Java Cookbook

Stopping a Thread

Problem

You need to stop a thread.

Solution

Don’t use the Thread.stop( ) method; instead, use a boolean tested at the top of the main loop in the run( ) method.

Discussion

While you can use the thread’s stop( ) method, Sun recommends against it. That’s because the method is so drastic that it can never be made to behave reliably in a program with multiple active threads. That is why, when you try to use it, the compiler will generate deprecation warnings. The recommended method is to use a boolean variable in the main loop of the run( ) method. The program in Example 24-5 prints a message endlessly until its shutDown( ) method is called; it then sets the controlling variable done to false, which terminates the loop. This causes the run( ) method to return, ending the thread. The ThreadStoppers program in the source directory for this chapter has a main program that instantiates and starts this class, and then calls the shutDown( ) method.

Example 24-5. StopBoolean.java

public class StopBoolean extends Thread {
    protected boolean done = false;
    public void run(  ) {
        while (!done) {
            System.out.println("StopBoolean running");
            try {
                sleep(720);
            } catch (InterruptedException ex) {
                // nothing to do 
            }
        }
        System.out.println("StopBoolean finished.");
    }
    public void shutDown(  ) {
        done = true;
    }
}

Running it looks like this:

StopBoolean running StopBoolean running StopBoolean running StopBoolean running StopBoolean running StopBoolean running StopBoolean running StopBoolean finished. ...