预备知识

概览

本文探究一下Java最基础的机制之一：线程同步
我们先讨论一些并发相关的术语和方法论，接着会提供一个简单例子来处理并发问题，可以帮助我们更好的理解wait()和notify()方法。

线程同步

多线程环境下，每个线程都可能去修改相同资源，如果线程没有被较好的管理，那就可能会出现并发问题。

多线程之间经常需要协同工作，最常见的方式是使用保护块（Guarded Blocks），它循环检查一个条件（通常初始值为true），直到条件发生变化才跳出循环继续执行。

public void guardedJoy() {
    // Simple loop guard. Wastes processor time. Don't do this!
    while(!joy) {}
    System.out.println("Joy has been achieved!");
}

但是使用Guarded Blocks的方法不停的检查循环条件实际上是一种资源浪费，更加高效的方法是调用Object.wait将当前线程挂起，直到有另一线程发起事件通知（尽管通知的事件不一定是当前线程等待的事件）。

public synchronized void guardedJoy() {
    // This guard only loops once for each special event, 
    // which may not be the event we're waiting for.
    while(!joy) {
        try {
            wait();
        } catch (InterruptedException e) {}
    }
    System.out.println("Joy and efficiency have been achieved!");
}

我们今天要讨论的就是wait()和notify()方法：

Object.wait() – 挂起线程，
Object.notify() – 唤醒线程

补充：
下面这张图是wait()和notify()在线程的生命周期作用域的图解：

线程的生命周期

可以看到有很多种方式可以控制生命周期，本文我们只关注wait()和notify()方法。

wait()方法

/**
 * Causes the current thread to wait until another thread invokes the
 * {@link java.lang.Object#notify()} method or the
 * {@link java.lang.Object#notifyAll()} method for this object.
 * In other words, this method behaves exactly as if it simply
 * performs the call {@code wait(0)}.
 * <p>
 * The current thread must own this object's monitor. The thread
 * releases ownership of this monitor and waits until another thread
 * notifies threads waiting on this object's monitor to wake up
 * either through a call to the {@code notify} method or the
 * {@code notifyAll} method. The thread then waits until it can
 * re-obtain ownership of the monitor and resumes execution.
 * <p>
 * As in the one argument version, interrupts and spurious wakeups are
 * possible, and this method should always be used in a loop:
 * <pre>
 *     synchronized (obj) {
 *         while (&lt;condition does not hold&gt;)
 *             obj.wait();
 *         ... // Perform action appropriate to condition
 *     }
 * </pre>
 * This method should only be called by a thread that is the owner
 * of this object's monitor. See the {@code notify} method for a
 * description of the ways in which a thread can become the owner of
 * a monitor.
 */

当一个线程调用wait方法时，它释放锁并挂起。然后另一个线程请求并获得这个锁并调用Object.notifyAll()通知所有等待该锁的线程（之后当前线程释放该锁），此时第一个线程收到通知获取到该锁，从wait()方法返回并继续执行。

wait()方法有三个重载方法：

wait()

wait方法会使当前线程无限期等待，直到另一个线程调用了当前对象的notify()或notifyAll()方法。

wait(long timeout)

调用该方法可指定一段时间的限期等待，之后会由系统自动唤醒该线程。
在未到达timeout时间前也可通过当前对象的notify()或notifyAll()方法唤醒。

wait(long timeout, int nanos)

这是另一个限期等待的重载方法，不同的是提供了更高精度的timeout。

notify() & notifyAll()

notify()方法被用来唤醒在等待对象的内置锁的线程，有两种唤醒方式：

notify()

/**
 * Wakes up a single thread that is waiting on this object's
 * monitor. If any threads are waiting on this object, one of them
 * is chosen to be awakened. The choice is arbitrary and occurs at
 * the discretion of the implementation. A thread waits on an object's
 * monitor by calling one of the {@code wait} methods.
 * <p>
 * The awakened thread will not be able to proceed until the current
 * thread relinquishes the lock on this object. The awakened thread will
 * compete in the usual manner with any other threads that might be
 * actively competing to synchronize on this object; for example, the
 * awakened thread enjoys no reliable privilege or disadvantage in being
 * the next thread to lock this object.
 * <p>
 * This method should only be called by a thread that is the owner
 * of this object's monitor. A thread becomes the owner of the
 * object's monitor in one of three ways:
 * <ul>
 * <li>By executing a synchronized instance method of that object.
 * <li>By executing the body of a {@code synchronized} statement
 *     that synchronizes on the object.
 * <li>For objects of type {@code Class,} by executing a
 *     synchronized static method of that class.
 * </ul>
 * <p>
 * Only one thread at a time can own an object's monitor.
 */

它只会唤醒一个线程。但由于它并不指定哪一个线程被唤醒，所以一般大量相似任务的多线程环境中使用。因为对于这类任务，我们其实并不关心哪一个线程被唤醒。

对于该方法，当前线程必须拥有当前对象的内置锁或监视器锁（intrinsic lock aks monitor lock），根据Java文档，可通过以下三种方式的任意一种：

在给定对象上执行了同步方法
在给定对象上执行了同步块逻辑
执行给定对象上的同步静态方法

注意某一时间只有一个活跃线程能获取到对象的内置锁

notifyAll()

/**
* Wakes up all threads that are waiting on this object's monitor. A
* thread waits on an object's monitor by calling one of the
* {@code wait} methods.
* <p>
* The awakened threads will not be able to proceed until the current
* thread relinquishes the lock on this object. The awakened threads
* will compete in the usual manner with any other threads that might
* be actively competing to synchronize on this object; for example,
* the awakened threads enjoy no reliable privilege or disadvantage in
* being the next thread to lock this object.
* <p>
* This method should only be called by a thread that is the owner
* of this object's monitor. See the {@code notify} method for a
* description of the ways in which a thread can become the owner of
* a monitor.
*/

