Java parked thread

Question

What is the difference between a parked thread and a waiting thread in java ? I've a jboss core dump and analysing it is showing a lot of parked threads.

Answer 1

Look at Javadoc the park() method:

Disables the current thread for thread scheduling purposes unless the permit is available. If the permit is available then it is consumed and the call returns immediately; otherwise the current thread becomes disabled for thread scheduling purposes and lies dormant until one of three things happens:

Some other thread invokes unpark with the current thread as the target; or Some other thread interrupts the current thread; or The call spuriously (that is, for no reason) returns. This method does not report which of these caused the method to return. Callers should re-check the conditions which caused the thread to park in the first place. Callers may also determine, for example, the interrupt status of the thread upon return.

So a parked thread is a thread blocked using LockSupport.park().

Answer 2

Both park() and wait() will result in a disabled thread. Making a disabled thread active again depends on how it was disabled.

A thread that has been disabled by calling LockSupport.park() will remain disabled until:

• some other thread calls unpark(), or
• some other thread calls interrupt(), or
• "the call spuriously (that is, for no reason) returns"

A thread that has been disabled by calling Object's wait() – which is equivalent to calling wait(0) – will remain disabled until:

• some other thread calls notify() or notifyAll(), or
• some other thread calls interrupt() on the disabled thread