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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 Java^TM Runtime Environment (JRE^TM), a java.security.AccessControlException is thrown in the stop, suspend, or resume method of java.lang.Thread class as shown in the following code:

        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 without any error with the Microsoft Virtual Machine (VM).

Cause

This exception is caused when a user call 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 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 expression of the implementation, thus resulting in an java.security.AccessControlException.

Resolution

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

Replace calls to stop, suspend, and resume with code that modifies a variable to indicate that the target thread should stop or suspend or resume.

The following examples show how your can replace stop, suspend and resume methods with the alternative code:

For example, assume that 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 java.security.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 blinker.stop by modifying the applet's stop, destroy, and run methods as shown in the following code:

    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();
        }
    }

For example consider an applet that has the mousePressed event handler which toggles the state of a thread called blinker as shown in the following code:

    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 blinker.suspend and blinker.resume by replacing the event handler with the following code:

    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?