Handling InterruptedException in Java

What is the difference between the following ways of handling InterruptedException? What is the best way to do it?

You've probably come to ask this question because you've called a method that throws InterruptedException.

First of all, you should see throws InterruptedException for what it is: A part of the method signature and a possible outcome of calling the method you're calling. So start by embracing the fact that an InterruptedException is a perfectly valid result of the method call.

Now, if the method you're calling throws such exception, what should your method do? You can figure out the answer by thinking about the following:

Does it make sense for the method you are implementing to throw an InterruptedException? Put differently, is an InterruptedException a sensible outcome when calling your method?

• If yes, then throws InterruptedException should be part of your method signature, and you should let the exception propagate (i.e. don't catch it at all).

  Example: Your method waits for a value from the network to finish the computation and return a result. If the blocking network call throws an InterruptedException your method can not finish computation in a normal way. You let the InterruptedException propagate.

  int computeSum(Server server) throws InterruptedException {    // Any InterruptedException thrown below is propagated    int a = server.getValueA();    int b = server.getValueB();    return a + b;}

• If no, then you should not declare your method with throws InterruptedException and you should (must!) catch the exception. Now two things are important to keep in mind in this situation:

  1. Someone interrupted your thread. That someone is probably eager to cancel the operation, terminate the program gracefully, or whatever. You should be polite to that someone and return from your method without further ado.

  2. Even though your method can manage to produce a sensible return value in case of an InterruptedException the fact that the thread has been interrupted may still be of importance. In particular, the code that calls your method may be interested in whether an interruption occurred during execution of your method. You should therefore log the fact an interruption took place by setting the interrupted flag: Thread.currentThread().interrupt()

  Example: The user has asked to print a sum of two values. Printing "Failed to compute sum" is acceptable if the sum can't be computed (and much better than letting the program crash with a stack trace due to an InterruptedException). In other words, it does not make sense to declare this method with throws InterruptedException.

  void printSum(Server server) {     try {         int sum = computeSum(server);         System.out.println("Sum: " + sum);     } catch (InterruptedException e) {         Thread.currentThread().interrupt();  // set interrupt flag         System.out.println("Failed to compute sum");     }}

By now it should be clear that just doing throw new RuntimeException(e) is a bad idea. It isn't very polite to the caller. You could invent a new runtime exception but the root cause (someone wants the thread to stop execution) might get lost.

Other examples:

Implementing Runnable: As you may have discovered, the signature of Runnable.run does not allow for rethrowing InterruptedExceptions. Well, you signed up on implementing Runnable, which means that you signed up to deal with possible InterruptedExceptions. Either choose a different interface, such as Callable, or follow the second approach above.

Calling Thread.sleep: You're attempting to read a file and the spec says you should try 10 times with 1 second in between. You call Thread.sleep(1000). So, you need to deal with InterruptedException. For a method such as tryToReadFile it makes perfect sense to say, "If I'm interrupted, I can't complete my action of trying to read the file". In other words, it makes perfect sense for the method to throw InterruptedExceptions.

String tryToReadFile(File f) throws InterruptedException {    for (int i = 0; i < 10; i++) {        if (f.exists())            return readFile(f);        Thread.sleep(1000);    }    return null;}

This post has been rewritten as an article here.

As it happens I was just reading about this this morning on my way to work in Java Concurrency In Practice by Brian Goetz. Basically he says you should do one of three things

1. Propagate the InterruptedException - Declare your method to throw the checked InterruptedException so that your caller has to deal with it.

2. Restore the Interrupt - Sometimes you cannot throw InterruptedException. In these cases you should catch the InterruptedException and restore the interrupt status by calling the interrupt() method on the currentThread so the code higher up the call stack can see that an interrupt was issued, and quickly return from the method. Note: this is only applicable when your method has "try" or "best effort" semantics, i. e. nothing critical would happen if the method doesn't accomplish its goal. For example, log() or sendMetric() may be such method, or boolean tryTransferMoney(), but not void transferMoney(). See here for more details.

3. Ignore the interruption within method, but restore the status upon exit - e. g. via Guava's Uninterruptibles. Uninterruptibles take over the boilerplate code like in the Noncancelable Task example in JCIP § 7.1.3.

What are you trying to do?

The InterruptedException is thrown when a thread is waiting or sleeping and another thread interrupts it using the interrupt method in class Thread. So if you catch this exception, it means that the thread has been interrupted. Usually there is no point in calling Thread.currentThread().interrupt(); again, unless you want to check the "interrupted" status of the thread from somewhere else.

Regarding your other option of throwing a RuntimeException, it does not seem a very wise thing to do (who will catch this? how will it be handled?) but it is difficult to tell more without additional information.