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JDK源码分析-AbstractQueuedSynchronizer(3)

概述

前文「 JDK源码分析-AbstractQueuedSynchronizer(2) 」分析了 AQS 在独占模式下获取资源的流程,本文分析共享模式下的相关操作。

其实二者的操作大部分是类似的,理解了前面对独占模式的分析,再分析共享模式就相对容易了。

共享模式

方法概述

与独占模式类似,共享模式下也有与之类似的相应操作,分别如下:

1. acquireShared(int arg):  以共享模式获取资源,忽略中断;

2. acquire Shared Interruptibly(int arg):  以共享模式获取资源,响应中断;

3. tryAcquire Shared Nanos(int arg, long nanosTimeout):  以共享模式获取资源, 响应中断,且有超时等待;

4. release Shared (int arg):  释放资源,唤醒后继节点,并确保传播

它们的操作与独占模式也比较类似,下面具体分析。

方法分析

1. 共享模式获取资源(忽略中断)

acquireShared:

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"><span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"><span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">public</span> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">final</span> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">void</span> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">acquireShared(<span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">int</span> arg)</span> </span>{</span>

<span style="padding:0px;margin:0px;max-width:1000%;font-style:italic;box-sizing:border-box !important;overflow-wrap:break-word !important;">// 返回值小于 0,表示获取失败</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">if</span> (tryAcquireShared(arg) &lt; <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">0</span>)</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> doAcquireShared(arg);</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;">}</span>



<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;">// 尝试以共享模式获取资源(返回值为 int 类型)</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"><span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"><span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">protected</span> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">int</span> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">tryAcquireShared(<span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">int</span> arg)</span> </span>{</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">throw</span> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">new</span> UnsupportedOperationException();</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;">}</span>

与独占模式的 tryAcquire 方法 类似,tryAcquireShared 方法在 AQS 中也抛出异常,由子类实现其逻辑。

不同的地方在于, tryAcquire 方法的返回结果是 boolean 类型,表示获取成功与否;而  tryAcquireShared 的返回结果是 int 类型,分别为:

1) 负数:表示获取失败;

2) 0:表示获取成功,但后续共享模式的获取会失败;

3) 正数:表示获取成功,后续共享模式的获取可能会成功(需要进行检测)。

tryAcquireShared 获取成功,则直接返回;否则执行  doAcquireShared 方法:

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"><span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"><span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">private</span> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">void</span> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">doAcquireShared(<span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">int</span> arg)</span> </span>{</span>

<span style="padding:0px;margin:0px;max-width:1000%;font-style:italic;box-sizing:border-box !important;overflow-wrap:break-word !important;"> // 把当前线程封装成共享模式的 Node 节点,插入主队列末尾</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">final</span> Node node = addWaiter(Node.SHARED);</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">boolean</span> failed = <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">true</span>;</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">try</span> {</span>

<span style="padding:0px;margin:0px;max-width:1000%;font-style:italic;box-sizing:border-box !important;overflow-wrap:break-word !important;"> // 中断标志位</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">boolean</span> interrupted = <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">false</span>;</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">for</span> (;;) {</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">final</span> Node p = node.predecessor();</span>

<span style="padding:0px;margin:0px;max-width:1000%;font-style:italic;box-sizing:border-box !important;overflow-wrap:break-word !important;"> // 若前驱节点为头节点,则尝试获取资源</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">if</span> (p == head) {</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">int</span> r = tryAcquireShared(arg);</span>

<span style="padding:0px;margin:0px;max-width:1000%;font-style:italic;box-sizing:border-box !important;overflow-wrap:break-word !important;"> // 这里表示当前线程成功获取到了资源</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">if</span> (r &gt;= <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">0</span>) {</span>

