AQS分析（AbstractQueuedSynchronizer）（三）

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1、AQS是什么

　　AQS同步器是Java并发编程的基础，从资源共享的角度分成独占和共享两种模式，像ReentrantLock、ThreadPoolExecutor、CountDownLatch等都是基于AQS来实现的，如图：

2、AQS同步队列的基本结构

　　AQS维护了一个头节点（head）和一个尾节点（tail）结构的双向链表，当一个线程获取锁失败时，会将该线程打包成一个Node节点，挂到同步队列尾节点    

private transient volatile Node head;//同步队列头结点
private transient volatile Node tail;//同步队列尾结点
private volatile int state;//同步状态 

 2.1、同步队列内部结构

 

2.2、获取同步状态失败，将当前线程和节点模式打包成一个节点，放入同步队列尾部 

2.3、前驱节点是头节点并且获取同步状态成功，设置当前节点为头节点，并将前驱节点指针清空，方便GC回收

2.1、Node节点类（双向链表挂在同步器中）  

static final class Node {static final Node SHARED = new Node();//共享模式static final Node EXCLUSIVE = null;//独占模式static final int CANCELLED =  1;//线程已取消static final int SIGNAL    = -1;//后继线程需要取消挂起static final int CONDITION = -2;//线程正在等待条件static final int PROPAGATE = -3;volatile int waitStatus;volatile Node prev;//前驱结点volatile Node next;//后继结点volatile Thread thread;//当前线程Node nextWaiter;final boolean isShared() {return nextWaiter == SHARED;}//获取前驱节点final Node predecessor() throws NullPointerException {Node p = prev;if (p == null)throw new NullPointerException();elsereturn p;}Node() {}//当前线程和节点模式Node(Thread thread, Node mode) {this.nextWaiter = mode;this.thread = thread;}Node(Thread thread, int waitStatus) {this.waitStatus = waitStatus;this.thread = thread;}
}

2.2、state同步状态（AQS的重要成员变量）

//同步状态    
private volatile int state;
//获取同步状态
protected final int getState() {return state;
}
//设置同步状态
protected final void setState(int newState) {state = newState;
}
//CAS设置同步状态
protected final boolean compareAndSetState(int expect, int update) {// See below for intrinsics setup to support thisreturn unsafe.compareAndSwapInt(this, stateOffset, expect, update);
}

3、子类需重写的方法

 AQS使用了模板方法设计模式，核心框架JDK已经写好，子类（自定义同步器）只需重写如下几个方法，即可实现不同的同步器：

//用于判断当前方法是否被线程独占,独占锁需重写
protected boolean isHeldExclusively() {throw new UnsupportedOperationException();
}//独占式获取锁
protected boolean tryAcquire(int arg) {throw new UnsupportedOperationException();
}//独占式释放锁
protected boolean tryRelease(int arg) {throw new UnsupportedOperationException();
}//共享式获取锁
protected int tryAcquireShared(int arg) {throw new UnsupportedOperationException();
}//共享式释放锁
protected int tryReleaseShared(int arg) {throw new UnsupportedOperationException();
}

