Generating a JAXB class that implements an interface

It might be overkill for your situation, but I have done this using AspectJ (we were already using aspects on that project so we already had the dependency and the exposure).

You'd declare an aspect along the lines of:

public aspect MyAspect
{
    declare parents: 
        com.foo.generated.Adult
    implements com.foo.Person;

    declare parents: 
        com.foo.generated.Kid
    implements com.foo.Person;
}

Which will add the interface com.foo.Person to the classes com.foo.generated.Adult and com.foo.generated.Kid

Might be overkill for your purpose, but it worked for us.

The answer that worked for me was Jim Hurne's example of using the JAXB2 Basics plugin. But the documentation he linked appears to be no longer available so for reference, this is how I configured the maven plugin:

        <plugin>
            <groupId>org.jvnet.jaxb2.maven2</groupId>
            <artifactId>maven-jaxb2-plugin</artifactId>
            <version>0.8.2</version>
            <executions>
                <execution>
                    <goals>
                        <goal>generate</goal>
                    </goals>
                    </execution>
            </executions>
            <configuration>
                <extension>true</extension>
                <args>
                    <arg>-Xinheritance</arg>
                </args>
                <bindingDirectory>src/main/resources/xjb</bindingDirectory>
                <bindingIncludes>
                    <include>**.xml</include> <!-- This Should reference the binding files you use to configure the inheritance -->
                </bindingIncludes>
                <schemaDirectory>src/main/resources/xsd</schemaDirectory>
                <generateDirectory>${project.build.directory}/generated-sources/jaxb</generateDirectory>
                <generatePackage>mypackage</generatePackage>
                <plugins>
                    <plugin>
                        <groupId>org.jvnet.jaxb2_commons</groupId>
                        <artifactId>jaxb2-basics</artifactId>
                        <version>0.5.3</version>
                    </plugin>
                </plugins>
            </configuration>
        </plugin>

It is possible to achieve this for this simple case without using a 3rd Party plugin using the JAXB RI Vendor Extensions xjc:superInterface customization. It should be noted this approach will cause all generated classes to implement the interface.

Example bindings file:

<jxb:bindings version="1.0" 
  xmlns:jxb="http://java.sun.com/xml/ns/jaxb" 
  xmlns:xs="http://www.w3.org/2001/XMLSchema" 
  xmlns:xjc="http://java.sun.com/xml/ns/jaxb/xjc" 
  jxb:extensionBindingPrefixes="xjc">
    <jxb:bindings schemaLocation="adult.xsd" node="/xs:schema">
    <jxb:globalBindings>
        <xjc:superInterface name="com.example.model.Person"/>
    </jxb:globalBindings>
    </jxb:bindings>
</jxb:bindings>

You then just need to run xjc specifying the binding file and setting the -extension flag. Much quicker/simpler than bringing in JAXB2Basics!

I was initially sceptical that this would work as the documentation states:

The customization allows you to specify the fully qualified name of the Java interface that is to be used as the root interface of all the generated interfaces. This customization has no effect unless you are specifically generating interfaces with the globalBindings generateValueClass="false" switch.

But when I tried it out with a bindings similar to the one above (without specifying generateValueClass="false", and generating classes, not interfaces), it gave me the output I required - all my generated classes implemented the common interface.

Unfortunately, it looks like the interface-injection plugin mentioned in some of the other answers is no longer well-supported. In fact, I'm having trouble finding the JAR for download.

Thankfully, the JAXB2 Basics Plugins provides a similar mechanism for adding an interface to the generated JAXB stubs (see the Inheritance plugin).

The Inheritance plugin documentation has an example showing what the XML schema file might look like. However, since you cannot modify the schema, you can use an external bindings file instead:

<?xml version="1.0"?>
<jxb:bindings version="1.0" 
  xmlns:jxb="http://java.sun.com/xml/ns/jaxb" 
  xmlns:xs="http://www.w3.org/2001/XMLSchema" 
  xmlns:xjc="http://java.sun.com/xml/ns/jaxb/xjc"
  xmlns:inheritance="http://jaxb2-commons.dev.java.net/basic/inheritance"
  jxb:extensionBindingPrefixes="xjc">

    <jxb:bindings schemaLocation="xsd/adult.xsd">
      <jxb:bindings node="//xs:complexType[@name='Person']">
        <inheritance:implements>mypackage.Hello</inheritance:implements> 
      </jxb:bindings>
    </jxb:bindings>

</jxb:bindings>

The JAXB2 Basics Plugins documentation includes instructions for using the plugin with Ant and Maven. You can also use it straight from the command line, but the command is a bit messy (due to the number of jars you have to add to the classpath):

java -jar jaxb-xjc.jar 
     -classpath jaxb2-basics-0.5.3.jar,jaxb2-basics-runtime-0.5.3.jar,
                jaxb2-basics-tools-0.5.3.jar,commons-beanutils-0.5.3.jar,
                commons-lang.jar,commons-logging.jar
     -p mypackage.myxml -extension -Xinheritance xsd/adult.xsd -b binding.xjb

The JAXB2 Basics Plugins provides a number of other utilities which you might also find useful (such as autogeneration of equals, hashCode, and toString methods).