Condition接口原理与应用

一. Condition接口简介

 任意一个Java对象，都拥有一组监视器方法（定义在java.lang.Object上），主要包括wait()、wait(long timeout)、notify()以及notifyAll()方法，这些方法与synchronized同步关键字配合，可以实现等待/通知模式。Condition接口也提供了类似Object的监视器方法，与Lock配合可以实现等待/通知模式，但是这两者在使用方式以及功能特性上还是有差别的。

 通过对比Object的监视器方法和Condition接口，可以更详细地了解Condition的特性，对比如下：

对比项	Object Monitor Methods	Condition
前置条件	获取对象的锁	1.调用Lock.lock()获取 2.调用Lock.newCondition()获取Condition对象
调用方式	直接调用，如:object.wait()	直接调用，如:condition.await()
等待队列个数	一个	多个
当前线程释放锁并进入等待状态	支持	支持
当前线程释放锁并进入等待状态，在等待状态中不响应终端	不支持	支持
当前线程释放锁并进入超时等待状态	支持	支持
当前线程释放锁并进入等待状态到将来的某个时间	不支持	支持
唤醒等待队列中的一个线程	支持	支持
唤醒等待队列中的全部线程	支持	支持

1.1 Condition接口与示例

 Condition定义了等待/通知两种类型的方法，当前线程调用这些方法时，需要提前获取到Condition对象关联的锁。Condition对象是由Lock对象（调用Lock对象的newCondition()方法）创建出来的，换句话说，Condition是依赖Lock对象的。

 Condition的使用方式比较简单，需要注意在调用方法前获取锁，如下代码所示：

    Lock lock = new ReentrantLock();
    Condition condition = lock.newCondition();
    
    public void conditionWait() throws InterruptedException {
        lock.lock();
        try {
            condition.await();
        } finally {
            lock.unlock();
        }
    }
    public void conditionSignal() throws InterruptedException {
        lock.lock();
        try {
            condition.signal();
        } finally {
            lock.unlock();
        }
    }

 如示例所示，一般都会将Condition对象作为成员变量。当调用await()方法后，当前线程会释放锁并在此等待，而其他线程调用Condition对象的signal()方法，通知当前线程后，当前线程才从await()方法返回，并且在返回前已经获取了锁。

 Condition定义的（部分）方法以及描述如下表：

方法名称	描述
void await() throws InterruptedException;	当线程进入等待状态直到被通知(signal)或中断，当前线程将进入运行状态且从await()方法返回的情况，包括：其他线程调用该Condition的signal()或signalAll()方法，而当前线程被选中唤醒； 1. 其他线程(调用interrupt()方法)中断当前线程； 2. 如果当前等待线程从await()方法返回，那么表明该线程已经获取了Condition对象所对应的锁
void awaitUninterruptibly();	当前线程进入等待状态直到被通知，从方法名称上可以看出该方法对中断不敏感
long awaitNanos(long nanosTimeout) throws InterruptedException;	当前线程进入等待状态直到被通知、中断或者超时。返回值表示剩余的时间，如果在nanosTimeout纳秒之前被唤醒，那么返回值就是(nanosTimeout - 实际耗时)。如果返回值是0或者负数，那么可以认定已经超时了
boolean awaitUntil(Date deadline) throws InterruptedException;	当前线程进入等待状态直到被通知、中断或者到某个时间。如果没有到指定时间就被通知，方法返回true，否则，表示到了指定时间，返回false
void signal();	唤醒一个等待在Condition上的线程，该线程从等待方法返回前必须获得与Condition相关联的锁
void signalAll();	唤醒所有等待在Condition上的线程，能够从等待方法返回的线程必须获得与Condition相关联的锁

