Developer

Interrupting Java threads

Multithreading presents a whole set of new challenges to the programmer that, if not correctly addressed, can lead to unexpected behavior and hard-to-find errors. In this article we address one of those challenges: how to interrupt a running thread.


Writing multithreaded programs in Java, with its built-in support for threads, is fairly straightforward. However, multithreading presents a whole set of new challenges to the programmer that, if not correctly addressed, can lead to unexpected behavior and subtle, hard-to-find errors. In this article, we address one of those challenges: how to interrupt a running thread.

Background
Interrupting a thread means stopping what it is doing before it has completed its task, effectively aborting its current operation. Whether the thread dies, waits for new tasks, or goes on to the next step depends on the application.

Although it may seem simple at first, you must take some precautions in order to achieve the desired result. There are some caveats you must be aware of as well.

First of all, forget the Thread.stop method. Although it indeed stops a running thread, the method is unsafe and was deprecated, which means it may not be available in future versions of the Java.

Another method that can be confusing for the unadvised is Thread.interrupt. Despite what its name may imply, the method does not interrupt a running thread (more on this later), as Listing A demonstrates. It creates a thread and tries to stop it using Thread.interrupt. The calls to Thread.sleep() give plenty of time for the thread initialization and termination. The thread itself does not do anything useful.

If you run the code in Listing A, you should see something like this on your console:
Starting thread...
Thread is running...
Thread is running...
Thread is running...
Interrupting thread...
Thread is running...
Thread is running...
Thread is running...
Stopping application...

Even after Thread.interrupt() is called, the thread continues to run for a while.

Really interrupting a thread
The best, recommended way to interrupt a thread is to use a shared variable to signal that it must stop what it is doing. The thread must check the variable periodically, especially during lengthy operations, and terminate its task in an orderly manner. Listing B demonstrates this technique.

Running the code in Listing B will generate output like this (notice how the thread exits in an orderly fashion):
Starting thread...
Thread is running...
Thread is running...
Thread is running...
Asking thread to stop...
Thread exiting under request...
Stopping application...

Although this method requires some coding, it is not difficult to implement and give the thread the opportunity to do any cleanup needed, which is an absolute requirement for any multithreaded application. Just be sure to declare the shared variable as volatile or enclose any access to it into synchronized blocks/methods.

So far, so good! But what happens if the thread is blocked waiting for some event? Of course, if the thread is blocked, it can't check the shared variable and can't stop. There are plenty of situations when that may occur, such as calling Object.wait(), ServerSocket.accept(), and DatagramSocket.receive(), to name a few.

They all can block the thread forever. Even if a timeout is employed, it may not be feasible or desirable to wait until the timeout expires, so a mechanism to prematurely exit the blocked state must be used.

Unfortunately there is no such mechanism that works for all cases, but the particular technique to use depends on each situation. In the following sections, I'll give solutions for the most common cases.

Interrupting a thread with Thread.interrupt()
As demonstrated in Listing A, the method Thread.interrupt() does not interrupt a running thread. What the method actually does is to throw an interrupt if the thread is blocked, so that it exits the blocked state. More precisely, if the thread is blocked at one of the methods Object.wait, Thread.join, or Thread.sleep, it receives an InterruptedException, thus terminating the blocking method prematurely.

So, if a thread blocks in one of the aforementioned methods, the correct way to stop it is to set the shared variable and then call the interrupt() method on it (notice that it is important to set the variable first). If the thread is not blocked, calling interrupt() will not hurt; otherwise, the thread will get an exception (the thread must be prepared to handle this condition) and escape the blocked state. In either case, eventually the thread will test the shared variable and stop. Listing C is a simple example that demonstrates this technique.

As soon as Thread.interrupt() is called in Listing C, the thread gets an exception so that it escapes the blocked state and determines that it should stop. Running this code produces output like this:
Starting thread...
Thread running...
Thread running...
Thread running...
Asking thread to stop...
Thread interrupted...
Thread exiting under request...
Stopping application...

Interrupting an I/O operation
But what happens if the thread is blocked on an I/O operation? I/O can block a thread for a considerable amount of time, particularly if network communication is involved. For example, a server may be waiting for a request, or a network application may be waiting for an answer from a remote host.

If you're using channels, available with the new I/O API introduced in Java 1.4, the blocked thread will get a ClosedByInterruptException exception. If that is the case, the logic is the same as that used in the third example—only the exception is different.

But you might be using the traditional I/O available since Java 1.0, since the new I/O is so recent and requires more work. In this case, Thread.interrupt() doesn't help, since the thread will not exit the blocked state. Listing D demonstrates that behavior. Although the interrupt() method is called, the thread does not exit the blocked state.

Fortunately, the Java Platform provides a solution for that case by calling the close() method of the socket the thread is blocked in. In this case, if the thread is blocked in an I/O operation, the thread will get a SocketException exception, much like the interrupt() method causes an InterruptedException to be thrown.

The only caveat is that a reference to the socket must be available so that its close() method can be called. That means the socket object must also be shared. Listing E demonstrates this case. The logic is the same as in the examples presented so far.

And here's the sample output you can expect from running Listing E:
Starting thread...
Waiting for connection...
Asking thread to stop...
accept() failed or interrupted...
Thread exiting under request...
Stopping application...

Multithreading is a powerful tool, but it presents its own set of challenges. One of these is how to interrupt a running thread. If properly implemented, these techniques make interrupting a thread no more difficult than using the built-in operations already provided by the Java Platform.

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