AbstractQueuedSynchronizer类(AQS)
AbstractQueuedSynchronizer抽象同步队列是一个抽象类，简称AQS，是实现同步器的基础组件，并发包中锁的底层就是使用AQS实现的
AQS的数据结构：逻辑结构：双向队列，存储结构：链式存储，即包含头尾指针head、tail及节点Node。Node结构体包含前驱节点prev、后继节点next及data
一、Node（静态内部类）
1、变量与构造方法
//标记线程是请求共享资源时被阻塞挂起后放入AQS队列的 static final Node SHARED = new Node(); //标记线程是请求独占资源时被挂起后放入AQS队列的 static final Node EXCLUSIVE = null; //waitStatus == 1：表示线程已取消 static final int CANCELLED = 1; //waitStatus == -1：线程阻塞（park）需要被唤醒(unpark) static final int SIGNAL = -1; //waitStatus == -2:线程在条件队列中等待 static final int CONDITION = -2; //waitStatus == -3释放共享资源时需要通知其他节点 static final int PROPAGATE = -3; //状态 volatile int waitStatus; //前一个节点 volatile Node prev; // volatile Node next; //后一个节点 volatile Thread thread; // Node nextWaiter; Node() { //无参构造方法 创建头结点或者共享模型节点 } Node(Thread thread, Node mode) { // Used by addWaiter 等待队列节点 this.nextWaiter = mode; this.thread = thread; } Node(Thread thread, int waitStatus) { // Used by Condition 条件队列节点 this.waitStatus = waitStatus; this.thread = thread; }
Node根据构造方法可分为三类
Node()：头节点和共享模式mode类型Node.SHARED（单例）
Node(Thread thread, Node mode):AQS中的阻塞队列中的节点，mode是类型——仅为共享模式节点Node.SHARED及独占模式Node.EXCLUSIVE，waitStatus != -2(Node.CONDITION)
Node(Thread thread, int waitStatus):ConditionObject中条件队列中的的节点，此时mode == null（Node.EXCLUSIVE），waitStatus == -2（Node.CONDITION）
2.方法；只有两个方法
/** * Returns true if node is waiting in shared mode. */判断是否是共享模式下的节点 final boolean isShared() { return nextWaiter == SHARED; } /** * Returns previous node, or throws NullPointerException if null. * Use when predecessor cannot be null. The null check could * be elided, but is present to help the VM. * 返回pre 为空时空指针异常 * @return the predecessor of this node */ final Node predecessor() throws NullPointerException { Node p = prev; if (p == null) throw new NullPointerException(); else return p; }
二、AQS
1.变量和构造方法
//双链表头节点 延迟初始化 只能setHead修改 头结点waitStatus != Node.CANCELLED private transient volatile Node head; //双链表尾节点 延迟初始化 只能enq()入队时修改 private transient volatile Node tail; //The synchronization state.同步锁状态 private volatile int state; private static final Unsafe unsafe = Unsafe.getUnsafe(); private static final long stateOffset; private static final long headOffset; private static final long tailOffset; private static final long waitStatusOffset; private static final long nextOffset; static { try { stateOffset = unsafe.objectFieldOffset (AbstractQueuedSynchronizer.class.getDeclaredField("state")); headOffset = unsafe.objectFieldOffset (AbstractQueuedSynchronizer.class.getDeclaredField("head")); tailOffset = unsafe.objectFieldOffset (AbstractQueuedSynchronizer.class.getDeclaredField("tail")); waitStatusOffset = unsafe.objectFieldOffset (Node.class.getDeclaredField("waitStatus")); nextOffset = unsafe.objectFieldOffset (Node.class.getDeclaredField("next")); } catch (Exception ex) { throw new Error(ex); } } protected AbstractQueuedSynchronizer() { }
2.部分重要的方法
1).Node enq(Node node):入队方法，头尾节点的初始化；及尾插法建立链表入队。是private方法，有封装方法
/** * 节点入队，包含头尾节点初始化 */ private Node enq(final Node node) { for (;;) { Node t = tail; if (t == null) { // Must initialize if (compareAndSetHead(new Node())) tail = head;//初始化，tail == head == new Node(); } else { node.prev = t; if (compareAndSetTail(t, node)) { t.next = node; return t;//标准的尾插法建立链表 } } } }
2. Node addWaiter(Node node):enq方法的封装，是private方法，封装方法分为两类：共享模式，独占模式
