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);    }  }