深入理解Java-Condition - Condition实现原理 - 《Java 笔记》

一、基本使用
二、源码分析

Java Condition
Condition接口提供了与Object阻塞(wait())与唤醒(notify()或notifyAll())相似的功能，只不过Condition接口提供了更为丰富的功能，如：限定等待时长等。Condition需要与Lock结合使用，需要通过锁对象获取Condition。

一、基本使用

基于Condition实现生产者、消费者模式。代码基本与Object#wait()和Object#notify()类似，只不过这里使用Lock替换了synchronized关键字。

生产者

public class Producer implements Runnable {
    private Lock lock;
    private Condition condition;
    private Queue<String> queue;
    private int maxSize;
    public Producer(Lock lock, Condition condition, Queue<String> queue, int maxSize) {
        this.lock = lock;
        this.condition = condition;
        this.queue = queue;
        this.maxSize = maxSize;
    }
    @Override
    public void run() {
        int i = 0;
        for (; ; ) {
            lock.lock();
            // 如果满了，则阻塞
            while (queue.size() == maxSize) {
                System.out.println("生产者队列满了，等待...");
                try {
                    condition.await();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            try {
                TimeUnit.SECONDS.sleep(2);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            queue.add("一个消息:" + ++i);
            System.out.printf("生产者%s生产了一个消息:%s\n", Thread.currentThread().getName(), i);
            condition.signal();
            lock.unlock();
        }
    }
}

消费者

public class Consumer implements Runnable {
    private Lock lock;
    private Condition condition;
    private Queue<String> queue;
    private int maxSize;
    public Consumer(Lock lock, Condition condition, Queue<String> queue, int maxSize) {
        this.lock = lock;
        this.condition = condition;
        this.queue = queue;
        this.maxSize = maxSize;
    }
    @Override
    public void run() {
        for (; ; ) {
            lock.lock();
            while (queue.isEmpty()) {
                System.out.println("消费者队列为空，等待...");
                try {
                    condition.await();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            try {
                TimeUnit.SECONDS.sleep(1);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            String obj = queue.remove();
            System.out.printf("消费者%s消费一个消息：%s\n", Thread.currentThread().getName(), obj);
            condition.signal();
            lock.unlock();
        }
    }
}

测试类

public class ConditionProducerConsumer {
    public static void main(String[] args) {
        Lock lock = new ReentrantLock();
        Condition condition = lock.newCondition();
        Queue<String> queue = new LinkedBlockingQueue<>();
        int maxSize = 10;
        Producer producer = new Producer(lock, condition, queue, maxSize);
        Consumer consumer = new Consumer(lock, condition, queue, maxSize);
        new Thread(producer).start();
        new Thread(consumer).start();
    }
}

二、源码分析

上述示例中使用的Lock是ReentrantLock，关于它的lock方法与unlock方法的原理详见ReentrantLock实现原理。上述示例中的Condition对象是调用了Lock#newCondition()方法，源码如下：

public class ReentrantLock implements Lock, java.io.Serializable {
    ...
        public Condition newCondition() {
        return sync.newCondition();
    }
    abstract static class Sync extends AbstractQueuedSynchronizer {
        ...
            final ConditionObject newCondition() {
            return new ConditionObject();
        }
        ...
        }
    ...
}

上述的ConditionObject定义在AQS中，如下:

public abstract class AbstractQueuedSynchronizer
    extends AbstractOwnableSynchronizer
    implements java.io.Serializable {
    ...
     public class ConditionObject implements Condition, java.io.Serializable {
          ...
     }
 ...
}

首先来分析下Condition#await()方法

public final void await() throws InterruptedException {
    if (Thread.interrupted())
        throw new InterruptedException();
    Node node = addConditionWaiter();
    int savedState = fullyRelease(node);
    int interruptMode = 0;
    while (!isOnSyncQueue(node)) {
        LockSupport.park(this);
        if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
            break;
    }
    if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
        interruptMode = REINTERRUPT;
    if (node.nextWaiter != null) // clean up if cancelled
        unlinkCancelledWaiters();
    if (interruptMode != 0)
        reportInterruptAfterWait(interruptMode);
}
private Node addConditionWaiter() {
    Node t = lastWaiter;
    // If lastWaiter is cancelled, clean out.
    if (t != null && t.waitStatus != Node.CONDITION) {
        unlinkCancelledWaiters();
        t = lastWaiter;
    }
    Node node = new Node(Thread.currentThread(), Node.CONDITION);
    if (t == null)
        firstWaiter = node;
    else
        t.nextWaiter = node;
    lastWaiter = node;
    return node;
}

根据AQS队列的特性，若有多个线程执行lock#lock()方法，会将处于阻塞状态的线程维护到一个双向链表中，如下：

假设当前是线程A获取到锁，其他线程执行lock#lock()方法时，将会构建成一个上述链表。
若获取锁的线程（线程A）执行Condition#await()方法，则会将当前线程添加至Condition队列中，如下：

然后在调用fullyRelease()方法时会释放当前线程的锁，然后唤醒处于阻塞队列中的下一个线程：

在调用isOnSyncQueue()方法时会检查当前节点是否在同步队列中，若不存在，则会调用LockSupport.park()进行阻塞。
假设当前线程A是生产者线程，调用await()方法后，会释放锁，并且将当前线程加入到Condition队列中。此时，消费者能获取到锁资源，然后继续执行。假设线程B是消费者线程，当添加一个元素后会调用condition#signal()方法，定义如下：

public final void signal() {
    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);
}
final boolean transferForSignal(Node node) {
    /*
         * If cannot change waitStatus, the node has been cancelled.
         */
    if (!compareAndSetWaitStatus(node, Node.CONDITION, 0))
        return false;
    /*
         * Splice onto queue and try to set waitStatus of predecessor to
         * indicate that thread is (probably) waiting. If cancelled or
         * attempt to set waitStatus fails, wake up to resync (in which
         * case the waitStatus can be transiently and harmlessly wrong).
         */
    Node p = enq(node);
    int ws = p.waitStatus;
    if (ws > 0 || !compareAndSetWaitStatus(p, ws, Node.SIGNAL))
        LockSupport.unpark(node.thread);
    return true;
}

执行signal()方法，会将Condition队列中的第一个节点移除，将其变为同步队列中的尾结点，如下：
Condition实现原理 - 图4
至此，完成了Condition队列转换为同步队列的过程。后续流程基本就是重复以上操作。
详细介绍了单个Condition队列的执行流程，其实一个Lock中可以有多个Condition队列，比如：JUC中提供的LinkedBlockingDeque、ArrayBlockingQueue等。