/** * Creates and enqueues node for current thread and given mode. * AQS阻塞队列 mode区分共享资源与独占资源 * @param mode Node.EXCLUSIVE for exclusive, Node.SHARED for shared * @return the new node */ private Node addWaiter(Node mode) { Node node = new Node(Thread.currentThread(), mode); // 尾节点不为空时，初次入队尝试跳过enq方法直接入队，如果CAS失败,调用enq入队，enq方法会无限重试CAS for(;;) Node pred = tail; if (pred != null) { node.prev = pred; if (compareAndSetTail(pred, node)) { pred.next = node; return node; } } enq(node); return node; }
3.共享资源模式主要方法：
void acquireShared(int arg)：请求资源，已经包含了释放资源的代码（doReleaseShared）
public final void acquireShared(int arg) { //共享模式下尝试请求资源，成功直接返回，失败入AQS阻塞队列（tryAcquireShared是个空方法，设计模式:模板模式，具体实现延迟到子类） if (tryAcquireShared(arg) < 0) doAcquireShared(arg); } private void doAcquireShared(int arg) { //创建共享模式Node节点插入阻塞队列，并返回Node节点 final Node node = addWaiter(Node.SHARED); boolean failed = true; try { boolean interrupted = false; for (;;) { final Node p = node.predecessor(); if (p == head) { //这里其实是释放资源时逻辑，doReleaseShared()释放资源时，会选择unpark第二个节点，即前驱节点为head头结点的节点（队列的FIFO特点） //此节点尝试请求资源 int r = tryAcquireShared(arg); if (r >= 0) { //请求成功时，释放资源，这里面有个递归实现，会唤醒所有线程等待共享资源的线程（doReleaseShared()）,后面会有图说明 setHeadAndPropagate(node, r); p.next = null; // help GC //还原线程的中断标志 if (interrupted) selfInterrupt(); failed = false; return; } } if (shouldParkAfterFailedAcquire(p, node) && parkAndCheckInterrupt()) //shouldParkAfterFailedAcquire设置Node.waiterStatus == Node.SIGNAL //parkAndCheckInterruptLockSupport.park阻塞线程，当返回时判断是否中断引起的返回 interrupted = true; } } finally { if (failed) //清空首尾节点 cancelAcquire(node); } } private void setHeadAndPropagate(Node node, int propagate) { Node h = head; //第二节点设置为head节点，thread == prev == null setHead(node); if (propagate > 0 || h == null || h.waitStatus < 0 || (h = head) == null || h.waitStatus < 0) { Node s = node.next; if (s == null || s.isShared()) //释放资源 doReleaseShared(); } } private void doReleaseShared() { for (;;) { Node h = head; if (h != null && h != tail) { //阻塞队列不为空 int ws = h.waitStatus; if (ws == Node.SIGNAL) { //头结点head阻塞ws == -1 if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0)) //修改头结点ws = 0 continue; // loop to recheck cases unparkSuccessor(h); } else if (ws == 0 && !compareAndSetWaitStatus(h, 0, Node.PROPAGATE)) //头结点为初始状态ws == 0则修改头结点ws = -3 continue; // loop on failed CAS } if (h == head) // loop if head changed break; } } private void unparkSuccessor(Node node) { int ws = node.waitStatus; if (ws < 0) //节点阻塞修改ws = 0 初始状态 compareAndSetWaitStatus(node, ws, 0); Node s = node.next; if (s == null || s.waitStatus > 0) { //后继节点为空或者线程已取消 s = null; for (Node t = tail; t != null && t != node; t = t.prev) if (t.waitStatus <= 0) //取头结点后阻塞的节点 s = t; } if (s != null) //线程唤醒 LockSupport.unpark(s.thread); } private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) { int ws = pred.waitStatus; if (ws == Node.SIGNAL) //前驱节点是阻塞park，直接返回true return true; if (ws > 0) { //线程已取消 do { //删除队列中线程已取消的所有前驱节点 node.prev = pred = pred.prev; } while (pred.waitStatus > 0); pred.next = node; } else { //前驱节点ws=Node.SIGNAL compareAndSetWaitStatus(pred, ws, Node.SIGNAL); } return false; } private final boolean parkAndCheckInterrupt() { //park当前线程阻塞 LockSupport.park(this); //返回当前线程中断标志 return Thread.interrupted(); }
共享模式AQS阻塞队列的链表结构变化
4.独占模式与共享模式大部分逻辑相同，但
1).独占模式节点类型：Node.EXCLUSIVE
2).少了递归逻辑及仅唤醒头结点的后继节点（仅唤醒一个线程，共享模式是唤醒共享资源上所有线程）