 获取一个Condition必须通过Lock的newCondition()方法。下面通过一个有界队列的示例来深入了解Condition的使用方式。有界队列是一种特殊的队列，当队列为空时，队列的获取操作将会阻塞获取线程，直到队列中有新增元素，当队列已满时，队列的插入操作将会阻塞插入线程，直到队列出现“空位”，如代码清单5-21所示。

import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

public class BoundedQueue<T> {
    private Object[] items;
    // 添加的下标，删除的下标和数组当前数量
    private int addIndex, removeIndex, count;
    private Lock lock = new ReentrantLock();
    private Condition notEmpty = lock.newCondition();
    private Condition notFull = lock.newCondition();
    public BoundedQueue(int size) {
    items = new Object[size];
    }
    // 添加一个元素，如果数组满，则添加线程进入等待状态，直到有"空位"
    public void add(T t) throws InterruptedException {
        lock.lock();try {
        while (count == items.length)
            notFull.await();
            items[addIndex] = t;
            if (++addIndex == items.length)
            addIndex = 0;
            ++count;
            notEmpty.signal();
        } finally {
            lock.unlock();
        }
    }
    // 由头部删除一个元素，如果数组空，则删除线程进入等待状态，直到有新添加元素
    public T remove() throws InterruptedException {
        lock.lock();
        try {
            while (count == 0)
                notEmpty.await();
            Object x = items[removeIndex];
            if (++removeIndex == items.length)
                removeIndex = 0;
            --count;
            notFull.signal();
            return (T) x;
        } finally {
            lock.unlock();
        }
    }
}

 上述示例中，BoundedQueue通过add(T t)方法添加一个元素，通过remove()方法移出一个元素。以添加方法为例。

 首先需要获得锁，目的是确保数组修改的可见性和排他性。当数组数量等于数组长度时，表示数组已满，则调用notFull.await()，当前线程随之释放锁并进入等待状态。如果数组数量不等于数组长度，表示数组未满，则添加元素到数组中，同时通知等待在notEmpty上的线程，数组中已经有新元素可以获取。

 在添加和删除方法中使用while循环而非if判断，目的是防止过早或意外的通知，只有条件符合才能够退出循环。回想之前提到的等待/通知的经典范式，二者是非常类似的。

---

二. Condition原理分析

 ConditionObject是同步器AbstractQueuedSynchronizer的内部类，因为Condition的操作需要获取相关联的锁，所以作为同步器的内部类也较为合理。每个Condition对象都包含着一个队列（以下称为等待队列），该队列是Condition对象实现等待/通知功能的关键。

 下面将分析Condition的实现，主要包括：等待队列、等待和通知，下面提到的Condition如果不加说明均指的是ConditionObject。

2.1 等待队列

 等待队列是一个FIFO的队列，在队列中的每个节点都包含了一个线程引用，该线程就是在Condition对象上等待的线程，如果一个线程调用了Condition.await()方法，那么该线程将会释放锁、构造成节点加入等待队列并进入等待状态。事实上，节点的定义复用了同步器中节点的定义，也就是说，同步队列和等待队列中节点类型都是同步器的静态内部类AbstractQueuedSynchronizer.Node。

 一个Condition包含一个等待队列，Condition拥有首节点（firstWaiter）和尾节点（lastWaiter）。当前线程调用Condition.await()方法，将会以当前线程构造节点，并将节点从尾部加入等待队列，等待队列的基本结构如图所示。

等待队列基本结构

 如图所示，Condition拥有首尾节点的引用，而新增节点只需要将原有的尾节点nextWaiter指向它，并且更新尾节点即可。上述节点引用更新的过程并没有使用CAS保证，原因在于调用await()方法的线程必定是获取了锁的线程，也就是说该过程是由锁来保证线程安全的。

 在Object的监视器模型上，一个对象拥有一个同步队列和等待队列，而并发包中的Lock（更确切地说是同步器）拥有一个同步队列和多个等待队列，其对应关系如图所示。

同步队列与等待队列

 如图所示，Condition的实现是同步器的内部类，因此每个Condition实例都能够访问同步器提供的方法，相当于每个Condition都拥有所属同步器的引用。

2.2 等待

 调用Condition的await()方法（或者以await开头的方法），会使当前线程进入等待队列并释放锁，同时线程状态变为等待状态。当从await()方法返回时，当前线程一定获取了Condition相关联的锁。

