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
|
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 |
/** * 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()); else acquire( 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 ; } } |
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。
|
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 |
/** * 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。
到此这篇关于java非公平锁知识点实例详解的文章就介绍到这了,更多相关java非公平锁如何理解内容请搜索以前的文章或继续浏览下面的相关文章希望大家以后多多支持！
原文链接：https://www.py.cn/java/jichu/34414.html