概述

上文剖析了独占模式下的acquire实现，本章剖析一下共享模式下的acquireShared实现

acquireShared

每个线程都会尝试去获取共享锁，只有获取失败的才会进入doAcquireShared办法

  public final void acquireShared(int arg) {        if (tryAcquireShared(arg) < 0)            doAcquireShared(arg);    }

doAcquireShared

这里共享锁与独占锁的实现相似
首先，也是判断以后节点是否是head节点的无效后继节点，如果是的话，以后节点就回去尝试获取一次共享锁，如果获取胜利就调用setHeadAndPropagate持续流传，如果不是head节点的无效后继节点，就判断以后节点是否应该阻塞，剩下的逻辑与acquire相似就不再剖析，重点关注一下setHeadAndPropagate的实现

private void doAcquireShared(int arg) {        final Node node = addWaiter(Node.SHARED);        boolean failed = true;        try {            boolean interrupted = false;            for (;;) {                final Node p = node.predecessor();                if (p == head) {                    int r = tryAcquireShared(arg);                    if (r >= 0) {                        setHeadAndPropagate(node, r);                        p.next = null; // help GC                        if (interrupted)                            selfInterrupt();                        failed = false;                        return;                    }                }                if (shouldParkAfterFailedAcquire(p, node) &&                    parkAndCheckInterrupt())                    interrupted = true;            }        } finally {            if (failed)                cancelAcquire(node);        }

setHeadAndPropagate