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并发操作

并发操作之——公平锁和非公平锁的区别

并发操作之——公平锁和非公平锁的区别

• 并发操作
• 一、公平锁和非公平锁的区别
• 二、ReenTrantLock源码
• 总结

一、公平锁和非公平锁的区别

公平锁是先入先出原则，先申请的先加锁，轮流加锁
非公平锁是每个线程都去抢锁，谁抢到锁，谁就加锁。

二、ReenTrantLock源码

非公平锁

     static final class NonfairSync extends Sync {
        private static final long serialVersionUID = 7316153563782823691L;
        protected final boolean tryAcquire(int acquires) {
            return nonfairTryAcquire(acquires);
        }
    }

公平锁

    static final class FairSync extends Sync {
        private static final long serialVersionUID = -3000897897090466540L;
        /**
         * Fair version of tryAcquire.  Don't grant access unless
         * recursive call or no waiters or is first.
         */
        @ReservedStackAccess
        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;
        }
    }

tryRelease

        protected final boolean tryRelease(int releases) {
            int c = getState() - releases;
            if (Thread.currentThread() != getExclusiveOwnerThread())
                throw new IllegalMonitorStateException();
            boolean free = false;
            if (c == 0) {
                free = true;
                setExclusiveOwnerThread(null);
            }
            setState(c);
            return free;
        }

总结

公平锁和非公平锁的区别以及ReenTrantLock源码

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