Name: boT120536 Date: 02/20/2001


java version "1.3.0"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.3.0-C)
Java HotSpot(TM) Client VM (build 1.3.0-C, mixed mode)

The JScrollPane constructor isn't thread-safe, which violates the
Swing single thread rule. Calling the JScrollPane constructor in
two different threads simultaneously can result in a NullPointerException.

First off, the single-thread rule is:
 
"Once a Swing component has been realized, all code that might affect or
depend on the state of that component should be executed in the
event-dispatching thread."
 
The important thing is the phrase "once a component has been realized",
which is explained as: "Realized means that the component's paint()
method has been or might be called." So if your code looks something
like:
 
  Component foo = new BlahComponent (); // line 1
  foo.setVisible (true); // line 2
 
then line 1 is before the component is realized (so can be run by any
thread), and line 2 realizes the component (so needs to be run by the
event-handling thread).
 
If calling a component's constructor counts as realizing it, then
there's not much point to this wording of the single-threaded rule!
 
So as I see it, either the Swing documentation needs to be rewritten to
remove the phrase "Once a Swing component has been realized" from the
single-thread rule, or (which would be preferable!) the JScrollPane
class needs to be rewritten so the constructor is thread safe.

--- source code and detailed discussion follows ---

package ajeffrey.teaching.test;

import javax.swing.JFrame;
import javax.swing.JScrollPane;
import javax.swing.JTextArea;

/**
 * A class which tests Sun's JScrollPane class.
 * It creates two threads, both of which create a new scroll pane.
 * When run under Windows 9x, JDK1.3, it can cause a null pointer
 * exception (there's a race condition, so it only happens intermittently),
 * for example:
 * <pre>
 * java.lang.NullPointerException
 * at javax.swing.plaf.metal.BumpBuffer.fillBumpBuffer(Unknown Source)
 * at javax.swing.plaf.metal.BumpBuffer.&lt;init&gt;(Unknown Source)
 * at javax.swing.plaf.metal.MetalBumps.createBuffer(Unknown Source)
 * at javax.swing.plaf.metal.MetalBumps.setBumpColors(Unknown Source)
 * at javax.swing.plaf.metal.MetalBumps.&lt;init&gt;(Unknown Source)
 * at javax.swing.plaf.metal.MetalScrollBarUI.installDefaults(UnknownSource)
 * at javax.swing.plaf.basic.BasicScrollBarUI.installUI(Unknown Source)
 * at javax.swing.JComponent.setUI(Unknown Source)
 * at javax.swing.JScrollBar.updateUI(Unknown Source)
 * at javax.swing.JScrollBar.&lt;init&gt;(Unknown Source)
 * at javax.swing.JScrollBar.&lt;init&gt;(Unknown Source)
 * at javax.swing.JScrollPane$ScrollBar.&lt;init&gt;(Unknown Source)
 * at javax.swing.JScrollPane.createVerticalScrollBar(Unknown Source)
 * at javax.swing.JScrollPane.&lt;init&gt;(Unknown Source)
 * at javax.swing.JScrollPane.&lt;init&gt;(Unknown Source)
 * at ajeffrey.teaching.test.SimpleGUI.&lt;init&gt;(ajeffrey/teaching/test/TestJScrollPane.java:93)
 * at ajeffrey.teaching.test.TestJScrollPane.run(ajeffrey/teaching/test/TestJScrollPane.java:82)
 * at java.lang.Thread.run(Unknown Source)
 * </pre>
 * This violates the <i>single thread rule</i> of the Swing API,
 * described in
 * <a href="http://java.sun.com/products/jfc/tsc/articles/threads/threads1.html">Threads and Swing</a>:
 * <blockquote>
 * Once a Swing component has been realized, all code that
 * might affect or depend on the state of that component should be
 * executed in the event-dispatching thread.
 * </blockquote>
 * In particular, the constructors for each Swing component are required to be thread-safe.
 * Unfortunately, this isn't true of <code>JScrollPane</code>, because it relies on
 * <code>javax.swing.plaf.metal.MetalBumps</code>, which in turn uses a hidden
 * <code>BumpBuffer</code> class, which uses the following method to initialize its
 * <code>component</code> field to be a <code>Canvas</code> whose parent is <code>frame</code>:
 * <pre>
 * protected void createComponent() {
 * if (frame == null) {
 * frame = new Frame( "bufferCreator" );
 * }
 * if (component == null ) {
 * component = new Canvas();
 * frame.add( component, BorderLayout.CENTER );
 * }
 * // fix for 4185993 (moved this outside if block)
 * frame.addNotify();
 * }
 * </pre>
 * Unfortunately, this has a race condition... First thread A executes createComponent(),
 * and sets <code>frame = new Frame(...)</code> and <code>component = new Canvas ()</code>.
 * Then before thread A gets a chance to call <code>frame.add (...)</code>, thread B
 * calls createComponent, finds that neither <code>frame</code> nor <code>component</code>
 * are null, and so returns, without having properly initialized <code>component</code>
 * (it still doesn't have a parent, so calling <code>createImage</code> on it causes a
 * <code>NullPointerException</code>.
 *
 * @author <a href="http://fpl.cs.depaul.edu/ajeffrey">Alan Jeffrey</a>
 * @version 1.0.0, 30 January 2001
 */
public class TestJScrollPane implements Runnable {

    public static void main (String[] args) {
        // Create two simple GUIs in two different threads, and show them.
        Runnable task = new TestJScrollPane ();
        Thread threadA = new Thread (task);
        Thread threadB = new Thread (task);
        threadA.start ();
        threadB.start ();
    }

    public void run () { new SimpleGUI ().show (); }

}

class SimpleGUI implements Runnable {

    // Build a simple GUI
    protected final JFrame mainFrame = new JFrame ();
    protected final JTextArea textArea = new JTextArea (25, 40);

    // This constructor is not thread-safe:
    protected final JScrollPane textPane = new JScrollPane (textArea);

    public SimpleGUI () {
        // A very simple GUI indeed.
     mainFrame.getContentPane ().add (textPane);
        mainFrame.setTitle ("Testing JScrollPane for thread safety");
    }

    public void show () {
        // To be extra goody-goody, show () makes sure the Swing event handler
        // thread calls pack() and setVisible(true).
        javax.swing.SwingUtilities.invokeLater (this);
    }

    public void run () {
        // This should only be called by the event handler thread.
        mainFrame.pack ();
        mainFrame.setVisible (true);
    }

}
(Review ID: 116086)
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