Do Not Use synchronized on a Lock Object

java.no_synchronize_on_lock_object

Severity

low

Remediation Complexity

trivial

Remediation Risk

low

Remediation Effort

low

Resource

Reliability

Language

Java

Description

Reports synchronized blocks that lock on a java.util.concurrent.locks.Lock instance. Lock objects have their own lock()/unlock() protocol and should not be used with intrinsic synchronization.

Rationale

Lock objects from java.util.concurrent.locks provide a richer locking API (try-lock, timed lock, interruptible lock, read-write locks) than intrinsic monitors. Using synchronized on a Lock object only acquires the object’s intrinsic monitor, not the Lock itself. This means other code calling lock.lock() will not see the synchronization, creating a subtle concurrency bug.

// Bad -- acquires the intrinsic monitor, not the Lock
Lock lock = new ReentrantLock();
synchronized (lock) {
    doWork();
}

Remediation

Use the Lock API directly:

// Good -- uses the Lock protocol
Lock lock = new ReentrantLock();
lock.lock();
try {
    doWork();
} finally {
    lock.unlock();
}

References

java.util.concurrent.locks.Lock Javadoc