CODEWITHSUNDEEP
javamultithreadingconditional-statementsreentrantlock
codereviewanskquestions
codereviewanskquestions

Reputation: 13998

Reentrance Lock Condition

public class MyLockConditionTest {
    private final Lock alock = new ReentrantLock();
    private final Condition condition = alock.newCondition();
    private String message = null;

    public void waitForCallback() {
         alock.lock();
         try {
            // wait for callback from a remote server
            condition.await();
            doSomething(message);
         } finally {
              alock.unlock();
         }
    }

    // another thread will call this method as a callback to wake up the thread called condition.await()
    public void onCallbackReceived(String message) {
         alock.lock();
         try {
            this.message = message
            condition.signal();
         } finally {
              alock.unlock();
         }
    }
}

I have this code using ReentrantLock and Condition to implement a class to wait on certain callbacks from a remote server. I tested this code and seems working but I have a few questions.

  1. Why do I need to do alock.lock() / unlock() in onCallbackReceived(). Without calling lock() / unlock(), I was getting an IllegalState exception. I am confused because the lock is held by the caller of waitForCallback() when onCallbackReceived() invoked by another thread so alock.lock() in onCallbackReceived() will always fail.

  2. Do I need to wrap condition.await() in waitForCallback() with while loop?

    while(message==null) condition.await();

Upvotes: 2

Views: 522

Answers (1)

John Vint
John Vint

Reputation: 40256

Why do I need to do alock.lock() / unlock() in onCallbackReceived(). Without calling lock() / unlock(), I was getting an IllegalState exception.

You are trying to signal a condition in which you do not own the lock. The only way you would hold the lock is if onCallbackReceived is called from doSomething which there is no indication that is occurring.

Do I need to wrap condition.await() in waitForCallback() with while loop?

Yes, imagine you have 5 threads stopped on the condition and 5 threads blocked on lock. Only one thread can wake up. What if when a waiting thread finally wakes up another thread had null'd that field out? You will need to check again to ensure the predicate is still true.

Upvotes: 1

Related Questions