Thread.AQS

AQS

• AbstractQueuedSynchronizer队列同步器，Lock接口实现的核心，可自定义同步器。

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• CLH锁实现图，将每一个线程构形成Node节点，加入链表，新加入线程在列表队列最后，每次头结点获取到所，后续节点继续获取锁。
• AQS暴露操做方法，隐藏实现细节，集成AQS，重写获取锁方法，可自定义同步锁。
• 内部经过voliate修饰的int变量，unsafe方法提供的compareAndSet方法提供同步，逐步包装成同步队列，同步锁。
• 原理总结，用AQS的volatile的同步特性，设置int锁数量，unsafe对象的CAS同步设置方法，构造同步队列，最后unsafe的park，unpark直接操做线程挂起释放。

static final class Node {
       
        static final Node SHARED = new Node();
        static final Node EXCLUSIVE = null;
        static final int CANCELLED =  1;
 
        static final int SIGNAL    = -1;
     
        static final int CONDITION = -2;
  
        static final int PROPAGATE = -3;
        volatile int waitStatus;
        volatile Node prev;
        volatile Node next;
        volatile Thread thread;
    
        Node nextWaiter;
 
    }

• 以Lock的实现Reentranlock为例

public class ReentrantLock implements Lock, java.io.Serializable {
    private static final long serialVersionUID = 7373984872572414699L;
    private final Sync sync;
    //实现AQS重写锁
    //Lock的全部操做，经过代理内部类调用AQS,先看非公平锁
    abstract static class Sync extends AbstractQueuedSynchronizer {
        private static final long serialVersionUID = -5179523762034025860L;
        abstract void lock();
        final boolean nonfairTryAcquire(int acquires) {
            final Thread current = Thread.currentThread();
            int c = getState();
            if (c == 0) {
                if (compareAndSetState(0, acquires)) {
                    setExclusiveOwnerThread(current);
                    return true;
                }
            }
            else if (current == getExclusiveOwnerThread()) {
                int nextc = c + acquires;
                if (nextc < 0) // overflow
                    throw new Error("Maximum lock count exceeded");
                setState(nextc);
                return true;
            }
            return false;
        }
        protected final boolean tryRelease(int releases) {
            int c = getState() - releases;
            if (Thread.currentThread() != getExclusiveOwnerThread())
                throw new IllegalMonitorStateException();
            boolean free = false;
            if (c == 0) {
                free = true;
                setExclusiveOwnerThread(null);
            }
            setState(c);
            return free;
        }
    }
    static final class NonfairSync extends Sync {
        private static final long serialVersionUID = 7316153563782823691L;
        //2. sync的非公平实现，快速调用父类AQS的CAS设置state是，若是失败，进入acquire构造Node节点，加入CLK
        //
        final void lock() {
            if (compareAndSetState(0, 1))
                setExclusiveOwnerThread(Thread.currentThread());
            else
                acquire(1);
        }
        
        //3. AQS的acruird，tryAcquire交给子类自定实现，addWaiter加入CLK节点，acquireQueued，全部节点自旋，获取同步状态
        protected final boolean tryAcquire(int acquires) {
            return nonfairTryAcquire(acquires);
        }
    }
    public ReentrantLock() {
        sync = new NonfairSync();
    }
    public ReentrantLock(boolean fair) {
        sync = fair ? new FairSync() : new NonfairSync();
    }
    //1. 外部调用Lock
    public void lock() {
        sync.lock();
    }
    public void lockInterruptibly() throws InterruptedException {
        sync.acquireInterruptibly(1);
    }
    public boolean tryLock() {
        return sync.nonfairTryAcquire(1);
    }
    public boolean tryLock(long timeout, TimeUnit unit)
            throws InterruptedException {
        return sync.tryAcquireNanos(1, unit.toNanos(timeout));
    }
    public Condition newCondition() {
        return sync.newCondition();
    }
}

    //4. AQS中的实现，tryAcquire交给子类
    public final void acquire(int arg) {
        if (!tryAcquire(arg) &&
            acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
            selfInterrupt();
    }
      //5. ReentranLock里Sync非公平锁实现
      final boolean nonfairTryAcquire(int acquires) {
            final Thread current = Thread.currentThread();
            //获取AQS中state
            int c = getState();
            if (c == 0) {
                //AQS的CAS同步设置state，并设置持有线程
                if (compareAndSetState(0, acquires)) {
                    //设置持有线程
                    setExclusiveOwnerThread(current);
                    return true;
                }
            }
            //可重入关键，同一线程可重复获取锁
            else if (current == getExclusiveOwnerThread()) {
                int nextc = c + acquires;
                if (nextc < 0) // overflow
                    throw new Error("Maximum lock count exceeded");
                setState(nextc);
                return true;
            }
            return false;
        }
    //6. 加入CLK链表尾部，经过AQS,CAS设置tail节点，构造链表见自定容器Stack
    private Node addWaiter(Node mode) {
        Node node = new Node(Thread.currentThread(), mode);
        // Try the fast path of enq; backup to full enq on failure
        Node pred = tail;
        if (pred != null) {
            node.prev = pred;
            //快速设置tail，若是失败进入enq
            if (compareAndSetTail(pred, node)) {
                pred.next = node;
                return node;
            }
        }
        enq(node);
        return node;
    }
    //死循环设置尾节点，直到成功，以死循环的方式，同步设置tail节点
    private Node enq(final Node node) {
        for (;;) {
            Node t = tail;
            if (t == null) { // Must initialize
                if (compareAndSetHead(new Node()))
                    tail = head;
            } else {
                node.prev = t;
                if (compareAndSetTail(t, node)) {
                    t.next = node;
                    return t;
                }
            }
        }
    }
    //7. 最后，每一个构造的节点，都以自旋的方式，获取前一节点，若是前一节点是头节点，而且获取到锁，把当前本身设置成头结点，获的锁。
    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);
                    p.next = null; // help GC
                    failed = false;
                    return interrupted;
                }
                //parkAndCheckInterrupt 调用LockSupport挂起线程，进入阻塞状态
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    interrupted = true;
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }
    //unlock,调用AQS释放
    public final boolean release(int arg) {
        if (tryRelease(arg)) {
            Node h = head;
            if (h != null && h.waitStatus != 0)
            //LockSupport的unparking，释放线程挂起状态    
            unparkSuccessor(h);
            return true;
        }
        return false;
    }
     //1. 交给子类实现
     protected final boolean tryRelease(int releases) {
            int c = getState() - releases;
            if (Thread.currentThread() != getExclusiveOwnerThread())
                throw new IllegalMonitorStateException();
            boolean free = false;
            if (c == 0) {
                free = true;
                setExclusiveOwnerThread(null);
            }
            setState(c);
            return free;
        }