 如果从队列（同步队列和等待队列）的角度看await()方法，当调用await()方法时，相当于同步队列的首节点（获取了锁的节点）移动到Condition的等待队列中。

 Condition的await()方法，如下代码所示。

    public final void await() throws InterruptedException {
        if (Thread.interrupted())
            throw new InterruptedException();
        // 当前线程加入等待队列
        Node node = addConditionWaiter();
        // 释放同步状态，也就是释放锁
        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)
            unlinkCancelledWaiters();
        if (interruptMode != 0)
            reportInterruptAfterWait(interruptMode);
    }

 调用该方法的线程成功获取了锁的线程，也就是同步队列中的首节点，该方法会将当前线程构造成节点并加入等待队列中，然后释放同步状态，唤醒同步队列中的后继节点，然后当前线程会进入等待状态。

 当等待队列中的节点被唤醒，则唤醒节点的线程开始尝试获取同步状态。如果不是通过其他线程调用Condition.signal()方法唤醒，而是对等待线程进行中断，则会抛出InterruptedException。

 如果从队列的角度去看，当前线程加入Condition的等待队列，该过程如图示。

当前线程加入等待队列

 如图所示，同步队列的首节点并不会直接加入等待队列，而是通过addConditionWaiter()方法把当前线程构造成一个新的节点并将其加入等待队列中。

2.3 通知

 调用Condition的signal()方法，将会唤醒在等待队列中等待时间最长的节点（首节点），在唤醒节点之前，会将节点移到同步队列中。
Condition的signal()方法，如代码如下所示：

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

 调用该方法的前置条件是当前线程必须获取了锁，可以看到signal()方法进行了isHeldExclusively()检查，也就是当前线程必须是获取了锁的线程。接着获取等待队列的首节点，将其移动到同步队列并使用LockSupport唤醒节点中的线程。

 节点从等待队列移动到同步队列的过程如图所示。

节点从等待队列移动到同步队列

 通过调用同步器的enq(Node node)方法，等待队列中的头节点线程安全地移动到同步队列。当节点移动到同步队列后，当前线程再使用LockSupport唤醒该节点的线程。

 被唤醒后的线程，将从await()方法中的while循环中退出（isOnSyncQueue(Node node)方法返回true，节点已经在同步队列中），进而调用同步器的acquireQueued()方法加入到获取同步状态的竞争中。

 成功获取同步状态（或者说锁）之后，被唤醒的线程将从先前调用的await()方法返回，此时该线程已经成功地获取了锁。

 Condition的signalAll()方法，相当于对等待队列中的每个节点均执行一次signal()方法，效果就是将等待队列中所有节点全部移动到同步队列中，并唤醒每个节点的线程。

---

三. Condition在生产者/消费者模式中的应用

生产者-消费者模式是一个十分经典的多线程并发协作的模式，弄懂生产者-消费者问题能够让我们对并发编程的理解加深。所谓生产者-消费者问题，实际上主要是包含了两类线程，一种是生产者线程用于生产数据，另一种是消费者线程用于消费数据，为了解耦生产者和消费者的关系，通常会采用共享的数据区域，就像是一个仓库，生产者生产数据之后直接放置在共享数据区中，并不需要关心消费者的行为；而消费者只需要从共享数据区中去获取数据，就不再需要关心生产者的行为。但是，这个共享数据区域中应该具备这样的线程间并发协作的功能：

1. 如果共享数据区已满的话，阻塞生产者继续生产数据放置入内；
2. 如果共享数据区为空的话，阻塞消费者继续消费数据；

3.1 使用Lock的Condition的await/signal的消息通知机制实现生产者和消费者的功能

实现源码如下：

package com.sunny.jdk.concurrent.condition.app;

import java.util.LinkedList;
import java.util.List;
import java.util.Random;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

