多线程学习笔记四之Condition实现分析

目录

• 简介
  • 等待队列
  • nextWaiter
• 源码分析
  • await()
  • signal()
  • signalAll()
• 总结

简介

  在使用内置锁synchronized时，通过调用java.lang.Objec中定义的监视器方法，主要有wait()、wait(long timeout)、notify()和notifyAll()方法，可以实现等待/通知模式。Codition接口中也定义了类似的监视器方法，与显示锁Lock配合使用也可以实现等待/通知模式。
  当线程需要利用Condition对象进行等待时，需要提前获取到Condition对象关联的显示锁Lock对象，使用案例如下：

    Lock lock = new ReentrantLock();
    Condition condition = lock.newCondition();

	//等待
    public void coditionWait() throws InterruptedException {
        lock.lock();
        try {
            condition.await();
        }finally {
            lock.unlock();
        }
    }

	//通知
    public void coditionSignal() throws InterruptedException {
        lock.lock();
        try {
            condition.signal();
        }finally {
            lock.unlock();
        }
    }

  Condition接口由同步器AbstractQueuedSynchronizer内部类ConditionObject提供实现，而显示锁Lock对象实现时内部类Sync会继承AQS,从而把Condition对象与Lock对象关联起来。

等待队列

  在上一篇博客中介绍到为了处理多个线程竞争同一把锁，同步器AQS中维护了一个先入先出的双向同步队列，让竞争失败的线程进入同步队列等待。同样，AQS在实现Condition接口也维护了一个先入先出的单向等待队列，当一个与Lock对象关联的Condition对象调用await方法，获得锁的线程就要释放锁，并推出同步队列head头节点，进入condition等待队列。condition队列规定了头节点firstWaiter和尾节点lastWaiter。

public class ConditionObject implements Condition, java.io.Serializable {
        private static final long serialVersionUID = 1173984872572414699L;
        /** First node of condition queue. */
        private transient Node firstWaiter;
        /** Last node of condition queue. */
        private transient Node lastWaiter;
}

AQS中构建等待队列复用了内部类Node结点类

    static final class Node {
		//等待状态
        volatile int waitStatus;

		//前驱结点
        volatile Node prev;
	
		//后继节点
        volatile Node next;
		
		//等待获取锁的线程
        volatile Thread thread;
		
		//condition队列的后继节点
        Node nextWaiter;      
    }

nextWaiter

  从上图可以发现，Condition等待队列是一个先入先出的单向链表，从链表尾部加入元素，头部移出链表。使用nextWaiter指向下一个等待节点，构成链表的基本元素是节点Node，复用了AQS中的Node类，nextWaiter并不单单在Condition链表指向下一个等待节点。这是Node类定义nextWaiter的注释：

Link to next node waiting on condition, or the special value SHARED. Because condition queues are accessed only when holding in exclusive mode, we just need a simple linked queue to hold nodes while they are waiting on conditions. They are then transferred to the queue to re-acquire. And because conditions can only be exclusive,we save a field by using special value to indicate sharedmode.

大意是只有独占锁才会关联Condition队列，通过nextWaiter变量在构成同步队列节点标识同步锁是独占锁还是共享锁，从以下方法可以看出AQS使用nextWaiter来表示锁：

	/** Marker to indicate a node is waiting in shared mode */
    static final Node SHARED = new Node();
    /** Marker to indicate a node is waiting in exclusive mode */
    static final Node EXCLUSIVE = null;

	//判断是否是共享锁
    final boolean isShared() {
        return nextWaiter == SHARED;
    }

	//构建同步队列节点，nextWaiter标识同步锁是独占锁还是共享锁
	Node(Thread thread, Node mode) {     // Used by addWaiter
        this.nextWaiter = mode;
        this.thread = thread;
    }

	//构建等待队列节点，nextWaiter指向单向链表下一个节点
    Node(Thread thread, int waitStatus) { // Used by Condition
        this.waitStatus = waitStatus;
        this.thread = thread;
    }

