Reports double-checked locking.

Double-checked locking tries to initialize a field on demand and in a thread-safe manner while avoiding the cost of synchronization. Unfortunately it is not thread-safe when used on a field that is not declared volatile. When using Java 1.4 or later, double-checked locking doesn't work even with volatile fields. Read the article linked above for the detailed explanation of the problem.

Example of an incorrect double-checked locking:


  class Foo {
    private Helper helper = null;
    public Helper getHelper() {
      if (helper == null)
        synchronized(this) {
          if (helper == null) helper = new Helper();
        }
        return helper;
      }
    }
    // other functions and members...
  }