Question

Here’s my tiny class:

import java.io.Serializable;
public abstract class SerializableCallback extends Callback
implements Serializable {
    private static final long serialVersionUID = 4544768712188843966L;
    public abstract void handleMessage(Message msg);
}

Here’s its even tinier parent Callback:

public abstract class Callback {
    public abstract void handleMessage(Message msg);
}

And here’s my test:

public void testSerialization() throws IOException, ClassNotFoundException {
    SerializableCallback c = new SerializableCallback() {
        private static final long serialVersionUID = -4852385037064234702L;
        @Override
        public void handleMessage(Message msg) {
            callbackMethod();
        }
    };

    FileOutputStream fos = new FileOutputStream(FILE);
    ObjectOutputStream out = new ObjectOutputStream(fos);
    out.writeObject(c); // fails
    out.close();

    FileInputStream fis = new FileInputStream(FILE);
    ObjectInputStream in = new ObjectInputStream(fis);
    Object object = in.readObject();
    @SuppressWarnings("unused")
    SerializableCallback d = (SerializableCallback) object;
}

private void callbackMethod() {}

The test gives me a NotSerializableException on the line indicated by a comment. Here’s a couple of things by which it can certainly not be caused:

1. Missing Serializable declaration
2. Non-serializable fields: there are no fields at all
3. Invisible non-arg constructor of closest non-serializable parent type

So what does cause the exception?

Answer 1

Your class doesn’t have a no-arg constructor. It has a constructor which takes one argument which is the this of the outer class. If you made testSerialization() static it might fix the problem.

But I suspect your immediate real problem is that your nest class has a reference to the outer class and the outer class is not Serializable.