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:

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".

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"

1.2.1.1. 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:

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:

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"

1.2.1.2. 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:

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.

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.

1.2.2. 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>

1.2.3. 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.


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