java 线程公平锁与非公平锁详解及实例代码

时间:2022-02-25 04:36:42

java 线程公平锁与非公平锁详解

在ReentrantLock中很明显可以看到其中同步包括两种,分别是公平的FairSync和非公平的NonfairSync。公平锁的作用就是严格按照线程启动的顺序来执行的,不允许其他线程插队执行的;而非公平锁是允许插队的。

默认情况下ReentrantLock是通过非公平锁来进行同步的,包括synchronized关键字都是如此,因为这样性能会更好。因为从线程进入了RUNNABLE状态,可以执行开始,到实际线程执行是要比较久的时间的。而且,在一个锁释放之后,其他的线程会需要重新来获取锁。其中经历了持有锁的线程释放锁,其他线程从挂起恢复到RUNNABLE状态,其他线程请求锁,获得锁,线程执行,这一系列步骤。如果这个时候,存在一个线程直接请求锁,可能就避开挂起到恢复RUNNABLE状态的这段消耗,所以性能更优化。

?
1
2
3
4
5
6
7
/**
 * Creates an instance of {@code ReentrantLock}.
 * This is equivalent to using {@code ReentrantLock(false)}.
 */
public ReentrantLock() {
  sync = new NonfairSync();
}

默认状态,使用的ReentrantLock()就是非公平锁。再参考如下代码,我们知道ReentrantLock的获取锁的操作是通过装饰模式代理给sync的。

?
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
/**
  * 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();
 }

下面参考一下FairSync和NonfairSync对lock方法的实现:

?
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
/**
  * 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);
   }
 }

当使用非公平锁的时候,会立刻尝试配置状态,成功了就会插队执行,失败了就会和公平锁的机制一样,调用acquire()方法,以排他的方式来获取锁,成功了立刻返回,否则将线程加入队列,知道成功调用为止。

感谢阅读,希望能帮助到大家,谢谢大家对本站的支持!

原文链接:http://blog.csdn.net/ethanwhite/article/details/55508357