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Instances of classes that implement either or both of the Lock and Condition interfaces of the java.util.concurrent.locks package are known as high-level concurrency objects. Using the intrinsic locks of such objects is a questionable practice even in cases where the code may appear to function correctly. Code that uses the intrinsic lock of a Lock object is likely to interact with code that uses the Lock interface. These two components will believe they are protecting data with the same lock, while they are, in fact, using two distinct locks. As such, the Lock will fail to protect any data.

Consequently, programs that interact with such objects must use only the high-level locking facilities provided by the interfaces; use of the intrinsic locks is prohibited. This problem generally arises when code is refactored from intrinsic locking to the java.util.concurrent dynamic-locking utilities.

Noncompliant Code Example (ReentrantLock)

The doSomething() method in this noncompliant code example synchronizes on the intrinsic lock of an instance of ReentrantLock rather than on the reentrant mutual exclusion Lock encapsulated by ReentrantLock.

private final Lock lock = new ReentrantLock();

public void doSomething() {
  synchronized(lock) {
    // ...
  }
}

Compliant Solution (lock() and unlock())

This compliant solution uses the lock() and unlock() methods provided by the Lock interface.

private final Lock lock = new ReentrantLock();

public void doSomething() {
  lock.lock();
  try {
    // ...
  } finally {
    lock.unlock();
  }
}

In the absence of a requirement for the advanced functionality of the java.util.concurrent package's dynamic-locking utilities, it is better to use other concurrency primitives such as synchronization and atomic classes.

Risk Assessment

Synchronizing on the intrinsic lock of high-level concurrency utilities can cause nondeterministic behavior resulting from inconsistent locking policies.

Bibliography

      08. Locking (LCK)