Minimize Your Lock Scopes


/** thread safe */
class Counter {

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

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

      this.count += amount;
    }
  }
}

guard doesn't access shared state
scope of synchronized is too large


/** 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;
    }
  }
}

reduce the lock to the critical section!