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:

Bean with Map

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

Bean with Map

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

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.


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