/**
 * <Description> 使用Condition的wait和signal实现消费者和生产者相互通知的功能<br>
 *
 * @author Sunny<br>
 * @version 1.0<br>
 * @taskId: <br>
 * @createDate 2018/08/21 13:17 <br>
 * @see com.sunny.jdk.concurrent.condition.app <br>
 */
public class ProducerConsumerApp {
    private static ReentrantLock lock = new ReentrantLock();
    private static Condition full = lock.newCondition();
    private static Condition empty = lock.newCondition();

    public static void main(String[] args) {
        LinkedList linkedList = new LinkedList();
        ExecutorService service = Executors.newFixedThreadPool(15);
        for (int i = 0; i < 5; i++) {
            service.submit(new Producer(linkedList, 8, lock));
        }
        for (int i = 0; i < 10; i++) {
            service.submit(new Consumer(linkedList, lock));
        }
    }

    static class Producer implements Runnable {

        private List<Integer> list;
        private int maxLength;
        private Lock lock;

        public Producer(List list, int maxLength, Lock lock) {
            this.list = list;
            this.maxLength = maxLength;
            this.lock = lock;
        }

        @Override
        public void run() {
            while (true) {
                lock.lock();
                try {
                    while (list.size() == maxLength) {
                        System.out.println("生产者" + Thread.currentThread().getName()
                                + "  list以达到最大容量【" + maxLength + "】，进行wait");
                        full.await();
                        System.out.println("生产者" + Thread.currentThread().getName() + "  退出wait");
                    }
                    Random random = new Random();
                    int i = random.nextInt();
                    System.out.println("生产者" + Thread.currentThread().getName() + " 生产数据" + i);
                    list.add(i);
                    empty.signalAll();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                } finally {
                    lock.unlock();
                }
            }
        }
    }


    static class Consumer implements Runnable {

        private List<Integer> list;
        private Lock lock;

        public Consumer(List list, Lock lock) {
            this.list = list;
            this.lock = lock;
        }

        @Override
        public void run() {
            while (true) {
                lock.lock();
                try {
                    while (list.isEmpty()) {
                        System.out.println("消费者" + Thread.currentThread().getName() + "  list为空，进行wait");
                        empty.await();
                        System.out.println("消费者" + Thread.currentThread().getName() + "  退出wait");
                    }
                    Integer element = list.remove(0);
                    System.out.println("消费者" + Thread.currentThread().getName() + "  消费数据：" + element);
                    full.signalAll();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                } finally {
                    lock.unlock();
                }
            }
        }
    }
}

---

四. Condition接口源码

/*
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */

package java.util.concurrent.locks;
import java.util.concurrent.TimeUnit;
import java.util.Date;

