How do you kill a Thread in Java

This is a quick tutorial on killing a Thread in Java.

In Java threads are not killed, but the stopping of a thread is done in a cooperative way. The thread is asked to terminate and the thread can then shutdown gracefully.

Often a volatile boolean field is used which the thread periodically checks and terminates when it is set to the corresponding value.

I would not use a boolean to check whether the thread should terminate. If you use volatile as a field modifier, this will work reliable, but if your code becomes more complex, for instead uses other blocking methods inside the while loop, it might happen, that your code will not terminate at all or at least takes longer as you might want.

• Certain blocking library methods support interruption.

Every thread has already a boolean flag interrupted status and you should make use of it. It can be implemented like this:

public void run() {
   try {
      while (!interrupted()) {
         // ...
      }
   } catch (InterruptedException consumed)
      /* Allow thread to exit */
   }
}
public void cancel() { interrupt(); }

Source code adapted from Java Concurrency in Practice. Since the cancel() method is public you can let another thread invoke this method as you wanted.

Observations to kill a Thread in Java

1. Thread.stop() will stop a thread if the security manager allows it.
2. Thread.stop() is dangerous. Having said that, if you are working in a JEE environment and you have no control over the code being called, it may be necessary; see Why is Thread.stop deprecated?
3. You should never stop stop a container worker thread. If you want to run code that tends to hang, (carefully) start a new daemon thread and monitor it, killing if necessary.
4. stop() creates a new ThreadDeathError error on the calling thread and then throws that error on the target thread. Therefore, the stack trace is generally worthless.
5. In JRE 6, stop() checks with the security manager and then calls stop1() that calls stop0(). stop0() is native code.
6. As of Java 13 Thread.stop() has not been removed (yet), but Thread.stop(Throwable) was removed in Java 11. (mailing list, JDK-8204243)