使用AbstractMonitor在Java中进行并行编程，如果满足条件，如何使线程离开监视器

Question

i'm trying to solve the sleeping barber problem in java i'm having a problem when trying to make a thread leave the monitor when a signle condition is met here's the code:

public void getHairCut(String threadName) {
    enter();
    if(!barberSleeping) {
        if(freeSeats == 0) {
        System.out.println(threadName + " waiting...");
            freeSeats--;;           
            hairCut.await();
        }
        else {
            System.out.println("waiting room is full " + threadName + " is leaving...");
            return;   // i'm having a problem here,i want the thread to return     from the method and release the monitor's lock to let other threads enter
        }
    }
    // some code

    leave();
}

thanks in advance.

Answer 1

In very brief the situation is as follows:

In order to be capable of waiting for a condition to be met a thread (say T0) must first acquire a lock for itself. After that it might (or might not) be suspended because some condition is not yet met by calling a wait-function. If this condition will be met at a later point (by another thread eg T1, because T0 is suspended), T1 might signal the suspended Thread T0 to reclaim its work. In order to signal the condition which was responsible for suspending T0, T1 also needs to acquire the one lock associated with the condition.

A corresponding code would look like:

// relevant prior assumptions

Lock myLock = new ReentrantLock();
Condition myCondition = myLock.newCondition();

// code that might suspend thread T0
try {

    myLock.lock();

    while (someCondition == false) {

        myCondition.await();

    }

} finally {

    myLock.unlock();

}

// code that signals T0 to wake up and continue with its wrkr where it stopped (which is exactly aone line after the call to myCondition.awayit())

try {

    myLock.lock();

    myCondition.signal();
} finally {

    myLock.unlock();
}

Answer 2

Can't you wrap the critical section in a try-finally block?

public void getHairCut(String threadName) {
    enter();
    try {
        if(!barberSleeping) {
            if(freeSeats == 0) {
            System.out.println(threadName + " waiting...");
                freeSeats--;;           
                hairCut.await();
            }
            else {
                System.out.println("waiting room is full " + threadName + " is leaving...");
                return;   // i'm having a problem here,i want the thread to return     from the method and release the monitor's lock to let other threads enter
            }
        }
        // some code
    }
    finally {
        leave(); // this would be called in any case
    }
}

The finally block will be executed in any case:

• when execution reaches the bottom
• when an exception is thrown
• when the code block is quit because of a return statement