java 线程公平锁与非公平锁详解及实例代码

  /**
   * Creates an instance of {@code ReentrantLock}.
   * This is equivalent to using {@code ReentrantLock(false)}.
   */
  public ReentrantLock() {
    sync = new NonfairSync();
  }

 /**
   * Acquires the lock.
   *
   * <p>Acquires the lock if it is not held by another thread and returns
   * immediately, setting the lock hold count to one.
   *
   * <p>If the current thread already holds the lock then the hold
   * count is incremented by one and the method returns immediately.
   *
   * <p>If the lock is held by another thread then the
   * current thread becomes disabled for thread scheduling
   * purposes and lies dormant until the lock has been acquired,
   * at which time the lock hold count is set to one.
   */
  public void lock() {
    sync.lock();
  }

 /**
   * Sync object for non-fair locks
   */
  static final class NonfairSync extends Sync {
    /**
     * Performs lock. Try immediate barge, backing up to normal
     * acquire on failure.
     */
    final void lock() {
      if (compareAndSetState(0, 1))
        setExclusiveOwnerThread(Thread.currentThread());
      else
        acquire(1);
    }
  }

  /**
   * Sync object for fair locks
   */
  static final class FairSync extends Sync {
    final void lock() {
      acquire(1);
    }
  }