JAXB has been removed from JavaSE starting in JDK 9, so we’ve had to make some changes to some of our code to work around this. We have some custom ANT tasks that use JAXB to process some XML. The task it used inside an ANT script using the <taskdef> tag as follows:

<taskdef name="myCustomTask" 
    classname="com.example.tasks.MyCustomTask" 
    classpath="MyCustomTask.jar"/>

Then laster the task us run using syntax like:

<myCustomTask />

If you try to run these ANT tasks on JDK 9 or higher, you just get a big ClassNotFound error when it tries to load the JAXB classes. So the obvious solution is to bundle the JAXB classes into our jar. This, however, only solves part of the problem. This will, indeed, allow the task to load, but when we try to run the task, it says

javax.xml.bind.JAXBException: Implementation of JAXB-API has not been found on module path or classpath.

Which is strange because we have the API (jaxb-api.jar) and implementation (jaxb-impl.jar, jaxb-core.jar, activation.jar) embedded inside our MyCustomTask.jar file, which should be available on the classpath.

After banging my head against this problem for a few hours, I discovered that the problem is the way which JAXB looks for the implementation. It uses the threads classloader for searching for an implementation, rather than the classloader for our task. When running inside an ANT task, this will be the root classpath for Ant, and not the classpath for my custom ant task.

For example, we have some code like:

JAXBContext componentContext = JAXBContext.newInstance(ComponentEntry.class);

This will fail to find the JAXB implementation (unless we included the JAXB jars in Ant’s classpath – which is not portable in our case, because ANT will usually be run inside an IDE like Netbeans).

I ultimately worked around this problem by wrapping all of my JAXB code inside my own spawned thread. E.g.

private String processFileWithJAXB(final File xmlFile, final boolean full) throws JAXBException {
        final JAXBException[] error = new JAXBException[1];
        final String[] result = new String[1];

        Thread t = new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    result[0] = processFileWithJAXBInternal(xmlFile, full);
                } catch (JAXBException ex) {
                    error[0] = ex;
                }
            }

        });
        ClassLoader cl = getClass().getClassLoader();
        t.setContextClassLoader(getClass().getClassLoader());
        t.start();

        try {
            t.join();
        } catch (InterruptedException ex) {
            Logger.getLogger(MyClass.class.getName()).log(Level.SEVERE, null, ex);
        }
        if (error[0] != null) {
            throw new JAXBException(error[0]);
        }
        return result[0];
    }

In this example, all of my JAXB stuff is inside the processFileWithJAXBInternal() method. The processFileWithJAXB method creates a thread, sets its context classloader to the current classes class loader, and runs it. And magically, it can find my bundled JAXB implementation.

Posting in my blog to help my memory as I’m bound to run into this issue again.