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I've found a way to set the private field without exposing it or giving it a setter: [Reflection](http://tutorials.jenkov.com/java-reflection/private-fields-and-methods.html).

Using external event listeners, I can get ahold of the `File` object. Then, inside the `beforeUnmarshal` method, and after checking that I got the correct object, I use reflection to get the private field, and with the `setAccessible` method, I can now control when I get access to the field _using reflection only_.

After lifting the access checks, it's only a matter of editing the value via `set`ting it, and reinstating the checks after that.

# Relevant changes

The following snippet includes the relevant changes:

    unmarshallerObj.setListener(new Unmarshaller.Listener() {
        @Override
        public void beforeUnmarshal(Object target, Object parent) {
            if (!(target instanceof MarshalMe))
                return;

            MarshalMe me = (MarshalMe) target;
            try {
                Field meBackingFile = MarshalMe.class.getDeclaredField("backingFile");
                meBackingFile.setAccessible(true);
                meBackingFile.set(me, xml);
                meBackingFile.setAccessible(false);
            } catch (NoSuchFieldException | IllegalAccessException ex) {
                // Intentionally left blank.
            }
        }
    });

# Including the edit in the sample program
Edit the file `JAXBDemo.java` by adding the following code:

    // Add to the import section
    import java.lang.reflect.Field;

    // Under this function
    public static MarshalMe readXML(File xml) {
        MarshalMe me = null;

        try {
            JAXBContext contextObj = JAXBContext.newInstance(MarshalMe.class);
            Unmarshaller unmarshallerObj = contextObj.createUnmarshaller();

            /* Add this code vvv */
            unmarshallerObj.setListener(new Unmarshaller.Listener() {
                @Override
                public void beforeUnmarshal(Object target, Object parent) {
                    if (!(target instanceof MarshalMe))
                        return;

                    MarshalMe me = (MarshalMe) target;
                    try {
                        Field meBackingFile = MarshalMe.class.getDeclaredField("backingFile");
                        meBackingFile.setAccessible(true);
                        meBackingFile.set(me, xml);
                        meBackingFile.setAccessible(false);
                    } catch (NoSuchFieldException | IllegalAccessException ex) {
                        // Intentionally left blank.
                    }
                }
            });
            /* Add this code ^^^ */

            me = (MarshalMe) unmarshallerObj.unmarshal(xml);
        } catch (JAXBException jaxbe) {
            jaxbe.printStackTrace();
        }

        return me;
    }

After adding the `import` and the code between the `/* Add this code */` lines, running the program again now outputs:

    dummy.hai
    me.xml

as expected.