<span style="padding:0px;margin:0px;max-width:1000%;font-style:italic;box-sizing:border-box !important;overflow-wrap:break-word !important;"> // 设置头节点,并传播状态(注意这里与独占模式不同)</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> setHeadAndPropagate(node, r);</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> p.next = <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">null</span>; <span style="padding:0px;margin:0px;max-width:1000%;font-style:italic;box-sizing:border-box !important;overflow-wrap:break-word !important;">// help GC</span></span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">if</span> (interrupted)</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> selfInterrupt();</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> failed = <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">false</span>;</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">return</span>;</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> }</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> }</span>

<span style="padding:0px;margin:0px;max-width:1000%;font-style:italic;box-sizing:border-box !important;overflow-wrap:break-word !important;"> // 是否应该休眠(与独占模式相同,不再赘述)</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">if</span> (shouldParkAfterFailedAcquire(p, node) &&</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> parkAndCheckInterrupt())</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> interrupted = <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">true</span>;</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> }</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> } <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">finally</span> {</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">if</span> (failed)</span>

<span style="padding:0px;margin:0px;max-width:1000%;font-style:italic;box-sizing:border-box !important;overflow-wrap:break-word !important;"> // 取消操作(与独占模式相同)</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> cancelAcquire(node);</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> }</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;">}</span>

doAcquireShared  方法会把当前线程封装成一个共享模式(SHARED)的节点,并插入主队列末尾。addWaiter(Node mode) 方法前文已经分析过,不再赘述。

该方法与  acquireQueued 方法的区别在于 setHeadAndPropagate 方法,把当前节点设置为头节点之后,还会有传播(propagate)行为:

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"><span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"><span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">private</span> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">void</span> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">setHeadAndPropagate</span>(<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;">Node node, <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">int</span> propagate</span>)</span> {</span>

<span style="padding:0px;margin:0px;max-width:1000%;font-style:italic;box-sizing:border-box !important;overflow-wrap:break-word !important;"> // 记录旧的头节点</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> Node h = head; <span style="padding:0px;margin:0px;max-width:1000%;font-style:italic;box-sizing:border-box !important;overflow-wrap:break-word !important;">// Record old head for check below</span></span>

<span style="padding:0px;margin:0px;max-width:1000%;font-style:italic;box-sizing:border-box !important;overflow-wrap:break-word !important;"> // 将 node 设置为头节点</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> setHead(node);</span>

<span style="padding:0px;margin:0px;max-width:1000%;font-style:italic;box-sizing:border-box !important;overflow-wrap:break-word !important;"> /*</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> * Try to signal next queued node if:</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> * Propagation was indicated by caller,</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> * or was recorded (as h.waitStatus either before</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> * or after setHead) by a previous operation</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> * (note: this uses sign-check of waitStatus because</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> * PROPAGATE status may transition to SIGNAL.)</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> * and</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> * The next node is waiting in shared mode,</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> * or we don't know, because it appears null</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> *</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> * The conservatism in both of these checks may cause</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> * unnecessary wake-ups, but&gt;<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> * racing acquires/releases, so most need signals now or soon</span></span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> * anyway.</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> */</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">if</span> (propagate &gt; <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">0</span> || h == <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">null</span> || h.waitStatus &lt; <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">0</span> ||</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> (h = head) == <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">null</span> || h.waitStatus &lt; <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">0</span>) {</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> Node s = node.next;</span>

<span style="padding:0px;margin:0px;max-width:1000%;font-style:italic;box-sizing:border-box !important;overflow-wrap:break-word !important;"> // 后继节点为空或共享模式唤醒</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">if</span> (s == <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">null</span> || s.isShared())</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> doReleaseShared();</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> }</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;">}</span>

doReleaseShared:

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"><span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"><span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">private</span> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">void</span> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">doReleaseShared</span>()</span> {</span>

