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1. 概述
前文「JDK 源码剖析 -AbstractQueuedSynchronizer(2)」剖析了 AQS 在独占模式下获取资源的流程,本文剖析共享模式下的相干操作。
其实二者的操作大部分是相似的,了解了后面对独占模式的剖析,再剖析共享模式就绝对容易了。
2. 共享模式
2.1 办法概述
与独占模式相似,共享模式下也有与之类似的相应操作,别离如下:
- acquireShared(int arg): 以共享模式获取资源,疏忽中断;
- acquireSharedInterruptibly(int arg): 以共享模式获取资源,响应中断;
- tryAcquireSharedNanos(int arg, long nanosTimeout): 以共享模式获取资源,响应中断,且有超时期待;
- releaseShared(int arg): 开释资源,唤醒后继节点,并确保流传。
它们的操作与独占模式也比拟相似,上面具体分析。
2.2 办法剖析
2.2.1 共享模式获取资源(疏忽中断)
acquireShared:
public final void acquireShared(int arg) {
// 返回值小于 0,示意获取失败
if (tryAcquireShared(arg) < 0)
doAcquireShared(arg);
}
// 尝试以共享模式获取资源(返回值为 int 类型)protected int tryAcquireShared(int arg) {throw new UnsupportedOperationException();
}
与独占模式的 tryAcquire 办法相似,tryAcquireShared 办法在 AQS 中也抛出异样,由子类实现其逻辑。
不同的中央在于,tryAcquire 办法的返回后果是 boolean 类型,示意获取胜利与否;而 tryAcquireShared 的返回后果是 int 类型,别离为:
- 正数:示意获取失败;
- 零:示意获取胜利,但后续共享模式的获取会失败;
- 负数:示意获取胜利,后续共享模式的获取可能会胜利(须要进行检测)。
若 tryAcquireShared 获取胜利,则间接返回;否则执行 doAcquireShared 办法:
private void doAcquireShared(int arg) {
// 把以后线程封装成共享模式的 Node 节点,插入主队列开端
final Node node = addWaiter(Node.SHARED);
boolean failed = true;
try {
// 中断标记位
boolean interrupted = false;
for (;;) {final Node p = node.predecessor();
// 若前驱节点为头节点,则尝试获取资源
if (p == head) {int r = tryAcquireShared(arg);
// 这里示意以后线程胜利获取到了资源
if (r >= 0) {
// 设置头节点,并流传状态(留神这里与独占模式不同)setHeadAndPropagate(node, r);
p.next = null; // help GC
if (interrupted)
selfInterrupt();
failed = false;
return;
}
}
// 是否应该休眠(与独占模式雷同,不再赘述)if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())
interrupted = true;
}
} finally {if (failed)
// 勾销操作(与独占模式雷同)cancelAcquire(node);
}
}
doAcquireShared 办法会把以后线程封装成一个共享模式(SHARED)的节点,并插入主队列开端。addWaiter(Node mode) 办法前文曾经剖析过,不再赘述。
该办法与 acquireQueued 办法的区别在于 setHeadAndPropagate 办法,把以后节点设置为头节点之后,还会有流传(propagate)行为:
private void setHeadAndPropagate(Node node, int propagate) {
// 记录旧的头节点
Node h = head; // Record old head for check below
// 将 node 设置为头节点
setHead(node);
/*
* Try to signal next queued node if:
* Propagation was indicated by caller,
* or was recorded (as h.waitStatus either before
* or after setHead) by a previous operation
* (note: this uses sign-check of waitStatus because
* PROPAGATE status may transition to SIGNAL.)
* and
* The next node is waiting in shared mode,
* or we don't know, because it appears null
*
* The conservatism in both of these checks may cause
* unnecessary wake-ups, but only when there are multiple
* racing acquires/releases, so most need signals now or soon
* anyway.
*/
if (propagate > 0 || h == null || h.waitStatus < 0 ||
(h = head) == null || h.waitStatus < 0) {
Node s = node.next;
// 后继节点为空或共享模式唤醒
if (s == null || s.isShared())
doReleaseShared();}
}
doReleaseShared:
private void doReleaseShared() {
/*
* Ensure that a release propagates, even if there are other
* in-progress acquires/releases. This proceeds in the usual
* way of trying to unparkSuccessor of head if it needs
* signal. But if it does not, status is set to PROPAGATE to
* ensure that upon release, propagation continues.
* Additionally, we must loop in case a new node is added
* while we are doing this. Also, unlike other uses of
* unparkSuccessor, we need to know if CAS to reset status
* fails, if so rechecking.
