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。
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/**
* Creates an instance of {@code ReentrantLock}.
* This is equivalent to using {@code ReentrantLock(false)}.
*/
public ReentrantLock() {
sync = new NonfairSync();
}
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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。
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/**
* 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();
}
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下面参考一下FairSync和NonfairSync对lock方法的实现:
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/**
* 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 );
}
}
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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。
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原文链接:http://blog.csdn.net/ethanwhite/article/details/55508357