<span style="padding:0px;margin:0px;max-width:1000%;font-style:italic;box-sizing:border-box !important;overflow-wrap:break-word !important;"> /*</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> * Ensure that a release propagates, even if there are other</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> * in-progress acquires/releases. This proceeds in the usual</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> * way of trying to unparkSuccessor of head if it needs</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> * signal. But if it does not, status is set to PROPAGATE to</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> * ensure that upon release, propagation continues.</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> * Additionally, we must loop in case a new node is added</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> * while we are doing this. Also, unlike other uses of</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> * unparkSuccessor, we need to know if CAS to reset status</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> * fails, if so rechecking.</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> */</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">for</span> (;;) {</span>

<span style="padding:0px;margin:0px;max-width:1000%;font-style:italic;box-sizing:border-box !important;overflow-wrap:break-word !important;"> // 这里的头节点已经是上面设置后的头节点了</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> Node h = head;</span>

<span style="padding:0px;margin:0px;max-width:1000%;font-style:italic;box-sizing:border-box !important;overflow-wrap:break-word !important;"> // 由于该方法有两个入口(setHeadAndPropagate 和 releaseShared),需考虑并发控制</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">if</span> (h != <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">null</span> && h != tail) {</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">int</span> ws = h.waitStatus;</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">if</span> (ws == Node.SIGNAL) {</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">if</span> (!compareAndSetWaitStatus(h, Node.SIGNAL, <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">0</span>))</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">continue</span>; <span style="padding:0px;margin:0px;max-width:1000%;font-style:italic;box-sizing:border-box !important;overflow-wrap:break-word !important;">// loop to recheck cases</span></span>

<span style="padding:0px;margin:0px;max-width:1000%;font-style:italic;box-sizing:border-box !important;overflow-wrap:break-word !important;"> // 唤醒后继节点</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> unparkSuccessor(h);</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> }</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">else</span> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">if</span> (ws == <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">0</span> &&</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> !compareAndSetWaitStatus(h, <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">0</span>, Node.PROPAGATE))</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">continue</span>; <span style="padding:0px;margin:0px;max-width:1000%;font-style:italic;box-sizing:border-box !important;overflow-wrap:break-word !important;">// loop&gt;<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> }</span></span></span>

<span style="padding:0px;margin:0px;max-width:1000%;font-style:italic;box-sizing:border-box !important;overflow-wrap:break-word !important;"> // 若头节点不变,则跳出循环;否则继续循环</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">if</span> (h == head) <span style="padding:0px;margin:0px;max-width:1000%;font-style:italic;box-sizing:border-box !important;overflow-wrap:break-word !important;">// loop if head changed</span></span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">break</span>;</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> }</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;">}</span>

该方法与独占模式下的获取方法 acquire 大体相似,不同在于该方法中,节点获取资源后会传播状态,即,有可能会继续唤醒后继节点。 值得注意的是:该方法有两个入口 setHeadAndPropagate 和 releaseShared, 可能有多个线程操作, 需考虑并发控制。

此外,本人对于将节点设置为 PROPAGATE 状态的理解还不是很清晰,网上说法也不止一种,待后续研究明白再补充。

2. 以共享模式获取资源(响应中断)

该方法与  acquireShared 类似:

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"><span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">public</span> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">final</span> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">void</span> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">acquireSharedInterruptibly(<span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">int</span> arg)</span></span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"><span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">throws</span> InterruptedException </span>{</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">if</span> (Thread.interrupted())</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">throw</span> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">new</span> InterruptedException();</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">if</span> (tryAcquireShared(arg) &lt; <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">0</span>)</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> doAcquireSharedInterruptibly(arg);</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;">}</span>

tryAcquireShared 方法前面已分析,若获取资源失败,则会执行 doAcquireSharedInterruptly 方法:

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"><span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">private</span> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">void</span> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">doAcquireSharedInterruptibly(<span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">int</span> arg)</span></span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"><span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">throws</span> InterruptedException </span>{</span>