public final void acquire(int arg) { if (!tryAcquire(arg) && acquireQueued(addWaiter(Node.EXCLUSIVE), arg))//Node.EXCLUSIVE类型节点 selfInterrupt(); } final boolean acquireQueued(final Node node, int arg) { boolean failed = true; try { boolean interrupted = false; for (;;) { final Node p = node.predecessor(); if (p == head && tryAcquire(arg)) { setHead(node);//此处将头结点后继节点设置为头结点，FIFO，释放先入队的节点 p.next = null; // help GC failed = false; return interrupted; } if (shouldParkAfterFailedAcquire(p, node) && parkAndCheckInterrupt()) interrupted = true; } } finally { if (failed) cancelAcquire(node); } }
三、ConditionObject
AQS中除了Node外，还包含一个内部类ConditionObject，主要功能信号量机制实现线程同步（与Object中wait notify类似，与操作系统中进程的PV操作保证进程同步）
ConditionObject定义了一个双向队列——条件队列，链式存储实现，节点还是Node，
1.变量及构造方法
/** 条件队列头结点 */ private transient Node firstWaiter; /** 条件队列尾节点 */ private transient Node lastWaiter; /** * 无参构造方法*/ public ConditionObject() { }
2.重要方法
1）入队方法await()：线程阻塞
public final void await() throws InterruptedException { if (Thread.interrupted()) throw new InterruptedException(); //创建新的节点插入到条件队列末尾并返回此节点 Node node = addConditionWaiter(); //释放当前线程的资源（锁），失败会抛出异常 int savedState = fullyRelease(node); int interruptMode = 0; //为条件队列节点（即ws == -2）时，直接返回false， //不为条件队列节点(即ws != -2)时，判断是否在AQS队列中 while (!isOnSyncQueue(node)) { LockSupport.park(this); //park返回的2种情况：线程被中断；从条件队列转入AQS队列，被unpark唤醒 if ((interruptMode = checkInterruptWhileWaiting(node)) != 0) //如果是中断引起的返回直接break；（unpark引起的返回由于，signal时节点从条件队列转入AQS队列中，根据while条件会跳出循环） break; } //尝试获取资源（锁），返回值是当前线程是否中断标志引起返回 && 0 != -1； if (acquireQueued(node, savedState) && interruptMode != THROW_IE) interruptMode = REINTERRUPT; //node无后继节点，从头结点遍历，去掉所有非ws ！=2 状态的队列节点 if (node.nextWaiter != null) // clean up if cancelled unlinkCancelledWaiters(); if (interruptMode != 0) //中断引起的返回时 //THROW_IE:抛出异常 //REINTREEUPT:还原中断状态 reportInterruptAfterWait(interruptMode); } private Node addConditionWaiter() { Node t = lastWaiter; // If lastWaiter is cancelled, clean out. if (t != null && t.waitStatus != Node.CONDITION) { //从头结点遍历，去掉所有不是ws != -2状态的队列节点 unlinkCancelledWaiters(); t = lastWaiter; } //新建ws = -2t条件队列节点，后尾插法 Node node = new Node(Thread.currentThread(), Node.CONDITION); if (t == null) firstWaiter = node; else t.nextWaiter = node; lastWaiter = node; return node; } final int fullyRelease(Node node) { boolean failed = true; try { int savedState = getState(); //释放当前线程占有的资源（锁）成功则ws:线程已取消，失败会抛出异常 if (release(savedState)) { failed = false; return savedState; } else { throw new IllegalMonitorStateException(); } } finally { if (failed) node.waitStatus = Node.CANCELLED; } }
2）出队方法signal()：线程唤醒
public final void signal() { //资源（锁）是否被当前线程占用，isHeldExclusively也是个模板方法，条件队列时，必须重写 if (!isHeldExclusively()) throw new IllegalMonitorStateException(); Node first = firstWaiter; if (first != null) //将条件队列头结点插入到AQS队列中 doSignal(first); } private void doSignal(Node first) { do { if ( (firstWaiter = first.nextWaiter) == null) lastWaiter = null; first.nextWaiter = null; } while (!transferForSignal(first) && (first = firstWaiter) != null); } final boolean transferForSignal(Node node) { /* * 修改条件队列节点状态ws = 0 */ if (!compareAndSetWaitStatus(node, Node.CONDITION, 0)) return false; /* * 插入到AQS队列，此时节点已转化为AQS节点状态：ws == 0，而不是条件队列节点：ws == -2 */ Node p = enq(node); int ws = p.waitStatus; //设置节点为park状态ws == -1，设置不成功直接唤醒当前线程 if (ws > 0 || !compareAndSetWaitStatus(p, ws, Node.SIGNAL)) LockSupport.unpark(node.thread); return true; }
四、总结
1.一个锁对应一个AQS阻塞队列，对应多个条件队列变量ConditionObject，每个条件变量又有自己的条件队列。
2.Node节点Data域包含3个变量，Thread thread；int waitStatus；Node nextWaiter;
thread表明节点对应线程，通常是创建节点时的当前线程；
waitStatus表明当前线程状态（0：初始；1：线程取消；-1：线程park状态；-2：线程位于条件队列中；-3：共享模式下传递状态）；
nextWaiter表明线程资源模式，用此属性标志共享资源模式（共享锁）还是独占资源模式（独占锁）