/**
 * {@code Condition} factors out the {@code Object} monitor
 * methods ({@link Object#wait() wait}, {@link Object#notify notify}
 * and {@link Object#notifyAll notifyAll}) into distinct objects to
 * give the effect of having multiple wait-sets per object, by
 * combining them with the use of arbitrary {@link Lock} implementations.
 * Where a {@code Lock} replaces the use of {@code synchronized} methods
 * and statements, a {@code Condition} replaces the use of the Object
 * monitor methods.
 *
 * <p>Conditions (also known as <em>condition queues</em> or
 * <em>condition variables</em>) provide a means for one thread to
 * suspend execution (to &quot;wait&quot;) until notified by another
 * thread that some state condition may now be true.  Because access
 * to this shared state information occurs in different threads, it
 * must be protected, so a lock of some form is associated with the
 * condition. The key property that waiting for a condition provides
 * is that it <em>atomically</em> releases the associated lock and
 * suspends the current thread, just like {@code Object.wait}.
 *
 * <p>A {@code Condition} instance is intrinsically bound to a lock.
 * To obtain a {@code Condition} instance for a particular {@link Lock}
 * instance use its {@link Lock#newCondition newCondition()} method.
 *
 * <p>As an example, suppose we have a bounded buffer which supports
 * {@code put} and {@code take} methods.  If a
 * {@code take} is attempted on an empty buffer, then the thread will block
 * until an item becomes available; if a {@code put} is attempted on a
 * full buffer, then the thread will block until a space becomes available.
 * We would like to keep waiting {@code put} threads and {@code take}
 * threads in separate wait-sets so that we can use the optimization of
 * only notifying a single thread at a time when items or spaces become
 * available in the buffer. This can be achieved using two
 * {@link Condition} instances.
 * <pre>
 * class BoundedBuffer {
 *   <b>final Lock lock = new ReentrantLock();</b>
 *   final Condition notFull  = <b>lock.newCondition(); </b>
 *   final Condition notEmpty = <b>lock.newCondition(); </b>
 *
 *   final Object[] items = new Object[100];
 *   int putptr, takeptr, count;
 *
 *   public void put(Object x) throws InterruptedException {
 *     <b>lock.lock();
 *     try {</b>
 *       while (count == items.length)
 *         <b>notFull.await();</b>
 *       items[putptr] = x;
 *       if (++putptr == items.length) putptr = 0;
 *       ++count;
 *       <b>notEmpty.signal();</b>
 *     <b>} finally {
 *       lock.unlock();
 *     }</b>
 *   }
 *
 *   public Object take() throws InterruptedException {
 *     <b>lock.lock();
 *     try {</b>
 *       while (count == 0)
 *         <b>notEmpty.await();</b>
 *       Object x = items[takeptr];
 *       if (++takeptr == items.length) takeptr = 0;
 *       --count;
 *       <b>notFull.signal();</b>
 *       return x;
 *     <b>} finally {
 *       lock.unlock();
 *     }</b>
 *   }
 * }
 * </pre>
 *
 * (The {@link java.util.concurrent.ArrayBlockingQueue} class provides
 * this functionality, so there is no reason to implement this
 * sample usage class.)
 *
 * <p>A {@code Condition} implementation can provide behavior and semantics
 * that is
 * different from that of the {@code Object} monitor methods, such as
 * guaranteed ordering for notifications, or not requiring a lock to be held
 * when performing notifications.
 * If an implementation provides such specialized semantics then the
 * implementation must document those semantics.
 *
 * <p>Note that {@code Condition} instances are just normal objects and can
 * themselves be used as the target in a {@code synchronized} statement,
 * and can have their own monitor {@link Object#wait wait} and
 * {@link Object#notify notification} methods invoked.
 * Acquiring the monitor lock of a {@code Condition} instance, or using its
 * monitor methods, has no specified relationship with acquiring the
 * {@link Lock} associated with that {@code Condition} or the use of its
 * {@linkplain #await waiting} and {@linkplain #signal signalling} methods.
 * It is recommended that to avoid confusion you never use {@code Condition}
 * instances in this way, except perhaps within their own implementation.
 *
 * <p>Except where noted, passing a {@code null} value for any parameter
 * will result in a {@link NullPointerException} being thrown.
 *
 * <h3>Implementation Considerations</h3>
 *
 * <p>When waiting upon a {@code Condition}, a &quot;<em>spurious
 * wakeup</em>&quot; is permitted to occur, in
 * general, as a concession to the underlying platform semantics.
 * This has little practical impact on most application programs as a
 * {@code Condition} should always be waited upon in a loop, testing
 * the state predicate that is being waited for.  An implementation is
 * free to remove the possibility of spurious wakeups but it is
 * recommended that applications programmers always assume that they can
 * occur and so always wait in a loop.
 *
 * <p>The three forms of condition waiting
 * (interruptible, non-interruptible, and timed) may differ in their ease of
 * implementation on some platforms and in their performance characteristics.
 * In particular, it may be difficult to provide these features and maintain
 * specific semantics such as ordering guarantees.
 * Further, the ability to interrupt the actual suspension of the thread may
 * not always be feasible to implement on all platforms.
 *
 * <p>Consequently, an implementation is not required to define exactly the
 * same guarantees or semantics for all three forms of waiting, nor is it
 * required to support interruption of the actual suspension of the thread.
 *
 * <p>An implementation is required to
 * clearly document the semantics and guarantees provided by each of the
 * waiting methods, and when an implementation does support interruption of
 * thread suspension then it must obey the interruption semantics as defined
 * in this interface.
 *
 * <p>As interruption generally implies cancellation, and checks for
 * interruption are often infrequent, an implementation can favor responding
 * to an interrupt over normal method return. This is true even if it can be
 * shown that the interrupt occurred after another action that may have
 * unblocked the thread. An implementation should document this behavior.
 *
 * @since 1.5
 * @author Doug Lea
 */
public interface Condition {

