What does java.lang.Thread.interrupt() do?

Could you explain what java.lang.Thread.interrupt() does when invoked?

Solution 1:

Thread.interrupt() sets the interrupted status/flag of the target thread. Then code running in that target thread MAY poll the interrupted status and handle it appropriately. Some methods that block such as Object.wait() may consume the interrupted status immediately and throw an appropriate exception (usually InterruptedException)

Interruption in Java is not pre-emptive. Put another way both threads have to cooperate in order to process the interrupt properly. If the target thread does not poll the interrupted status the interrupt is effectively ignored.

Polling occurs via the Thread.interrupted() method which returns the current thread's interrupted status AND clears that interrupt flag. Usually the thread might then do something such as throw InterruptedException.

EDIT (from Thilo comments): Some API methods have built in interrupt handling. Of the top of my head this includes.

• Object.wait(), Thread.sleep(), and Thread.join()
• Most java.util.concurrent structures
• Java NIO (but not java.io) and it does NOT use InterruptedException, instead using ClosedByInterruptException.

EDIT (from @thomas-pornin's answer to exactly same question for completeness)

Thread interruption is a gentle way to nudge a thread. It is used to give threads a chance to exit cleanly, as opposed to Thread.stop() that is more like shooting the thread with an assault rifle.

Solution 2:

What is interrupt ?

An interrupt is an indication to a thread that it should stop what it is doing and do something else. It's up to the programmer to decide exactly how a thread responds to an interrupt, but it is very common for the thread to terminate.

How is it implemented ?

The interrupt mechanism is implemented using an internal flag known as the interrupt status. Invoking Thread.interrupt sets this flag. When a thread checks for an interrupt by invoking the static method Thread.interrupted, interrupt status is cleared. The non-static Thread.isInterrupted, which is used by one thread to query the interrupt status of another, does not change the interrupt status flag.

Quote from Thread.interrupt() API:

Interrupts this thread. First the checkAccess method of this thread is invoked, which may cause a SecurityException to be thrown.

If this thread is blocked in an invocation of the wait(), wait(long), or wait(long, int) methods of the Object class, or of the join(), join(long), join(long, int), sleep(long), or sleep(long, int), methods of this class, then its interrupt status will be cleared and it will receive an InterruptedException.

If this thread is blocked in an I/O operation upon an interruptible channel then the channel will be closed, the thread's interrupt status will be set, and the thread will receive a ClosedByInterruptException.

If this thread is blocked in a Selector then the thread's interrupt status will be set and it will return immediately from the selection operation, possibly with a non-zero value, just as if the selector's wakeup method were invoked.

If none of the previous conditions hold then this thread's interrupt status will be set.

Check this out for complete understanding about same :

http://download.oracle.com/javase/tutorial/essential/concurrency/interrupt.html