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Objects which are pooled and potentially reused should not be used for synchronization. If they are, it can cause unrelated threads to deadlock with unhelpful stacktraces. Specifically, String literals, and boxed primitives such as Integers should not be used as lock objects because they are pooled and reused. The story is even worse for Boolean objects, because there are only two instances of Boolean, Boolean.TRUE and Boolean.FALSE and every class that uses a Boolean will be referring to one of the two.
Noncompliant Code Example

private static final Boolean bLock = Boolean.FALSE;
private static final Integer iLock = Integer.valueOf(0);
private static final String sLock = "LOCK";

public void doSomething() {

  synchronized(bLock) {  // Noncompliant
    // ...
  }
  synchronized(iLock) {  // Noncompliant
    // ...
  }
  synchronized(sLock) {  // Noncompliant
    // ...
  }

Compliant Solution

private static final Object lock1 = new Object();
private static final Object lock2 = new Object();
private static final Object lock3 = new Object();

public void doSomething() {

  synchronized(lock1) {
    // ...
  }
  synchronized(lock2) {
    // ...
  }
  synchronized(lock3) {
    // ...
  }

See


 CERT, LCK01-J - Do not synchronize on objects that may be reused