    /**
     * Causes the current thread to wait until it is signalled or
     * {@linkplain Thread#interrupt interrupted}.
     *
     * <p>The lock associated with this {@code Condition} is atomically
     * released and the current thread becomes disabled for thread scheduling
     * purposes and lies dormant until <em>one</em> of four things happens:
     * <ul>
     * <li>Some other thread invokes the {@link #signal} method for this
     * {@code Condition} and the current thread happens to be chosen as the
     * thread to be awakened; or
     * <li>Some other thread invokes the {@link #signalAll} method for this
     * {@code Condition}; or
     * <li>Some other thread {@linkplain Thread#interrupt interrupts} the
     * current thread, and interruption of thread suspension is supported; or
     * <li>A &quot;<em>spurious wakeup</em>&quot; occurs.
     * </ul>
     *
     * <p>In all cases, before this method can return the current thread must
     * re-acquire the lock associated with this condition. When the
     * thread returns it is <em>guaranteed</em> to hold this lock.
     *
     * <p>If the current thread:
     * <ul>
     * <li>has its interrupted status set on entry to this method; or
     * <li>is {@linkplain Thread#interrupt interrupted} while waiting
     * and interruption of thread suspension is supported,
     * </ul>
     * then {@link InterruptedException} is thrown and the current thread's
     * interrupted status is cleared. It is not specified, in the first
     * case, whether or not the test for interruption occurs before the lock
     * is released.
     *
     * <p><b>Implementation Considerations</b>
     *
     * <p>The current thread is assumed to hold the lock associated with this
     * {@code Condition} when this method is called.
     * It is up to the implementation to determine if this is
     * the case and if not, how to respond. Typically, an exception will be
     * thrown (such as {@link IllegalMonitorStateException}) and the
     * implementation must document that fact.
     *
     * <p>An implementation can favor responding to an interrupt over normal
     * method return in response to a signal. In that case the implementation
     * must ensure that the signal is redirected to another waiting thread, if
     * there is one.
     *
     * @throws InterruptedException if the current thread is interrupted
     *         (and interruption of thread suspension is supported)
     */
    void await() throws InterruptedException;

    /**
     * Causes the current thread to wait until it is signalled.
     *
     * <p>The lock associated with this condition is atomically
     * released and the current thread becomes disabled for thread scheduling
     * purposes and lies dormant until <em>one</em> of three things happens:
     * <ul>
     * <li>Some other thread invokes the {@link #signal} method for this
     * {@code Condition} and the current thread happens to be chosen as the
     * thread to be awakened; or
     * <li>Some other thread invokes the {@link #signalAll} method for this
     * {@code Condition}; or
     * <li>A &quot;<em>spurious wakeup</em>&quot; occurs.
     * </ul>
     *
     * <p>In all cases, before this method can return the current thread must
     * re-acquire the lock associated with this condition. When the
     * thread returns it is <em>guaranteed</em> to hold this lock.
     *
     * <p>If the current thread's interrupted status is set when it enters
     * this method, or it is {@linkplain Thread#interrupt interrupted}
     * while waiting, it will continue to wait until signalled. When it finally
     * returns from this method its interrupted status will still
     * be set.
     *
     * <p><b>Implementation Considerations</b>
     *
     * <p>The current thread is assumed to hold the lock associated with this
     * {@code Condition} when this method is called.
     * It is up to the implementation to determine if this is
     * the case and if not, how to respond. Typically, an exception will be
     * thrown (such as {@link IllegalMonitorStateException}) and the
     * implementation must document that fact.
     */
    void awaitUninterruptibly();

