Stopping a thread

On the previous pages, we focussed on how to start a thread in Java. We saw that after creating the Thread object, calling start() asynchronously starts the corresponding thread. In our example, the run() method contained an infinite loop. But in real life, we generally want our thread to stop. So how do we make our thread stop?

Firstly, the thing you shouldn't do is call Thread.stop(). This method, now deprecated, was intended to stop a given thread abruptly. But the problem with this is that the caller can't generally determine whether or not the given thread is at a safe point to be stopped. (This isn't just a Java phenomenon: in general, the underlying operating system calls that Thread.stop() makes to abruptly stop the thread are also deprecated for this reason.)

So how can we stop a thread safely? In general:

There are a couple of ways that we would typically do so.

Use a "stop request" variable

A common solution is to use an explicit "stop request" variable, which we check on each pass through the loop. This technique is suitable provided that we can check the variable frequently enough:

private volatile boolean stopRequested = false;

public void run() {
  while (!stopRequested) {
    ...
  }
}

public void requestStop() {
  stopRequested = true;
}

Note that we must declare the stopRequested variable as volatile, because it is accessed by different threads.

Use Thread.interrupt()

The above pattern is generally suitable if the variable stopRequested can be polled frequently. However, there is an obvious problem if the method blocks for a long time, e.g. by calling Thread.sleep() or waiting on an object. In general, such blocking methods are interruptible. For more information, see the section on thread interruption.

Organise for the thread to receive a queued "stop" message

In a typical producer-consumer pattern, where a thread blocks waiting on a queue (e.g. a BlockingQueue), we could organise for the thread to interpret some special object on the queue to mean "please shut down". To ask the thread to shut down, we therefore post the special message object to the queue. The advantage of this method is that the thread will finish processing messages already on the queue before shutting down.

See also

Among other things, see the section on thread safety for an introduction to some of the measures that need to be taken to make threads safely interact with one another, and hence why stopping a thread abruptly isn't safe.