Having a lock on a class, other than the enclosing class of the code block, can unintentionally prevent the locked class from being used properly when other classes effectively lock on the same resource. From a maintainability perspective, it can be time-consuming to ensure the synchronized blocks are working as expected. Hence, locking on a class other than the enclosing class of the synchronized code block is discouraged by Error Prone. Locking on the enclosing class or an instance is a preferred practice.

For example, a lock on Other.class rather than Example.class will trigger an Error Prone warning:

class Example {
  void method() {
    synchronized (Other.class) {
    }
  }
}

A lock on the instance or the enclosing class of the synchronized code block will not trigger the warning:

class Example {
  void method() {
    synchronized (Example.class) {
    }
    synchronized (this) {
    }
  }
}