    /**
     * Causes the current thread to wait until it is signalled or interrupted,
     * or the specified waiting time elapses.
     *
     * <p>The lock associated with this condition is atomically
     * released and the current thread becomes disabled for thread scheduling
     * purposes and lies dormant until <em>one</em> of five things happens:
     * <ul>
     * <li>Some other thread invokes the {@link #signal} method for this
     * {@code Condition} and the current thread happens to be chosen as the
     * thread to be awakened; or
     * <li>Some other thread invokes the {@link #signalAll} method for this
     * {@code Condition}; or
     * <li>Some other thread {@linkplain Thread#interrupt interrupts} the
     * current thread, and interruption of thread suspension is supported; or
     * <li>The specified waiting time elapses; or
     * <li>A &quot;<em>spurious wakeup</em>&quot; occurs.
     * </ul>
     *
     * <p>In all cases, before this method can return the current thread must
     * re-acquire the lock associated with this condition. When the
     * thread returns it is <em>guaranteed</em> to hold this lock.
     *
     * <p>If the current thread:
     * <ul>
     * <li>has its interrupted status set on entry to this method; or
     * <li>is {@linkplain Thread#interrupt interrupted} while waiting
     * and interruption of thread suspension is supported,
     * </ul>
     * then {@link InterruptedException} is thrown and the current thread's
     * interrupted status is cleared. It is not specified, in the first
     * case, whether or not the test for interruption occurs before the lock
     * is released.
     *
     * <p>The method returns an estimate of the number of nanoseconds
     * remaining to wait given the supplied {@code nanosTimeout}
     * value upon return, or a value less than or equal to zero if it
     * timed out. This value can be used to determine whether and how
     * long to re-wait in cases where the wait returns but an awaited
     * condition still does not hold. Typical uses of this method take
     * the following form:
     *
     *  <pre> {@code
     * boolean aMethod(long timeout, TimeUnit unit) {
     *   long nanos = unit.toNanos(timeout);
     *   lock.lock();
     *   try {
     *     while (!conditionBeingWaitedFor()) {
     *       if (nanos <= 0L)
     *         return false;
     *       nanos = theCondition.awaitNanos(nanos);
     *     }
     *     // ...
     *   } finally {
     *     lock.unlock();
     *   }
     * }}</pre>
     *
     * <p>Design note: This method requires a nanosecond argument so
     * as to avoid truncation errors in reporting remaining times.
     * Such precision loss would make it difficult for programmers to
     * ensure that total waiting times are not systematically shorter
     * than specified when re-waits occur.
     *
     * <p><b>Implementation Considerations</b>
     *
     * <p>The current thread is assumed to hold the lock associated with this
     * {@code Condition} when this method is called.
     * It is up to the implementation to determine if this is
     * the case and if not, how to respond. Typically, an exception will be
     * thrown (such as {@link IllegalMonitorStateException}) and the
     * implementation must document that fact.
     *
     * <p>An implementation can favor responding to an interrupt over normal
     * method return in response to a signal, or over indicating the elapse
     * of the specified waiting time. In either case the implementation
     * must ensure that the signal is redirected to another waiting thread, if
     * there is one.
     *
     * @param nanosTimeout the maximum time to wait, in nanoseconds
     * @return an estimate of the {@code nanosTimeout} value minus
     *         the time spent waiting upon return from this method.
     *         A positive value may be used as the argument to a
     *         subsequent call to this method to finish waiting out
     *         the desired time.  A value less than or equal to zero
     *         indicates that no time remains.
     * @throws InterruptedException if the current thread is interrupted
     *         (and interruption of thread suspension is supported)
     */
    long awaitNanos(long nanosTimeout) throws InterruptedException;

    /**
     * Causes the current thread to wait until it is signalled or interrupted,
     * or the specified waiting time elapses. This method is behaviorally
     * equivalent to:
     *  <pre> {@code awaitNanos(unit.toNanos(time)) > 0}</pre>
     *
     * @param time the maximum time to wait
     * @param unit the time unit of the {@code time} argument
     * @return {@code false} if the waiting time detectably elapsed
     *         before return from the method, else {@code true}
     * @throws InterruptedException if the current thread is interrupted
     *         (and interruption of thread suspension is supported)
     */
    boolean await(long time, TimeUnit unit) throws InterruptedException;

