Waiting with two locks held is likely to cause a deadlockJAVA-E0139

Waiting on a monitor while two locks are held may cause deadlock. This can also happen with Lock and Condition primitives from the java.util.concurrent package.

Bad Practice

Nesting synchronized blocks and waiting on one value may cause deadlocks:

synchronized(obj1) {
    
    // ...
    
    synchronized(obj2) {
        obj2.wait();

        // ...
    
    }

    // ...
}

Waiting on obj2 does not release the lock on obj1. If any other code locks obj2 before locking obj1, a deadlock will occur.

Similarly, it is not a good idea to hold multiple locks and call await on a Condition variable:

Lock l = new ReentrantLock();
Lock l2 = new ReentrantLock();
Condition c = l.newCondition();
Condition c2 = l2.newCondition();

l.lock();
l2.lock();

// ...

c.await();

// ...

l2.unlock();
l.unlock();

Calling c.await() will only release l, not l2. Performing a wait only releases the lock associated with the wait operation, not any other locks.

Recommended

If you must nest locks, be very careful about usage, and test your code rigorously. Deadlocks are dead serious business. It is almost always a bad idea to perform a wait operation when multiple locks are already held.

References

• SpotBugs - TLW_TWO_LOCK_WAIT
• CWE-833 - Deadlock