(Brought over from SE)

The problem

I'm coding a Java Swing application dealing with XML files, so I'm using JAXB in order to marshal classes into documents and unmarshal the other way around.

I want to include a private field in the class that gets marshalled, which will store the backing file the class is based on (if any), in the form of a File object. This way, I can determine if a backing file is in use, so when saving via a Save command, if the backing file is available, I can just marshal the class directly to that file, instead of obtaining it via a "Save file" dialog.

However, it seems that with the tools available in JAXB, I cannot get the File object from the Unmarshaller while opening it. How can I tackle the situation so that I can set that variable correctly?

As this variable is internal, I don't want to include a setter or expose it so that other classes can't change it.

Background

Being aware of class event callbacks and external listeners, I know I can use a class event callback to set a class instance private field either before or after unmarshalling, but it seems I can't retrieve the file object being in use by the Unmarshaller from inside that callback.

On the other hand, with an external listener I could get ahold of the File object being unmarshalled, as it would be at the same level with the unmarshal method call, but now the private field would either need to be public or include a setter in order for it to be set.

Sample code

The following is a minimal, reproducible example, split in two files: JAXBTest.java and MarshalMe.java, both placed at the same level.

MarshalMe.java

import java.io.File;

import javax.xml.bind.annotation.XmlRootElement;

@XmlRootElement
public class MarshalMe {
    private File backingFile;

    public File getBackingFile() {
        return backingFile;
    }

    // Dummy function that sets the backing file beforehand.
    public void processSth() {
        backingFile = new File("dummy.hai");
    }
}

JAXBDemo.java

import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;

import javax.xml.bind.JAXBContext;
import javax.xml.bind.JAXBException;
import javax.xml.bind.Marshaller;
import javax.xml.bind.Unmarshaller;

public class JAXBTest {
    public static void writeXML(MarshalMe me, File xml) {
        try {
            JAXBContext contextObj = JAXBContext.newInstance(MarshalMe.class);
            Marshaller marshallerObj = contextObj.createMarshaller();
            marshallerObj.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);

            marshallerObj.marshal(me, new FileOutputStream(xml));
        } catch (JAXBException jaxbe) {
            jaxbe.printStackTrace();
        } catch (FileNotFoundException fnfe) {
            fnfe.printStackTrace();
        }
    }

    public static MarshalMe readXML(File xml) {
        MarshalMe me = null;

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

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

        return me;
    }

    public static void main(String[] args) {
        MarshalMe src = new MarshalMe();
        src.processSth();
        System.out.println(src.getBackingFile());

        File meFile = new File("me.xml");
        writeXML(new MarshalMe(), meFile);

        MarshalMe retrieved = readXML(meFile);
        System.out.println(retrieved.getBackingFile());
    }
}

Expected output

Running with Java 1.8 (or later, provided a JAXB library and runtime implementation is accesible), the minimal, reproducible example outputs:

dummy.hai
null

when I expect the output to be

dummy.hai
me.xml

as the class is initially written in a XML file named me.xml before being read back.

posted about 19 hours ago

CC BY-SA 4.0

aura-lsprog-86‭

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