Node节点根据构造方法可分为三类
Node()：AQS阻塞队列的头节点head和共享模式nextWaiter类型Node.SHARED（单例）
Node(Thread thread, Node mode):AQS阻塞队列中的节点，mode是类型——共享模式Node.SHARED及独占模式Node.EXCLUSIVE，waitStatus != -2(Node.CONDITION)
Node(Thread thread, int waitStatus):ConditionObject中条件队列中的的节点，此时mode == null（Node.EXCLUSIVE），waitStatus == -2（Node.CONDITION）
3.LockSupport.park(this)阻塞的线程可被中断唤醒和LockSupport.unpark(this)唤醒;所以AQS中提供两种实现：忽略interrupt()返回并还原中断标志;不忽略interrupt()返回直接抛出异常
4.可以发现AQS类中，定义了state变量，但是对此变量没有任何操作，原因在于AQS是一个抽象顶级类，采用模板模式的设计方法将一些方法的实现延迟到了子类，而这些方法的实现就包含了对state变量的操作，并发包中锁的原理是通过计数器实现的，state就是这个计数器，state大小代表了锁的状态，通过unsafe类中CAS操作保证对state增减的原子性即保证线程对锁获取释放的原子性。
state == 0；锁未被占有
state > 0 ;锁被占用，state数代表可重入锁的重入数
state < 0 ;锁的重入数超过最大限度（int类型）导致了溢出
另外重写的方法还可以破坏队列FIFO特性（公平锁），实现非公平锁。具体破坏：例如当锁未被占用时，线程获取锁时，公平锁是插入到阻塞队列，队列头结点获取锁；但具体实现可以不插入阻塞队列，当前线程直接获取锁
//独占模式（独占锁）：尝试获取锁 protected boolean tryAcquire(int arg) { throw new UnsupportedOperationException(); } //独占模式（独占锁）：尝试释放锁 protected boolean tryRelease(int arg) { throw new UnsupportedOperationException(); } //共享模式（共享锁）：尝试获取锁 protected int tryAcquireShared(int arg) { throw new UnsupportedOperationException(); } //共享模式（共享锁）：尝试释放共享锁 protected boolean tryReleaseShared(int arg) { throw new UnsupportedOperationException(); } //使用条件队列时必须实现的方法：当前线程是否持有锁 protected boolean isHeldExclusively() { throw new UnsupportedOperationException(); }
五、实现例子
不可重入的独占锁
public class NonReentrantLock { private static class Sync extends AbstractQueuedSynchronizer{ @Override protected boolean isHeldExclusively(){ return getState() == 1; } @Override public boolean tryAcquire(int acquires){ assert acquires == 1; if (compareAndSetState(0,1)){ setExclusiveOwnerThread(Thread.currentThread()); return true; } return false; } @Override protected boolean tryRelease(int releases) { assert releases == 1; if (getState() == 0){ throw new IllegalMonitorStateException(); } setExclusiveOwnerThread(null); setState(0); return true; } Condition newCondition(){ return new ConditionObject(); } } private final Sync sync = new Sync(); public void lock(){ sync.acquire(1); } public boolean tryLock(){ return sync.tryAcquire(1); } public void unlock(){ sync.release(1); } public Condition newCondition(){ return sync.newCondition(); } public boolean isLocked(){ return sync.isHeldExclusively(); } public void lockInterruptibly() throws InterruptedException{ sync.acquireInterruptibly(1); } public boolean tryLock(long timeout, TimeUnit unit) throws InterruptedException{ return sync.tryAcquireNanos(1, unit.toNanos(timeout)); } } public class AQSTest { final static NonReentrantLock lock = new NonReentrantLock(); final static Condition notFull = lock.newCondition(); final static Condition notEmpty = lock.newCondition(); final static Queue<String> queue = new LinkedBlockingQueue<>(); final static int QUEUESIZE = 10; public static void main(String[] args) throws InterruptedException { Thread producer = new Thread(new Runnable() { @Override public void run() { lock.lock(); try { while (queue.size() == QUEUESIZE){ notEmpty.await(); } queue.add("ele"); notFull.signalAll(); }catch (Exception e){ e.printStackTrace(); }finally { lock.unlock(); } } }); Thread consumer = new Thread(new Runnable() { @Override public void run() { lock.lock(); try { while (0 == queue.size()){ notFull.await(); } String ele = queue.poll(); notEmpty.signalAll(); }catch (Exception e){ e.printStackTrace(); }finally { lock.unlock(); } } }); producer.start(); consumer.start(); } }
参考自《java并发编程之美》
