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