Hide Your Locks


/** thread safe */
class Counter {

  /** guarded by 'this' */
  private long count;

  void increment(long amount) {
    if (amount < 1) {
      throw new IllegalArgumentException("Only increments!");
    }

    synchronized (this) {
      this.count += amount;
    }
  }
}

as a default, this is the reference of the lock
=> everybody who has access to that object reference can interfere


/** thread safe */
class Counter {

  /** guarded by 'lock' */
  private long count;

  private final Object lock = new Object();

  void increment(long amount) {
    if (amount < 1) {
      throw new IllegalArgumentException("Only increments!");
    }

    synchronized (this.lock) {
      this.count += amount;
    }
  }
}

with a private lock object, the lock monitor stays local
update documentation!