如何在 java 中不使用 wait() 或 sleep() 使线程等待

Question

I am writing a class called MyReentrantLock and I have to create a function called acquire() that locks a thread if it's not locked and if it is indeed locked then it has to wait until it unlocks to lock it again. It's kind of confusing and I can't use wait() or sleep(), only AtomicBoolean and Thread class in java.

public class MyReentrantLock implements Lock{

    AtomicBoolean locked = new AtomicBoolean(false);
    long IdOfThreadCurrentlyHoldingLock;
    @Override
    public void acquire() {

        if(!locked.compareAndSet(false,true)){
           WHAT DO I WRITE HERE!!
        }

        locked.compareAndSet(false, true);
        IdOfThreadCurrentlyHoldingLock=Thread.currentThread().getId();

    }


    @Override
    public boolean tryAcquire() {
        if(!locked.compareAndSet(false,true))
            return false;
        else {
            acquire();
            return true;
        }
    }

    @Override
    public void release() {
       locked.compareAndSet(true,false);
       if(!locked.compareAndSet(true,false) || IdOfThreadCurrentlyHoldingLock!=Thread.currentThread().getId()){
           throw new IllegalReleaseAttempt();
       }

    }

Answer 1

Replace your if with a while . You may keep the body empty, even though it might be advisable to sleep some milliseconds.

The reason is: Your method acquire neither throws an exception nor does it return a result (success/failure). Thus the calling code would assume when the call to acquire is done, the thread can continue doing the sensitive job.

So you cannot exit the method without having the lock, even if that means to wait indefinitely. A more advanced method would allow to define a timeout but then also needs to communicate whether the lock is aquired or the timeout was reached.