What kind of behaviour causes an interrupted exception?

Question

I'm relatively new to Threading in Java and I've noticed that everytime I use Thread.sleep() I have to catch InterrupetdException.

What kind of behaviour causes this, and in simple applications where I have a monitor thread can I just Ignore the exception?

Answer 1

Well if some other Thread calls thread.interupt(), while the thread is sleeping, you'll get the Exception. And yes, you can probably just put try..catch arround the sleep() and ignore it ;)

Answer 2

The primary case is when someone calls Thread.interrupt() on your thread.

It may be safer to throw a RuntimeException if it happens when you're really not expecting it, but for very simple cases you can probably ignore it.

Answer 3

InterruptedException is a checked exception so unfortunately you cannot just ignore it. In most simple cases you do not have to do anything in the catch clause because you are sure that it will not happen.

From the API

Thrown when a thread is waiting, sleeping, or otherwise paused for a long time and another thread interrupts it using the interrupt method in class Thread.

Answer 4

From the javadocs:

Class InterruptedException

Thrown when a thread is waiting, sleeping, or otherwise paused for a long time and another thread interrupts it using the interrupt method in class Thread.

Hope that answers your question.

Answer 5

As others have said, it is caused by some other thread calling interupt() on the Thread object that is sleeping.

What this means in plain english, is that some other thread has decided to cancel the sleeping thread. The try/catch block is there so you can gracefully handle the cancellation of the thread, and safely clean up any resources, or shut down whatever operation it was doing correctly.

If you don't actually need to do any of that, then yes, you still need an empty catch block. But that's Java for you...

Answer 6

It happens when something calls interrupt() on the thread. This article by Brian Goetz explains the interruption mechanism and how you should handle InterruptedExceptions:

"The most common response to InterruptedException is to swallow it -- catch it and do nothing (or perhaps log it, which isn't any better) -- as we'll see later in Listing 4. Unfortunately, this approach throws away important information about the fact that an interrupt occurred, which could compromise the application's ability to cancel activities or shut down in a timely manner."

Answer 7

Some advices from Java Concurrency in Practice:

• Propagate the exception (possibly after some task-specific cleanup), making your method an interruptible blocking method, too; or
• Restore the interruption status so that code higher up on the call stack can deal with it.
• Only code that implements a thread's interruption policy may swallow an interruption request. General-purpose task and library code should never swallow interruption requests.