    /**
     * Causes the current thread to wait until it is signalled or interrupted,
     * or the specified deadline elapses.
     *
     * <p>The lock associated with this condition is atomically
     * released and the current thread becomes disabled for thread scheduling
     * purposes and lies dormant until <em>one</em> of five things happens:
     * <ul>
     * <li>Some other thread invokes the {@link #signal} method for this
     * {@code Condition} and the current thread happens to be chosen as the
     * thread to be awakened; or
     * <li>Some other thread invokes the {@link #signalAll} method for this
     * {@code Condition}; or
     * <li>Some other thread {@linkplain Thread#interrupt interrupts} the
     * current thread, and interruption of thread suspension is supported; or
     * <li>The specified deadline elapses; or
     * <li>A &quot;<em>spurious wakeup</em>&quot; occurs.
     * </ul>
     *
     * <p>In all cases, before this method can return the current thread must
     * re-acquire the lock associated with this condition. When the
     * thread returns it is <em>guaranteed</em> to hold this lock.
     *
     *
     * <p>If the current thread:
     * <ul>
     * <li>has its interrupted status set on entry to this method; or
     * <li>is {@linkplain Thread#interrupt interrupted} while waiting
     * and interruption of thread suspension is supported,
     * </ul>
     * then {@link InterruptedException} is thrown and the current thread's
     * interrupted status is cleared. It is not specified, in the first
     * case, whether or not the test for interruption occurs before the lock
     * is released.
     *
     *
     * <p>The return value indicates whether the deadline has elapsed,
     * which can be used as follows:
     *  <pre> {@code
     * boolean aMethod(Date deadline) {
     *   boolean stillWaiting = true;
     *   lock.lock();
     *   try {
     *     while (!conditionBeingWaitedFor()) {
     *       if (!stillWaiting)
     *         return false;
     *       stillWaiting = theCondition.awaitUntil(deadline);
     *     }
     *     // ...
     *   } finally {
     *     lock.unlock();
     *   }
     * }}</pre>
     *
     * <p><b>Implementation Considerations</b>
     *
     * <p>The current thread is assumed to hold the lock associated with this
     * {@code Condition} when this method is called.
     * It is up to the implementation to determine if this is
     * the case and if not, how to respond. Typically, an exception will be
     * thrown (such as {@link IllegalMonitorStateException}) and the
     * implementation must document that fact.
     *
     * <p>An implementation can favor responding to an interrupt over normal
     * method return in response to a signal, or over indicating the passing
     * of the specified deadline. In either case the implementation
     * must ensure that the signal is redirected to another waiting thread, if
     * there is one.
     *
     * @param deadline the absolute time to wait until
     * @return {@code false} if the deadline has elapsed upon return, else
     *         {@code true}
     * @throws InterruptedException if the current thread is interrupted
     *         (and interruption of thread suspension is supported)
     */
    boolean awaitUntil(Date deadline) throws InterruptedException;

    /**
     * Wakes up one waiting thread.
     *
     * <p>If any threads are waiting on this condition then one
     * is selected for waking up. That thread must then re-acquire the
     * lock before returning from {@code await}.
     *
     * <p><b>Implementation Considerations</b>
     *
     * <p>An implementation may (and typically does) require that the
     * current thread hold the lock associated with this {@code
     * Condition} when this method is called. Implementations must
     * document this precondition and any actions taken if the lock is
     * not held. Typically, an exception such as {@link
     * IllegalMonitorStateException} will be thrown.
     */
    void signal();

    /**
     * Wakes up all waiting threads.
     *
     * <p>If any threads are waiting on this condition then they are
     * all woken up. Each thread must re-acquire the lock before it can
     * return from {@code await}.
     *
     * <p><b>Implementation Considerations</b>
     *
     * <p>An implementation may (and typically does) require that the
     * current thread hold the lock associated with this {@code
     * Condition} when this method is called. Implementations must
     * document this precondition and any actions taken if the lock is
     * not held. Typically, an exception such as {@link
     * IllegalMonitorStateException} will be thrown.
     */
    void signalAll();
}

---

参考《Java并发编程的艺术》