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java.security.AccessControlException thrown in stop, suspend, or resume method of java.lang.Thread

Symptoms

When running an applet in a browser using the Sun JRE, an AccessControlException is thrown in the stop, suspend, or resume method of java.lang.Thread.

        java.security.AccessControlException: access denied (java.lang.RuntimePermission modifyThread)
        at java.security.AccessControlContext.checkPermission(Unknown Source)
        at java.security.AccessController.checkPermission(Unknown Source)
        at java.lang.SecurityManager.checkPermission(Unknown Source)
        at sun.applet.AppletSecurity.checkAccess(Unknown Source)
        at java.lang.Thread.checkAccess(Unknown Source)
        at java.lang.Thread.stop(Unknown Source)
        at ....

The same applet runs under the Microsoft VM.

Cause

This exception is caused by calling these methods on a dead Thread object in the Sun JRE.

The Java class libraries in the Sun JRE have changed over time. Some APIs have been clarified, some have been deprecated, and some have had their implementation altered.

The result of calling stop, suspend, and resume on a dead Thread object was not well defined. In the Microsoft VM, they result in a no-op. However, in the Sun JRE, calling these methods on a dead Thread object invalidates the underlying invariant of the implementation, thus resulting in an AccessControlException.

Resolution

The Thread stop, suspend, and resume methods are inherently unsafe and have been deprecated in the Java 2 platform.

To work around this problem, replace calls to stop, suspend, and resume with code that simply modifies some variable to indicate that the target thread should stop/suspend/resume.

For example, suppose your applet contains the following methods:

    private Thread blinker;

    public void start() {
        blinker = new Thread(this);
        blinker.start();
    }

    public void stop() {
        blinker.stop();  // UNSAFE!
    }

    public void destroy() {
        blinker.stop();  // UNSAFE and WILL throw AccessControlException in the Sun JRE!
    }

    public void run() {
        Thread thisThread = Thread.currentThread();
        while (true) {
            try {
                thisThread.sleep(interval);
            } catch (InterruptedException e){
            }
            repaint();
        }
    }

You can avoid the use of Thread.stop by replacing the applet's stop, destroy, and run methods with:

    private volatile Thread blinker;

    public void stop() {
        blinker = null;
    }

    public void destroy() {
        blinker = null;
    }

    public void run() {
        Thread thisThread = Thread.currentThread();
        while (blinker == thisThread) {
            try {
                thisThread.sleep(interval);
            } catch (InterruptedException e){
            }
            repaint();
        }
    }

Another example: Suppose your applet contains the following mousePressed event handler, which toggles the state of a thread called blinker:

    private boolean threadSuspended;

    public void mousePressed(MouseEvent e) {
        e.consume();

        if (threadSuspended)
            blinker.resume();
        else
            blinker.suspend();  // DEADLOCK-PRONE!

        threadSuspended = !threadSuspended;
    }

    public void run()
    {
	 while (true) {
        try {
            Thread.currentThread().sleep(interval);
        } catch (InterruptedException e){
        }
        repaint();
    }

You can avoid the use of Thread.suspend and Thread.resume by replacing the event handler above with the following:

    private boolean volatile threadSuspended;

    public synchronized void mousePressed(MouseEvent e) {
        e.consume();

        threadSuspended = !threadSuspended;

        if (!threadSuspended)
            notify();
    }

    public void run() {
        while (true) {
            try {
                Thread.currentThread().sleep(interval);

                if (threadSuspended) {
                    synchronized(this) {
                        while (threadSuspended)
                            wait();
                    }
                }
            } catch (InterruptedException e){
            }
            repaint();
        }
    }

Related Information

Why Are Thread.stop, Thread.suspend, Thread.resume, and Runtime.runFinalizersOnExit Deprecated?