ReentrantLock源码解析

Sync（内部类）

非公平锁

继承

waitStatus状态描述SIGNAL = -1 //可被唤醒CANCELLED = 1 //取消，代表出现异常/中断引起的，需要移除CONDITION = -2 // 条件等待PROPAGATE = -3 // 传播0 - 初始状态Init状态

Lock

// 尝试获取锁protected final boolean tryAcquire(int acquires) {            return nonfairTryAcquire(acquires);        }

// 中断，此处为了唤醒已在队列中的线程static void selfInterrupt() {        Thread.currentThread().interrupt();    }

sync.release(1)

实现

lock.unlock()

nonfairSync.lock()

NonfairSync

公平锁

FairSync

ReentrantLock

private final boolean parkAndCheckInterrupt() {        // 阻塞线程，程序可在外部通过interrupt唤醒        LockSupport.park(this);        // 返回中断状态，并清除中断状态        return Thread.interrupted();    }

非公平式

AbstractQueuedSynchronizer（AQS）

fairSync.lock();

// 释放锁protected final boolean tryRelease(int releases) {            int c = getState() - releases;            // 如果当前线程不是当前持锁线程，抛异常            if (Thread.currentThread() != getExclusiveOwnerThread())                throw new IllegalMonitorStateException();            boolean free = false;            // 如果持锁state-1==0 则表明当前线程持有最后一把锁，进行释放            if (c == 0) {                free = true;                setExclusiveOwnerThread(null);            }            // 将锁状态恢复0            setState(c);            return free;        }

lock.lock()

包含

// 释放锁public final boolean release(int arg) {        if (tryRelease(arg)) {            Node h = head;            if (h != null && h.waitStatus != 0)                unparkSuccessor(h);            return true;        }        return false;    }

公平式

      // 公平方式，尝试获取锁，如果成功返回true，失败返回false        protected final boolean tryAcquire(int acquires) {            final Thread current = Thread.currentThread();            int c = getState();            if (c == 0) {                if (                    // 判断等待队列中是否有线程                    !font color=\"#ff3333\

区别：公平锁：先判断队列中是否有阻塞线程非公平锁：直接尝试获取锁

// 将节点置为取消private void cancelAcquire(Node node) {        if (node == null)            return;        node.thread = null;        Node pred = node.prev;       // 如果上个节点的状态>0 代表上个节点也是取消节点，则当前节点pre指向上上个节点（跳过上个节点），直         // 到找到<=0 的节点       // font color=\"#ff3333\

aqs.acquire(1);

// 查看队列是否有其他等待线程public final boolean hasQueuedPredecessors() {        Node t = tail;         Node h = head;        Node s;        // 头引用与尾引用未指向同一个节点        //  && （头引用的下个节点空 || 下个节点非当前节点）        return h != t &&            ((s = h.next) == null || s.thread != Thread.currentThread());    }