重置ObjectOutputStream时，摆动：EDT NullPointerException

Question

I am really struggling with following nasty Exception thrown within the Swing-Thread:

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at javax.swing.plaf.basic.BasicScrollBarUI.layoutHScrollbar(BasicScrollBarUI.java:762)
at javax.swing.plaf.basic.BasicScrollBarUI.layoutContainer(BasicScrollBarUI.java:870)
at java.awt.Container.layout(Container.java:1508)
at java.awt.Container.doLayout(Container.java:1497)
at java.awt.Container.validateTree(Container.java:1693)
at java.awt.Container.validateTree(Container.java:1702)
at java.awt.Container.validate(Container.java:1628)
at javax.swing.RepaintManager$3.run(RepaintManager.java:704)
at javax.swing.RepaintManager$3.run(RepaintManager.java:702)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:75)
at javax.swing.RepaintManager.validateInvalidComponents(RepaintManager.java:701)
at javax.swing.RepaintManager$ProcessingRunnable.run(RepaintManager.java:1719)
at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:311)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:749)
at java.awt.EventQueue.access$500(EventQueue.java:97)
at java.awt.EventQueue$3.run(EventQueue.java:702)
at java.awt.EventQueue$3.run(EventQueue.java:696)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:75)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:719)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:201)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:116)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:105)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:101)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:93)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:82)

Since I'm not able to reproduce the error for a SSCCE, let me just explain what the application is doing:

• The Main -function calls the Ui -Class extending JFrame , which builds the GUI. Also, a ServerSocket -Thread is started and listens for incoming connections.
• If a client connects, a new ServerWorker -Thread is started which handles incoming packages send through ObjectOutputStreams and received through ObjectInputStreams (on both sides).
• The client requests an NetIORequestUpdate filled with information from the server every second. The server then collects the information and stores them in fields of the new NetIOResponseUpdate .
• Since I need to reset the ObjectOutputStream before sending a changed NetIOResponseUpdate (because field values differ from the previously sent), os.reset() is performed.
• For the first time, this NetIOResponseUpdate is being send (and os.reset() excecuted), everything works like a charm. But after then, every call to the reset() -function throws above-noted exception.

The GUI or components are not modified while processing the NetIORequestUpdate and filling the NetIOResponseUpdate with information.

Still, if I skip resetting the ObjectOutputStream , above exception is not thrown.

I encapsulated every Swing-Component creation and modification with SwingUtilities.invokeLater .

Am I missing something? I would welcome any hint or advices regarding this problem. Please let me know if I should provide additional information or code.

Code-Examples

NetIOServerWorker

public class NetIOServerWorker implements Runnable {
    @Override
    public final void run() {
        // ... establish / accept connection and initialize Streams like:
        // outStream = new ObjectOutputStream(client.getOutputStream())
        //  inStream = new ObjectInputStream(client.getInputStream())

        while ((request = (NetIORequest) inStream.readObject()) != null) {
            // Processes the request, e.g. build response package
            response = request.process();

            send(response);

            outStream.reset(); // -- This nasty little piece of fluff
        }
    }
}

NetIORequestUpdate

public class NetIORequestUpdate implements NetIORequest, Serializable {


    private static final long serialVersionUID = -1578148718545111344L;

    @Override
    public final NetIOResponse process() {
        return new NetIOResponseUpdate();
    }

}

NetIOResponseUpdate

public class NetIOResponseUpdate implements NetIOResponse, Serializable {

    private static final long serialVersionUID = -7965101237058494053L;

    private NetIOPackage data;

    public NetIOResponseUpdate() {
        data = new NetIOPackage();
        data.setState(DataModel.getState());
    }

    @Override
    public void process() {

    }

}

GUI Creation

public final class Ui extends JFrame {
    // Constructor is private since this is a Singleton
    private Ui() {
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setPreferredSize(new Dimension(800, 600));

        // Start GUI-Construction
        invokeInDispatchThreadIfNeeded(new Runnable() {
            @Override
            public void run() {
                build();
                pack();
                setVisible(true);
            }
        });
    }

    private void build() {

        setBounds(100, 100, 754, 529);
        getContentPane().setLayout(new BorderLayout(0, 0));

        UiComponentStatusPanel pnlStatus = new UiComponentStatusPanel(); // extends JPanel
        JScrollPane scrollStatus = new JScrollPane(pnlStatus);
        getContentPane().add(scrollStatus, BorderLayout.CENTER);
    }

    public static void invokeInDispatchThreadIfNeeded(final Runnable runnable) {
        if (EventQueue.isDispatchThread()) {
            runnable.run();
        } else {
            SwingUtilities.invokeLater(runnable);
        }
    }
}

Answer 1

NullPointerException in java implies that an object is used without initialization. Now let's trace it back, reset is not the problem but outstream. Once outstream is not-null then reset would work. Check the line that initializes outstream and make sure that the parameter you are passing into that constructor is indeed returning something valid (client.getOutputStream()).It simply implies that the client is not returning a valid outPutStream which is either a problem from the client.. Trace it back in that direction (use a debugger) and let's see what you find

Answer 2

Thanks for helping me get on the right track. A solution has been found.

As it turned out, the error lies in an other branch of the project:

To 'callback' my GUI after changes in the data model have been made, I used an Interface UiNotifierEvent with a single function void refreshUi(); . Since i then add a reference to the Ui -Instance to a field uiCallback in the DataModel -Class, i also need to run uiCallback.refreshUI(); within the EDT. Even if the function body is empty.

This is perfectly logical. What is not, is that resetting the OutputStream has an influence on this exception.