从以上分析可以看出：AQS复用了Node类来构建同步队列和等待队列，Node用来构建同步队列节点，nextWaiter标识同步锁是独占锁还是共享锁；Node用来构建等待队列节点，nextWaiter指向单向链表下一个节点。刚开始看这一部分时，对我造成了很大的困扰，所以特地写出来。

源码分析

await()

  await实现等待考虑到了中断，若当前线程等待期间发生中断，抛出InterruptedException异常。线程在等待期间会被阻塞，直到发生中断或者Condition对象调用signal方法。基本流程：首先将node加入condition队列，然后释放锁，挂起当前线程等待唤醒，唤醒后线程重新进入同步队列并调用acquireQueued获取锁。流程图如下：

    public final void await() throws InterruptedException {
        if (Thread.interrupted())
            throw new InterruptedException();
		//将当前线程加入Condition等待队列			
        Node node = addConditionWaiter();
        int savedState = fullyRelease(node);
        int interruptMode = 0;
		//判断当前线程是否在同步队列中
        while (!isOnSyncQueue(node)) {
			//阻塞当前线程
            LockSupport.park(this);
			//在阻塞的过程中发生中断
            if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
                break;
        }
		//被其他线程唤醒，退出Condition等待队列加入同步队列
		//获取锁
        if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
            interruptMode = REINTERRUPT;
        if (node.nextWaiter != null) // clean up if cancelled
            unlinkCancelledWaiters();
        if (interruptMode != 0)
            reportInterruptAfterWait(interruptMode);
    }

• addConditionWaiter()
    以当前线程构成节点Node加入等待队列，因为加入Condition等待队列在释放锁之前，所以不需要考虑并发的情况，就不需要像加入同步队列采用循环加CAS的机制。

    private Node addConditionWaiter() {
        Node t = lastWaiter;
        // If lastWaiter is cancelled, clean out.
		//如果尾节点lastWaiter等待状态是CANCELLED，将队列所有CANCELLED节点清除
        if (t != null && t.waitStatus != Node.CONDITION) {
            unlinkCancelledWaiters();
            t = lastWaiter;
        }
		//以当前线程构成节点
        Node node = new Node(Thread.currentThread(), Node.CONDITION);
		//尾节点为空，等待队列为空，进行初始化，当前节点是等待队列的头节点
        if (t == null)
            firstWaiter = node;
		//否则添加到等待队列的尾部，当前节点是等待队列新的lastWaiter
        else
            t.nextWaiter = node;
        lastWaiter = node;
        return node;
    }


	//unlinkCancelledWaiters方法遍历CONDITION队列，删除状态为CANCELLED的节点。
	private void unlinkCancelledWaiters() {
		//首节点
        Node t = firstWaiter;
		//保存遍历节点前驱节点的引用
        Node trail = null;
		//单向链表从前往后遍历
        while (t != null) {
			//下一个节点
            Node next = t.nextWaiter;
			//节点t的waitStatus为CANCELLED
            if (t.waitStatus != Node.CONDITION) {
                 t.nextWaiter = null;
                if (trail == null)
                    firstWaiter = next;
                else
                     trail.nextWaiter = next;
                if (next == null)
                    lastWaiter = trail;
            }
            else
                trail = t;
            t = next;
        }
    }

• fullyRelease(Node node)
    完全释放锁,释放成功则返回，失败则将当前节点(在Condition队列)的状态设置成CANCELLED表示当前节点失效

	final int fullyRelease(Node node) {
	    boolean failed = true;
	    try {
			//获取同步状态
	        int savedState = getState();
			//如果是重入锁，要多次释放
	        if (release(savedState)) {
	            failed = false;
	            return savedState;
	        } else {
	            throw new IllegalMonitorStateException();
	        }
	    } finally {
	        if (failed)
	            node.waitStatus = Node.CANCELLED;
	    }
	}