<span style="padding:0px;margin:0px;max-width:1000%;font-style:italic;box-sizing:border-box !important;overflow-wrap:break-word !important;"> // 把当前线程封装成共享模式节点,并插入主队列</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">final</span> Node node = addWaiter(Node.SHARED);</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">boolean</span> failed = <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">true</span>;</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">try</span> {</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">for</span> (;;) {</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">final</span> Node p = node.predecessor();</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">if</span> (p == head) {</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">int</span> r = tryAcquireShared(arg);</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">if</span> (r &gt;= <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">0</span>) {</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> setHeadAndPropagate(node, r);</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> p.next = <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">null</span>; <span style="padding:0px;margin:0px;max-width:1000%;font-style:italic;box-sizing:border-box !important;overflow-wrap:break-word !important;">// help GC</span></span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> failed = <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">false</span>;</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">return</span>;</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> }</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> }</span>

<span style="padding:0px;margin:0px;max-width:1000%;font-style:italic;box-sizing:border-box !important;overflow-wrap:break-word !important;"> // 与 doAcquireShared 相比,区别在于这里抛出了异常</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">if</span> (shouldParkAfterFailedAcquire(p, node) &&</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> parkAndCheckInterrupt())</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">throw</span> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">new</span> InterruptedException();</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> }</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> } <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">finally</span> {</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">if</span> (failed)</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> cancelAcquire(node);</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> }</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;">}</span>

从代码可以看到,acquireSharedInterruptibly 方法与 acquireShared 方法几乎完全一样,不同之处仅在于前者会抛出 InterruptedException 异常响应中断;而后者仅记录标志位,获取结束后才响应。

3. 以共享模式获取资源(响应中断,且有超时)

代码如下( 该方法可与前文独占模式下的超时获取方法比较分析)

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"><span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">public</span> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">final</span> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">boolean</span> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">tryAcquireSharedNanos(<span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">int</span> arg, <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">long</span> nanosTimeout)</span></span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"><span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">throws</span> InterruptedException </span>{</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">if</span> (Thread.interrupted())</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">throw</span> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">new</span> InterruptedException();</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">return</span> tryAcquireShared(arg) &gt;= <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">0</span> ||</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> doAcquireSharedNanos(arg, nanosTimeout);</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;">}</span>

doAcquireSharedNanos: 

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"><span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">private</span> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">boolean</span> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">doAcquireSharedNanos(<span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">int</span> arg, <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">long</span> nanosTimeout)</span></span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"><span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">throws</span> InterruptedException </span>{</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">if</span> (nanosTimeout &lt;= <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">0L</span>)</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">return</span> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">false</span>;</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">final</span> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">long</span> deadline = System.nanoTime() + nanosTimeout;</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">final</span> Node node = addWaiter(Node.SHARED);</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">boolean</span> failed = <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">true</span>;</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">try</span> {</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">for</span> (;;) {</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">final</span> Node p = node.predecessor();</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">if</span> (p == head) {</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">int</span> r = tryAcquireShared(arg);</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">if</span> (r &gt;= <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">0</span>) {</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> setHeadAndPropagate(node, r);</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> p.next = <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">null</span>; <span style="padding:0px;margin:0px;max-width:1000%;font-style:italic;box-sizing:border-box !important;overflow-wrap:break-word !important;">// help GC</span></span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> failed = <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">false</span>;</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">return</span> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">true</span>;</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> }</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> }</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> nanosTimeout = deadline - System.nanoTime();</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">if</span> (nanosTimeout &lt;= <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">0L</span>)</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">return</span> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">false</span>;</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">if</span> (shouldParkAfterFailedAcquire(p, node) &&</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> nanosTimeout &gt; spinForTimeoutThreshold)</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> LockSupport.parkNanos(<span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">this</span>, nanosTimeout);</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">if</span> (Thread.interrupted())</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">throw</span> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">new</span> InterruptedException();</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> }</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> } <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">finally</span> {</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">if</span> (failed)</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> cancelAcquire(node);</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> }</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;">}</span>

该方法可与独占模式下的超时等待方法 tryAcquireNanos(int arg, long nanosTimeout) 进行对比,二者操作基本一致,不再详细分析。

