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javamultithreadingsynchronization
Swaranga Sarma
Swaranga Sarma

Reputation: 13403

Multithreading and Synchronization with Thread preemption

Suppose I have the following piece of code

public synchronized void method()
{
    if(something == null)
    {
        something = new SomeThing();
    }
    //do something 
}

Now suppose in a multithreaded environment, one thread [Thread 1] enters the method and was preempted just after it executed the new Something(); but before it was able to assign it to something. Then another thread [Thread 2] also tries to call the method. What exactly happens now? What happens to the lock that Thread 1 had acquired? Will Thread 1's steps be rolled back?

Upvotes: 3

Views: 1639

Answers (2)

Collin
Collin

Reputation: 12277

Thread 2 will not be able to enter the method until Thread 1 has exited it because it is synchronized.

Eventually the scheduler will get around to continuing with Thread 1, Thread 1 will execute new Something() and exit the method. Then, Thread 2 will be able to enter the function with the new Something() constructed.

The whole idea of the lock is that Thread 1 does not lose it until it is done with it. Thread 1 unlocks when it exits method(), then Thread 2 is able to acquire it.

Upvotes: 3

Tim Bender
Tim Bender

Reputation: 20442

Thread1 did not give up the lock, so it still owns it. When Thread2 prepares to take the lock it will discover that it has to wait and enter a BLOCKED state. The next time the OS schedules Thread1 it will finish execution and release the lock. This allows Thread2 to be schedulable again.

Upvotes: 7

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