developed various techniques to use XML namespaces in conjunction with DTD. Because of this, XJC is currently unable to reverse-engineer the use of XML namespace from DTD. If you compile DTDs that use those techniques, you'd either manuallly modify the generated code, or you can try a tool like Trang that can convert DTD into XML Schema in ways that better preserves XML namespaces.

8.2.1. Customizations

The customization syntax for DTD is roughly based on the ver.0.21 working draft of the JAXB specification, which is available at xml.coverpages.org. The deviations from this document are:

  • The whitespace attribute of the conversion element takes " preserve", " replace", and " collapse" instead of " preserve", " normalize", and " collapse" as specified in the document.

  • The interface customization just generates marker interfaces with no method.

8.2.2. Compiling DTD from Maven2

Example 92. The following POM snippest describes how to invoke XJC to compile DTD from a Maven 2 project:

<plugin>
  <groupId>org.jvnet.jaxb2.maven2</groupId>
  <artifactId>maven-jaxb2-plugin</artifactId>
  <executions>
    <execution>
      <goals>
        <goal>generate</goal>
      </goals>
      <configuration>
        <!--  if you want to put DTD somewhere else
        <schemaDirectory>src/main/jaxb</schemaDirectory>
        -->
        <extension>true</extension>
        <schemaLanguage>DTD</schemaLanguage>
        <schemaIncludes>
          <schemaInclude>*.dtd</schemaInclude>
        </schemaIncludes>
        <bindingIncludes>
          <bindingInclude>*.jaxb</bindingInclude>
        </bindingIncludes>
        <args>
          <arg>-Xinject-listener-code</arg>
        </args>
      </configuration>
    </execution>
  </executions>
  <dependencies>
    <dependency>
      <groupId>org.jvnet.jaxb2-commons</groupId>
      <artifactId>property-listener-injector</artifactId>
      <version>1.0</version>
    </dependency>
  </dependencies>
</plugin>

Example 93. The dependencies section inside the plugin element can be used to specify additional XJC plugins. If you'd like to use more recent version of the JAXB RI, you can specify a dependency to XJC here to do so, like this:

<dependency>
  <groupId>com.sun.xml.bind</groupId>
  <artifactId>jaxb-xjc</artifactId>
  <version>2.1.2</version>
</dependency>

The complete sample project is available from here.

8.3. Using JAXB from Maven

Example 94. If you are using Maven 2 as your build system, you can declare the following dependencies in your POM for using the JAXB RI. Replace the version with the version of your choice.

<dependency>
  <groupId>com.sun.xml.bind</groupId>
  <artifactId>jaxb-impl</artifactId>
  <version>2.1.8</version>
</dependency>

This artifact is available in the java.net Maven repository, so you also need the following <repository> declaration in your POM:

<repository>
  <id>java.net</id>
  <url>http://download.java.net/maven/1</url>
  <layout>legacy</layout>
</repository>

For a list of available artifacts and versions, see this. In addition to the runtime jar, XJC is available under the artifact Id "jaxb-xjc".

You can also invoke XJC through a Maven plugin by using the Maven2 JAXB 2.x plugin.

8.4. Designing a client/server protocol in XML

Occasionally, people try to define a custom protocol that allows multiple XML requests/responses to be sent over a single transport channel (example.) This section discusses the non-trivial interaction between XML and sockets, and how you can design a protocol correctly.

XML1.0 requires a conforming parser to read the entire data till end of the stream (because a parser needs to handle documents like <root/><!-- post root comment -->). As a result, a naive attempt to keep one OutputStream open and marshal objects multiple times fails.

Example 95. One easy way to work around this limitation is to design your protocol so that the data on the wire will look like the following:

<conversation>
  <!-- message 1 -->
  <message>
    ...
  </message>

  <!-- message 2 -->
  <message>
    ...
  </message>

  ...
</conversation>

The <conversation> start tag is sent immediately after the socket is opened. This works as a container to send multiple "messages", and this is also an excellent opportunity to do the hand-shaking (e.g., protocol-version='1.0' attribute.) Once the <conversation> tag is written, multiple messages can be marshalled as a tree into the channel, possibly with a large time lag in between. You can use the JAXB marshaller to produce such message. When the sender wants to disconnect the channel, it can do so by sending the </conversation> end tag, followed by the socket disconnection.

Of course, you can choose any tag names freely, and each message can have different tag names.

The receiver would use the StAX API and use XMLStreamReader to read this stream. You'd have to use this to process the first <conversation> start tag. After that, every time you call a JAXB unmarshaller, you'll get the next message.

For the concrete code, see the xml-channel example in the JAXB RI distribution.

8.5. Contributing to this Guide

This document is maintained by people in the community. If you'd like to have the edit access to this document, please let us know so that we can discuss it.

The master source of this document is written in HTML + a set of custom tags. The use of custom tags allow us to ensure a consistent look&feel, and take care of other mandane tasks like assigning chapter/section numbers and generating permalinks. The technology is based on Jelly, so more custom tags can be added to perform additional processing and the possibility is endless.

Once you have the commit access, you can update the documents and run the build script to generate the final HTMLs to verify the result. When you feel satisfied, commit the changes. The repository is polled every hour, so your change will be eventually picked up and published, but you can also force the update by sending an e-mail to .

The general mode of operation is that:

  • When you are answering forum questions, ask yourself if the content should instead go to the user's guide. You can then post the pointer to the document (trigging a posting by e-mail is handy in this case.)

  • It's OK to have TBDs and TODOs. This is an incremental effort. Having something is better than not having anything.

  • Try not to change section titles, as they are used for permalinks.

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