4. 释放资源,唤醒节点,传播状态

如下:

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"><span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"><span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">public</span> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">final</span> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">boolean</span> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">releaseShared(<span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">int</span> arg)</span> </span>{</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">if</span> (tryReleaseShared(arg)) {</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> doReleaseShared();</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">return</span> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">true</span>;</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> }</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">return</span> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">false</span>;</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;">}</span>

tryReleaseShared:

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"><span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"><span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">protected</span> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">boolean</span> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">tryReleaseShared(<span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">int</span> arg)</span> </span>{</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;"> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">throw</span> <span style="padding:0px;margin:0px;max-width:1000%;box-sizing:border-box !important;overflow-wrap:break-word !important;">new</span> UnsupportedOperationException();</span>

<span style="margin: 0px; padding: 0px; max-width: 1000%; box-sizing: border-box !important; overflow-wrap: break-word !important;">}</span>

doReleaseShared() 方法前面已经分析过了。本方法与独占模式的 release 方法类似,不同的地方在于“传播”二字。

场景分析

为了便于理解独占模式和共享模式下队列和节点的状态,下面简要举例分析。

场景如下:有 T0~T4 共 5 个线程按先后顺序获取资源,其中 T2 和 T3 为共享模式,其他均为独占模式。

就此场景分析:T0 先获取到资源(假设占用时间较长),而后 T1~T4 再获取则失败,会依次进入主队列。 此时主队列中各个节点的状态示意图如下:

JDK源码分析-AbstractQueuedSynchronizer(3)

之后,T0 操作完毕并释放资源,会将 T1 唤醒。T1(独占模式) 会从 acquireQueued(final Node node, int arg) 方法的循环中继续获取资源,这时会获取成功,并将 T1 设置为头节点 (T 被移除) 。此时主队列节点示意图如下:

JDK源码分析-AbstractQueuedSynchronizer(3)

此时,T1 获取到资源并进行相关操作。

而后,T1 操作完释放资源,并唤醒下一个节点 T2,T2(共享模式) 继续从 doAcquireShared(int) 方法的循环中执行。此时 T2 获取资源成功,将自身设为头节点(T1 被移除),由于后继节点 T3 也是共享模式,因此 T1 会继续唤醒 T3;T3 唤醒后的操作与 T2 相同,但后继节点 T4 不是共享模式,因此不再继续唤醒。此时队列节点状态示意图如下:

JDK源码分析-AbstractQueuedSynchronizer(3)

此时,T2 和 T3 同时获取到资源。

之后,当二者都释放资源后会唤醒 T4:

JDK源码分析-AbstractQueuedSynchronizer(3)

T4 获取资源的与 T1 类似。

PS: 该场景仅供参考,只为便于理解,若有不当之处敬请指正。

小结

本文分析了以共享模式获取资源的三种方式,以及释放资源的操作。 分别为:

1. acquireShared: 共享模式获取资源,忽略中断;

2.  acquire Shared Interruptibly: 共享模式获取资源,响应中断;

3.  tryAcquire Shared Nanos: 共享模式获取资源,响应中断,有超时;

4. release Shared : 释放资源,唤醒后继节点,并确保传播。

并简要分析一个场景下主队列中各个节点的状态。此外,AQS 中还有嵌套类 ConditionObject 及条件队列的相关操作,后面涉及到的时候再进行分析。

单独去分析 AQS 的源码比较枯燥,后文会结合 ReentrantLock、CountdownLatch 等常用并发工具类的源码进行分析。

上述解析是参考其他资料及个人理解,若有不当之处欢迎指正。

相关阅读:

JDK源码分析-AbstractQueuedSynchronizer(2)

JDK源码分析-AbstractQueuedSynchronizer(1)

Stay hungry, stay foolish.

JDK源码分析-AbstractQueuedSynchronizer(3)

原文  https://juejin.im/post/5d5561b3e51d4561cb5dde88
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