java非公平锁知识点实例详解 java非公平锁知识点实例详解

1、非公平锁不能保证锁的获取是按照请求锁的顺序进行的。这可能会导致某个或某些线程永远得不到锁。
2、CPU唤醒线程的费用可以降低，整体吞吐效率会很高。但是可能会有线程长时间甚至永远得不到锁，导致饿死。
实例

/*** Sync object for non-fair locks*/static final class NonfairSync extends Sync {private static final long serialVersionUID = 7316153563782823691L; /*** Performs lock.Try immediate barge, backing up to normal* acquire on failure.*/final void lock() {if (compareAndSetState(0, 1))setExclusiveOwnerThread(Thread.currentThread()); elseacquire(1); } protected final boolean tryAcquire(int acquires) {return nonfairTryAcquire(acquires); }} /*** Sync object for fair locks*/static final class FairSync extends Sync {private static final long serialVersionUID = -3000897897090466540L; final void lock() {acquire(1); } /*** Fair version of tryAcquire.Don't grant access unless* recursive call or no waiters or is first.*/protected final boolean tryAcquire(int acquires) {final Thread current = Thread.currentThread(); int c = getState(); if (c == 0) {if (!hasQueuedPredecessors() &&compareAndSetState(0, acquires)) {setExclusiveOwnerThread(current); return true; }}else if (current == getExclusiveOwnerThread()) {int nextc = c + acquires; if (nextc < 0)throw new Error("Maximum lock count exceeded"); setState(nextc); return true; }return false; }}

【java非公平锁知识点实例详解】知识点扩展：
非公平锁，顾名思义，各个线程获取到锁的顺序，不一定和它们申请的先后顺序一致，有可能后来的线程，反而先获取到了锁。
在实现上，公平锁在进行lock时，首先会进行tryAcquire()操作。在tryAcquire中，会判断等待队列中是否已经有别的线程在等待了。如果队列中已经有别的线程了，则tryAcquire失败，则将自己加入队列。如果队列中没有别的线程，则进行获取锁的操作。

/*** Fair version of tryAcquire. Don't grant access unless* recursive call or no waiters or is first.**/protected final boolean tryAcquire(int acquires) {final Thread current = Thread.currentThread(); int c = getState(); if (c == 0) {if (!hasQueuedPredecessors() &&compareAndSetState(0, acquires)) {setExclusiveOwnerThread(current); return true; }}else if (current == getExclusiveOwnerThread()) {int nextc = c + acquires; if (nextc < 0)throw new Error("Maximum lock count exceeded"); setState(nextc); return true; }return false; }

非公平锁，在进行lock时，会直接尝试进行加锁，如果成功，则获取到锁，如果失败，则进行和公平锁相同的动作。
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