*/
for (;;) {
// 这里的头节点曾经是下面设置后的头节点了
Node h = head;
// 因为该办法有两个入口(setHeadAndPropagate 和 releaseShared),需思考并发管制
if (h != null && h != tail) {
int ws = h.waitStatus;
if (ws == Node.SIGNAL) {if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))
continue; // loop to recheck cases
// 唤醒后继节点
unparkSuccessor(h);
}
else if (ws == 0 &&
!compareAndSetWaitStatus(h, 0, Node.PROPAGATE))
continue; // loop on failed CAS
}
// 若头节点不变,则跳出循环;否则持续循环
if (h == head) // loop if head changed
break;
}
}
该办法与独占模式下的获取办法 acquire 大体类似,不同在于该办法中,节点获取资源后会流传状态,即,有可能会持续唤醒后继节点。值得注意的是:该办法有两个入口 setHeadAndPropagate 和 releaseShared,可能有多个线程操作,需思考并发管制。
此外,自己对于将节点设置为 PROPAGATE 状态的了解还不是很清晰,网上说法也不止一种,待后续钻研明确再补充。
2.2.2 以共享模式获取资源(响应中断)
该办法与 acquireShared 相似:
public final void acquireSharedInterruptibly(int arg)
throws InterruptedException {if (Thread.interrupted())
throw new InterruptedException();
if (tryAcquireShared(arg) < 0)
doAcquireSharedInterruptibly(arg);
}
tryAcquireShared 办法后面已剖析,若获取资源失败,则会执行 doAcquireSharedInterruptly 办法:
private void doAcquireSharedInterruptibly(int arg)
throws InterruptedException {
// 把以后线程封装成共享模式节点,并插入主队列
final Node node = addWaiter(Node.SHARED);
boolean failed = true;
try {for (;;) {final Node p = node.predecessor();
if (p == head) {int r = tryAcquireShared(arg);
if (r >= 0) {setHeadAndPropagate(node, r);
p.next = null; // help GC
failed = false;
return;
}
}
// 与 doAcquireShared 相比,区别在于这里抛出了异样
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())
throw new InterruptedException();}
} finally {if (failed)
cancelAcquire(node);
}
}
从代码能够看到,acquireSharedInterruptibly 办法与 acquireShared 办法简直齐全一样,不同之处仅在于前者会抛出 InterruptedException 异样响应中断;而后者仅记录标记位,获取完结后才响应。
2.2.3 以共享模式获取资源(响应中断,且有超时)
代码如下(该办法可与前文独占模式下的超时获取办法比拟剖析):
public final boolean tryAcquireSharedNanos(int arg, long nanosTimeout)
throws InterruptedException {if (Thread.interrupted())
throw new InterruptedException();
return tryAcquireShared(arg) >= 0 ||
doAcquireSharedNanos(arg, nanosTimeout);
}
doAcquireSharedNanos:
private boolean doAcquireSharedNanos(int arg, long nanosTimeout)
throws InterruptedException {if (nanosTimeout <= 0L)
return false;
final long deadline = System.nanoTime() + nanosTimeout;
final Node node = addWaiter(Node.SHARED);
boolean failed = true;
try {for (;;) {final Node p = node.predecessor();
if (p == head) {int r = tryAcquireShared(arg);
if (r >= 0) {setHeadAndPropagate(node, r);
p.next = null; // help GC
failed = false;
return true;
}
}
nanosTimeout = deadline - System.nanoTime();
if (nanosTimeout <= 0L)
return false;
if (shouldParkAfterFailedAcquire(p, node) &&
nanosTimeout > spinForTimeoutThreshold)
LockSupport.parkNanos(this, nanosTimeout);
if (Thread.interrupted())
throw new InterruptedException();}
} finally {if (failed)
cancelAcquire(node);
}
}
该办法可与独占模式下的超时期待办法 tryAcquireNanos(int arg, long nanosTimeout) 进行比照,二者操作基本一致,不再详细分析。
2.2.4 开释资源,唤醒节点,流传状态
如下:
public final boolean releaseShared(int arg) {if (tryReleaseShared(arg)) {doReleaseShared();
return true;
}
return false;
}
tryReleaseShared:
protected boolean tryReleaseShared(int arg) {throw new UnsupportedOperationException();
}
doReleaseShared() 办法后面曾经剖析过了。本办法与独占模式的 release 办法相似,不同的中央在于“流传”二字。
3. 场景剖析
为了便于了解独占模式和共享模式下队列和节点的状态,上面简要举例剖析。
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,其余均为独占模式。
就此场景剖析:T0 先获取到资源(假如占用工夫较长),而后 T1~T4 再获取则失败,会顺次进入主队列。此时主队列中各个节点的状态示意图如下:
之后,T0 操作结束并开释资源,会将 T1 唤醒。T1(独占模式) 会从 acquireQueued(final Node node, int arg) 办法的循环中持续获取资源,这时会获取胜利,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:
此时,T1 获取到资源并进行相干操作。
而后,T1 操作完开释资源,并唤醒下一个节点 T2,T2(共享模式) 持续从 doAcquireShared(int) 办法的循环中执行。此时 T2 获取资源胜利,将本身设为头节点(T1 被移除),因为后继节点 T3 也是共享模式,因而 T1 会持续唤醒 T3;T3 唤醒后的操作与 T2 雷同,但后继节点 T4 不是共享模式,因而不再持续唤醒。此时队列节点状态示意图如下:
此时,T2 和 T3 同时获取到资源。
之后,当二者都开释资源后会唤醒 T4:
T4 获取资源的与 T1 相似。
PS: 该场景仅供参考,只为便于了解,若有不当之处敬请斧正。
4. 小结
本文剖析了以共享模式获取资源的三种形式,以及开释资源的操作。别离为:
- acquireShared: 共享模式获取资源,疏忽中断;
- acquireSharedInterruptibly: 共享模式获取资源,响应中断;
- tryAcquireSharedNanos: 共享模式获取资源,响应中断,有超时;
- releaseShared: 开释资源,唤醒后继节点,并确保流传。
并简要剖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 及条件队列的相干操作,前面波及到的时候再进行剖析。
独自去剖析 AQS 的源码比拟干燥,后文会联合 ReentrantLock、CountdownLatch 等罕用并发工具类的源码进行剖析。
上述解析是参考其余材料及集体了解,若有不当之处欢送斧正。
相干浏览:
JDK 源码剖析 -AbstractQueuedSynchronizer(1)
JDK 源码剖析 -AbstractQueuedSynchronizer(2)