该方法会唤醒所有在该对象上等待内置锁的线程。
被唤醒的线程正常执行下去直到完成任务。
但在允许唤醒的线程开始继续执行逻辑之前，我们通常会定义一个快速检查，以确定继续执行线程所需的条件，因为可能会出现这种被唤醒的线程没收到通知的情况（一个对象多个方法中都调用了wait()，但是notifyAll()可能只针对某个方法有意义）

生产者-消费者同步问题

在我们理解了上述叙述后，我们来看一个简单的生产者-消费者例子：

生产者应该发送一条数据给消费者
如果生产者还未生产完毕，消费者此时不能处理数据
相同的，如果消费者未处理完数据，生产者不能发送下一条数据

我们首先创建一个Drop类，它包含了生产者需要传送给消费者的数据，我们会使用wait()和notifyAll()方法来让两个线程之间共享数据：

public class Drop {
    // Message sent from producer to consumer.
    private String message;
    // True if consumer should wait for producer to send message,
    // false if producer should wait for consumer to retrieve message.
    private boolean empty = true;

    public synchronized String take() {
        // Wait until message is
        // available.
        while (empty) {
            try {
                wait();
            } catch (InterruptedException e) {}
        }
        // Toggle status.
        empty = true;
        // Notify producer that
        // status has changed.
        notifyAll();
        return message;
    }

    public synchronized void put(String message) {
        // Wait until message has been retrieved.
        while (!empty) {
            try { 
                wait();
            } catch (InterruptedException e) {}
        }
        // Toggle status.
        empty = false;
        // Store message.
        this.message = message;
        // Notify consumer that status
        // has changed.
        notifyAll();
    }
}

我们来分解一下：·

message变量表示需要被传送的数据
布尔类型的empty变量是生产者和消费者用来做同步使用的：
- 如果为true，消费者需要等待生产者生产完毕
- 如果为false，生产者需要等待消费者消费完毕
生产者使用put()方法发送消息给消费者
- 如果empty为false，调用wait()等待
- 如果empty为true，设置empty为false，设置message为传入的消息，并调用notifyAll()方法来唤醒其他线程表明有一个事件发生了，大家可以检查一下当前状态看看是否需要继续执行。
相似的，消费者使用take()方法接收消息
- 如果empty被生产者设置为false，那它就继续执行，否则调用wait()方法等待
- 当条件满足后（empty为false），设置empty为true，唤醒其他等待线程并返回接收消息

为什么要把wait()方法放入while语句中？

因为线程唤醒后当前方法的循环条件不一定发生了改变。

为什么要同步put()和take()方法？

假设o是用来调用wait的对象，当一个线程调用o.wait()，它必须要拥有o的内部锁（否则会抛出异常），获得d的内部锁的最简单方法是在一个synchronized方法里面调用wait()。

我们现在创建Producer和Consumer。

先看看Producer：

import java.util.Random;

public class Producer implements Runnable {
    private Drop drop;

    public Producer(Drop drop) {
        this.drop = drop;
    }

    public void run() {
        String importantInfo[] = {
            "Mares eat oats",
            "Does eat oats",
            "Little lambs eat ivy",
            "A kid will eat ivy too"
        };
        Random random = new Random();

        for (int i = 0;
             i < importantInfo.length;
             i++) {
            drop.put(importantInfo[i]);
            try {
                Thread.sleep(random.nextInt(5000));
            } catch (InterruptedException e) {}
        }
        drop.put("DONE");
    }
}

对Producer来说：

我们定义了一个消息数据数组，在循环内一个一个的生产出去
对于每个消息数据，我们只调用put()方法
最后我们休眠一个随机数来模拟耗时的操作

下面是Consumer的实现：

import java.util.Random;

public class Consumer implements Runnable {
    private Drop drop;

    public Consumer(Drop drop) {
        this.drop = drop;
    }

    public void run() {
        Random random = new Random();
        for (String message = drop.take();
             ! message.equals("DONE");
             message = drop.take()) {
            System.out.format("MESSAGE RECEIVED: %s%n", message);
            try {
                Thread.sleep(random.nextInt(5000));
            } catch (InterruptedException e) {}
        }
    }
}

实现很简单，就是在for循环中调用drop.take()方法直到收到最后一个数据。

我们来运行一下程序：

public class ProducerConsumerExample {
    public static void main(String[] args) {
        Drop drop = new Drop();
        (new Thread(new Producer(drop))).start();
        (new Thread(new Consumer(drop))).start();
    }
}

程序输出如下：

MESSAGE RECEIVED: Mares eat oats
MESSAGE RECEIVED: Does eat oats
MESSAGE RECEIVED: Little lambs eat ivy
MESSAGE RECEIVED: A kid will eat ivy too

可以看到，我们以正确的顺序收到了所有的消息数据并成功的在Producer和Consumer之间完成了数据共享。

总结

本文讨论了Java的一些核心概念，更具体地说，我们聚焦在怎么使用wait()和notify()来解决同步问题，最后我们以一个简单例子说明了这些概念的使用。

值得一提的是这些都是低层次的API（wait、notify、notifyAll）。
有一些更高层次的API通常更简单且更好用，比如JDK中的Lock、Condition。关于这些可以看下我整理的一些文章

测试代码

参考

Guarded Blocks
Oracle官方并发教程之Guarded Blocks
Oracle Java Tutorials "Intrinsic Locks and Synchronization"
Oracle Java Tutorials "Questions and Exercises: Concurrency"

从Guarded Block来看Java中的wait和notify方法