4、独占模式分析

 4.1、acquire独占锁获取（ReentrantLock的lock方法就是调用该方法）

1、tryAcquire（子类实现的方法，此时派上用场）

尝试获取同步状态，获取成功则直接使用

2、addWaiter

   将当前线程打包成一个独占模式节点，放入同步队列的尾部

3、acquireQueued

   进入等待状态，直到其他线程唤醒自己

//获取独占锁入口方法
public final void acquire(int arg) {if (!tryAcquire(arg) &&acquireQueued(addWaiter(Node.EXCLUSIVE), arg))selfInterrupt();
}
//放入队列
private Node addWaiter(Node mode) {Node node = new Node(Thread.currentThread(), mode);// 快速尝试将其放入尾部节点Node pred = tail;if (pred != null) {node.prev = pred;if (compareAndSetTail(pred, node)) {pred.next = node;return node;}}enq(node);//循环CAS方式将节点放入队列尾部return node;
}
//入队
private Node enq(final Node node) {for (; ; ) {//循环CAS添加尾部节点Node t = tail;if (t == null) { //队列为空,初始化一个空节点if (compareAndSetHead(new Node()))//CAS防止产生多个队列tail = head;} else {node.prev = t;if (compareAndSetTail(t, node)) {//CAS设置尾节点t.next = node;return t;}}}
}
//阻塞等待
final boolean acquireQueued(final Node node, int arg) {boolean failed = true;try {boolean interrupted = false;//是否被中断for (; ; ) {final Node p = node.predecessor();//获取前驱节点if (p == head && tryAcquire(arg)) {//前驱节点是头节点 且 自己获取到锁setHead(node);//将当前节点设置为头节点p.next = null; //便于GC回收以前的头节点failed = false;return interrupted;}if (shouldParkAfterFailedAcquire(p, node) &&//设置前驱节点状态parkAndCheckInterrupt())//阻塞线程interrupted = true;//被中断一次就设置为true}} finally {if (failed)cancelAcquire(node);}
}
//设置前驱节点
private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {int ws = pred.waitStatus;//前驱节点的状态if (ws == Node.SIGNAL)return true;if (ws > 0) {//目的是为了剔除取消的节点do {node.prev = pred = pred.prev;} while (pred.waitStatus > 0);//找到一个没有被取消的节点pred.next = node;} else {compareAndSetWaitStatus(pred, ws, Node.SIGNAL);//将前驱节点设置为SIGNAL}return false;
}
//阻塞线程
private final boolean parkAndCheckInterrupt() {LockSupport.park(this);//阻塞线程，直到被唤醒(发生中断或被其他线程调用unpark)return Thread.interrupted();//线程是否中断
}

4.2、release独占锁释放(ReentrantLock的unlock方法就是调用该方法)

1、tryRelease（子类实现的方法，自定义释放的逻辑）

 尝试获取同步状态，成功则继续

2、unparkSuccessor

　　找到头节点，唤醒后继线程   

public final boolean release(int arg) {if (tryRelease(arg)) {Node h = head;//找到头节点if (h != null && h.waitStatus != 0)unparkSuccessor(h);//唤醒后继线程return true;}return false;
}
//唤醒后继线程
private void unparkSuccessor(Node node) {/** If status is negative (i.e., possibly needing signal) try* to clear in anticipation of signalling.  It is OK if this* fails or if status is changed by waiting thread.*/int ws = node.waitStatus;if (ws < 0)compareAndSetWaitStatus(node, ws, 0);Node s = node.next;//获取后继节点if (s == null || s.waitStatus > 0) {s = null;for (Node t = tail; t != null && t != node; t = t.prev)if (t.waitStatus <= 0)s = t;}if (s != null)LockSupport.unpark(s.thread);//唤醒后继线程
}

5、共享模式分析

5.1、acquireSharedInterruptibly共享锁获取（Semaphore的acquire方法就是调用该方法）

1、线程是否中断，是则抛出异常

2、tryAcquireShared（子类实现的方法）

尝试获取资源，成功直接返回，失败进入下面流程

3、doAcquireSharedInterruptibly（和独占锁类似） 

将当前线程打包成共享节点，放入同步队列并阻塞，直到被唤醒并成功获取到资源才返回 

public final void acquireSharedInterruptibly(int arg)throws InterruptedException {if (Thread.interrupted())throw new InterruptedException();if (tryAcquireShared(arg) < 0)//子类实现的方法，一般用来判断是否还有资源doAcquireSharedInterruptibly(arg);//放入同步队列等待
}private void doAcquireSharedInterruptibly(int arg)throws InterruptedException {final Node node = addWaiter(Node.SHARED);//将当前线程打包成一个共享节点，放入同步队列尾部boolean failed = true;try {for (;;) {//自旋final Node p = node.predecessor();//获取前驱节点if (p == head) {//前驱节点是头节点int r = tryAcquireShared(arg);//尝试获取资源if (r >= 0) {//大于0代表有资源可用setHeadAndPropagate(node, r);//设置自己为head，还有剩余资源则唤醒后继线程p.next = null; // help GCfailed = false;return;}}if (shouldParkAfterFailedAcquire(p, node) &&//设置前驱节点状态parkAndCheckInterrupt())//阻塞线程，等待其他线程唤醒或线程被中断throw new InterruptedException();}} finally {if (failed)cancelAcquire(node);}
}

5.2、releaseShared共享锁释放（Semaphore的release方法就是调用该方法）

public final boolean releaseShared(int arg) {if (tryReleaseShared(arg)) {doReleaseShared();return true;}return false;
}private void doReleaseShared() {for (;;) {Node h = head;if (h != null && h != tail) {int ws = h.waitStatus;if (ws == Node.SIGNAL) {if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))continue;            // loop to recheck casesunparkSuccessor(h);//唤醒后继线程}else if (ws == 0 &&!compareAndSetWaitStatus(h, 0, Node.PROPAGATE))continue;                // loop on failed CAS}if (h == head)                   // loop if head changedbreak;}
}

 

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