Why is this an issue?

If Thread.sleep(...) is called when the current thread holds a lock, it could lead to performance and scalability issues, or even worse to deadlocks because the execution of the thread holding the lock is frozen. It’s better to call wait(...) on the monitor object to temporarily release the lock and allow other threads to run.

Noncompliant code example

public void doSomething(){
  synchronized(monitor) {
    while(notReady()){
      Thread.sleep(200);
    }
    process();
  }
  ...
}

Compliant solution

public void doSomething(){
  synchronized(monitor) {
    while(notReady()){
      monitor.wait(200);
    }
    process();
  }
  ...
}

Resources