• isOnSyncQueue(Node node)
    判断node节点是否被signal方法从condition队列转移到同步队列

    final boolean isOnSyncQueue(Node node) {
		//转移到同步队列，CONDITION状态会被清除
		//同步队列prev表示前驱结点，不为null
        if (node.waitStatus == Node.CONDITION || node.prev == null)
            return false;
		//同步队列next表示后继节点，不为null
        if (node.next != null) // If has successor, it must be on queue
            return true;
        /*
         * node.prev can be non-null, but not yet on queue because
         * the CAS to place it on queue can fail. So we have to
         * traverse from tail to make sure it actually made it.  It
         * will always be near the tail in calls to this method, and
         * unless the CAS failed (which is unlikely), it will be
         * there, so we hardly ever traverse much.
         */
		//遍历同步队列，一个一个找
        return findNodeFromTail(node);
    }

• checkInterruptWhileWaiting(Node node)
    检查当前线程在等待状态时中断状态，返回REINTERRUPT标志位，退出等待状态时调用selfInterrupt方法产生中断；返回THROW_IE标志位，线程退出等待状态时会抛出InterruptedException异常。

        //表示从等待状态退出时会重新产生一个中断，但不会抛出异常
        private static final int REINTERRUPT =  1;
        //从等待状态退出时抛出InterruptedException异常
        private static final int THROW_IE    = -1;

        /**
         * Checks for interrupt, returning THROW_IE if interrupted
         * before signalled, REINTERRUPT if after signalled, or
         * 0 if not interrupted.
         */
        private int checkInterruptWhileWaiting(Node node) {
            return Thread.interrupted() ?
                (transferAfterCancelledWait(node) ? THROW_IE : REINTERRUPT) :
                0;
        }

• reportInterruptAfterWait(int interruptMode)
    根据interruptMode对应的标志位响应中断

    private void reportInterruptAfterWait(int interruptMode)
        throws InterruptedException {
		//产生异常
        if (interruptMode == THROW_IE)
            throw new InterruptedException();
		//产生中断
        else if (interruptMode == REINTERRUPT)
            selfInterrupt();
    }

signal()

  检查当前线程是否占据独占锁，唤醒等待在当前Condition对象等待最久的线程（等待队列的头节点）

    public final void signal() {
		//检查当前线程是否占据独占锁，如果不是没有权限唤醒等待线程，抛出异常
        if (!isHeldExclusively())
            throw new IllegalMonitorStateException();
        Node first = firstWaiter;
        if (first != null)
            doSignal(first);
    }

	private void doSignal(Node first) {
        do {
            if ( (firstWaiter = first.nextWaiter) == null)
                lastWaiter = null;
            first.nextWaiter = null;
        } while (!transferForSignal(first) &&
                (first = firstWaiter) != null);
    }

• transferForSignal(Node node)
    将当前线程从Condition等待队列转移到同步队列中，看到这里应该明白为什么await方法以节点是否在同步队列（isOnSyncQueue(node)）做为循环条件了。

    final boolean transferForSignal(Node node) {
        /*
         * If cannot change waitStatus, the node has been cancelled.
         */
		//如果CAS设置失败，说明节点在signal之前被取消了，返回false
        if (!compareAndSetWaitStatus(node, Node.CONDITION, 0))
            return false;
        
		//CAS设置成功，入队
		//插入节点的前驱节点
        Node p = enq(node);
		//前驱节点的等待状态
        int ws = p.waitStatus;
		//如果p等待状态为CANECLLED或对p进行CAS设置失败，则唤醒线程，让node中线程进入acquireQueued方法。否则
		//由于前驱节点等待状态为signal，由同步器唤醒线程
        if (ws > 0 || !compareAndSetWaitStatus(p, ws, Node.SIGNAL))
            LockSupport.unpark(node.thread);
        return true;
    }

signalAll()

  将等待队列所有节点依次转移到同步队列末尾。

    public final void signalAll() {
        if (!isHeldExclusively())
            throw new IllegalMonitorStateException();
        Node first = firstWaiter;
        if (first != null)
            doSignalAll(first);
    }

    private void doSignalAll(Node first) {
        lastWaiter = firstWaiter = null;
        do {
			//first节点从condition队列移出
            Node next = first.nextWaiter;
            first.nextWaiter = null;
			//first节点加入同步队列
            transferForSignal(first);
			//更新first节点指向
            first = next;
        } while (first != null);
    }

总结

  以上是对AQS中内部类ConditionObject对Condition接口实现的简单分析。