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Can I combine the two techniques to produce a thread that may be safely ''stoppe

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Question: Can I combine the two techniques to produce a thread that may be safely "stopped" or "suspended"?

Answer: Yes; it's reasonably straightforward. The one subtlety is that the target thread may already be suspended at the time that another thread tries to stop it. If the stop method merely sets the state variable (blinker) to null, the target thread will remain suspended (waiting on the monitor), rather than exiting gracefully as it should. If the applet is restarted, multiple threads could end up waiting on the monitor at the same time, resulting in erratic behavior.
To rectify this situation, the stop method must ensure that the target thread resumes immediately if it is suspended. Once the target thread resumes, it must recognize immediately that it has been stopped, and exit gracefully. Here's how the resulting run and stop methods look:

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

public synchronized void stop() {
blinker = null;
notify();
}

If the stop method calls Thread.interrupt, as described above, it needn't call notify as well, but it still must be synchronized. This ensures that the target thread won't miss an interrupt due to a race condition.

this question/answer was published in SUN's article


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