try {
doSomething();
} catch(InterruptedException swallowed) {
// BAD BAD PRACTICE, TO IGNORE THIS EXCEPTION
// just logging is also not a useful option here....
}I have! I newer knew what the heck to do with those annoying InterruptedException when I simply wanted to call Thread.sleep(..). What is this InterruptedException? Why is it thrown? Why can't I ignore it?
Before I explain the whys, here is what you should do (if you don't re-throw):
try {
doSomething();
} catch(InterruptedException e) {
// Restore the interrupted status
Thread.currentThread().interrupt();
}What is the InterruptedException?
There is no way to simply stop a running thread in java (don't even consider using the deprecated method stop()). Stopping threads is cooperative in java. Calling Thread.interrupt() is a way to tell the thread to stop what it is doing. If the thread is in a blocking call, the blocking call will throw an InterruptedException, otherwise the interrupted flag of the tread will be set. A Thread or a Runnable that is interruptable should check from time to time Thread.currentThread().isInterrupted(). If it returns true, cleanup and return.
Why is it InterruptedException thrown?
The problem is that blocking calls like sleep() and wait(), can take very long till the check can be done. Therefore they throw an InterruptedException. However the isInterrupted is cleared when the InterruptedException is thrown! (I have some vague idea why this is the case, but for whatever reason this is done, that is how it is!)
Why can't InterruptedException be simply ignored?
It should be clear by now: because ignoring an InterruptedException means resetting the interrupted status of the thread. For example, worker threads take runnable from a queue and execute they may check the interrupted status periodically. If you swallow it the thread would not know that it was interrupted and would happily continue to run.
Some more thoughts
Unfortunately it is not specified that Thread.interrupt() can only be used for cancellation. It can be used for anything that requires to set a flag on a thread. So, ending your task or runnable early might be the wrong choice, if the interrupted status is used for something else. But common practice is to use it for cancellation. But even if it is used for something else, you code does not have the right to reset the interrupt flag (unless you are the owner of the thread).
To learn more read the nice article Dealing with InterruptedException by Brian Goetz.
Or read Java Concurrency in Practice By BriaBrian Goetz, Tim Peierls, Joshua Bloch, Joseph Bowbeer, David Holmes, and Doug Lea. It's a great book! If you program in java 5 this book is a must read!. I bought the book after I read Brians artikle. I have not yet read the entire book, but what I have read impresses me. It gives a lot of theorie and options. Very competent and complete, but I'm missing a set of simple patterns for concurrent programming. I'm looking forward to Doug Leas 3rd edition of Concurrent Programming in Java
Summary: If you don't know what to do with an InterruptedException call Thread.currentThread().interrupt()
