CODEWITHSUNDEEP
javamultithreadingconcurrencydeadlockjava.util.concurrent
Stormshadow
Stormshadow

Reputation: 7179

Does parking a JAVA thread cause any acquired monitors to be released

When thread execution is suspended by parking a thread, does it cause the thread to relinquish ownership of any acquired object monitors?

Put simply, can the following code deadlock if a thread (t1) acquires the monitor for the 'this' object and is parked while another thread (t2) that tries to unpark t1 by first trying to acquire the monitor of 'this' and blocking.

// Thread t1 executes this code first.
syncronized(this) {
    LockSupport.park();
}

// Thread t2 then executes this piece of code.
synchronized(this) {
    LockSupport.unpark(t1);
}

Upvotes: 5

Views: 367

Answers (2)

Solomon Slow
Solomon Slow

Reputation: 27190

That would make no sense.

Sometimes a thread needs to hold more than one lock at a time. Some awful piece of legacy code might do something like this:

synchronized(objectA) {
    lockB.lock();
    try {
        doSomethingThatRequiresOwnershipOfBothLocks();
    } finally {
        lockB.unlock();
    }
}

It would not solve any problems if the lockB.lock() call temporarily unlocked the monitor on objectA.

Upvotes: 0

Pavlo Viazovskyy
Pavlo Viazovskyy

Reputation: 937

There will be a deadlock because t1 is blocked and still owns lock on this object when t2 is trying to acquire the same lock.

Upvotes: 4

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