Java swing组件序列化

Question

I have to bugfix an old Java project. This sticks to Java 5. It can be compiled with Java 8, but then some GUI components do no more work correctly. The first thing i saw and really shocked me: This [censored] original developer had built a javax.swing package with many new classes, and many classes implemented again, which where already part of the standard library. So the first thing i did was moving the whole crap to another namespace. I used the Eclipse refactoring feature to rename the whole javax.swing and all its subpackages to somethingelse.swing. Now it does not compile any more. Hopefully now i have the chance to find the bugs.

The current bug i'm stuck is in this method:

private void writeObject(ObjectOutputStream s) throws IOException {
    s.defaultWriteObject();
    if (getUIClassID().equals(uiClassID)) {
        byte count = JComponent.getWriteObjCounter(this);
        JComponent.setWriteObjCounter(this, --count);
        if (count == 0 && ui != null) {
            ui.installUI(this);
        }
    }
}

The class this is part of is kind of a JComboBox with some extras. I do not really know what extras. It's not important. The problem here is: The methods JComponent.getWriteObjCounter() and setWriteObjCounter() are package-private. So the original code had access to these methods. The moved code now does not have. Since these are package-private, i did not find any documentation of it. So i do not have an idea for what all this is, besides the fact that it is a custom serializer for the Java serialization feature.

I do understand the first line. That is standard. Interestingly it only writes the default, it does not write anything else. So i strongly doubt if the whole method is necessary at all. But the remainder of the method i have no idea what it is supposed to do. It seems to install the UI delegate if a counter is counted down to 0. But then, why on writing? Why not on reading? And what is this counter? How is it handled? For what purpose is it?

The original Java code in the JComponent class here is:

static byte getWriteObjCounter(JComponent comp) {
    return (byte)((comp.flags >> WRITE_OBJ_COUNTER_FIRST) & 0xFF);
}

But that does not shed much light to it either. The attribute JComponent.flags is not accessible. Not from outside the javax.swing package. So again i have no idea what this is for.

As already said, i strongly doubt that this method makes any sense at all. But if anyone could tell me for what purpose this writeObjCounter is i would have a chance for a better guess :)

Many thanks Siegfried Gipp

Answer 1

Welcome in maintenance hell :-(

Look in source of JComponent at constant WRITE_OBJ_COUNTER_FIRST :

/** Bits 14-21 are used to handle nested writeObject calls. **/
private static final int WRITE_OBJ_COUNTER_FIRST = 14;

Hence, the getWriteObjCounter(JComponent) and setWriteObjCounter(JComponent, byte) methods somehow handle the nesting depth when serializing nested JComponent objects.

Also look at the comment in front of these 2 methods :

// These functions must be static so that they can be called from
// subclasses inside the package, but whose inheritance hierarhcy includes
// classes outside of the package below JComponent (eg, JTextArea).

Therefore it is important these methods have default package access, or otherwise Swing serialization of subclasses of JComponent would not be compilable anymore.