ReentrantReadWriteLock fails to get lock even when its state is Unlocked

Question

I am trying to get lock on a thread using following piece of code:

Lock lock = readLock ? getLock(key).readLock() : getLock(key).writeLock();
try {
    boolean locked = lock.tryLock(DEFAULT_TRY_TIME, DEFAULT_TRY_TIME_UNIT); //line 3
    // If false, lock is not acquired
    if (!locked) {
        throw new TryLockTimeoutException(
                key + ": Failed to acquire " + lock + " within " + DEFAULT_TRY_TIME_STRING);
    }
}

Line 3 returns false after 30 minutes hence TryLockTimeoutException is thrown with error as:

com.concurrent.TryLockTimeoutException: keyIp : Failed to acquire java.util.concurrent.locks.ReentrantReadWriteLock$WriteLock@74be2cee[Unlocked] within 30MINUTES
    at com.concurrent.NeAccessLockMap.acquireReadOrWriteLock(NeAccessLockMap.java:72)

Notice that lock state is shown as Unlocked in error.

I am not able to understand why would this happen? Why thread is not able to get lock even when the lock is free.

Answer 1

In your example you try to acquire a write lock, but the read lock is already locked, which prevents you from acquiring the write lock.

Because you can either have one or more read locks acquired, or a single write lock acquired, the write lock is marked as Unlocked when there are read locks acquired.

Try the following code:

ReentrantReadWriteLock readWriteLock = new ReentrantReadWriteLock();
new Thread() {
    @Override
    public void run() {
        readWriteLock.readLock().lock();
        try {
            // block the read lock
            Thread.sleep(5000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}.start();
if (!readWriteLock.writeLock().tryLock(1, TimeUnit.SECONDS)) {
    System.out.println(readWriteLock);
    System.out.println(readWriteLock.readLock());
    System.out.println(readWriteLock.writeLock());
}

would have output like:

java.util.concurrent.locks.ReentrantReadWriteLock@31221be2[Write locks = 0, Read locks = 1]
java.util.concurrent.locks.ReentrantReadWriteLock$ReadLock@377dca04[Read locks = 1]
java.util.concurrent.locks.ReentrantReadWriteLock$WriteLock@728938a9[Unlocked]