本章介绍 线程间的协作方式，主要包含wait()，notify()，notifyAll()方法的使用以及代码示例
一、等待/通知机制介绍
在多线程环境下，为了保证线程安全，我们使用同步锁来保证任何时刻只有一个线程可以访问共享资源。而在实际场景中，除了线程互斥，我们也希望线程间可以通过协作来解决问题，而java并没有手段使得线程间直接的交互，通常的做法是通过一个共享变量来实现线程间的通讯，例如：A线程需要读取B线程发送的信息，代码如下：
public class Demo1 {
private static String msg;
public static void main(String[] args) {
work();
}
public static void work() {
Thread t1 = new Thread(() -> {
System.out.println(msg);
});
Thread t2 = new Thread(() -> {
msg = "t2 set the msg";
});
t1.start();
t2.start();
}
}
结果输出：
null 或者 t2 set the msg
结果说明：这个例子中线程t1，t2通过msg来通信，t1需要读取t2设置的msg，由于线程的执行无法预测，我们不能保证t1一定能读取t2设置的msg，所以会输出如上的结果。
如果我们想保证t1能够读取到t2设置的值，可以调整为如下代码：
public class Demo1 {
private static String msg;
public static void main(String[] args) {
work();
}
public static void work() {
Thread t1 = new Thread(() -> {
while (msg == null) {
}
System.out.println(msg + "!!!");
});
Thread t2 = new Thread(() -> {
msg = "t2 set the msg";
});
t1.start();
t2.start();
}
}
结果输出：
t2 set the msg!!!
结果说明：t1通过死循环的方式直到msg有值为止，这种方式虽然能满足需求，但是非常耗CPU资源。
为了更好的实现线程间的交互，java提供了等待/通知机制，本质上也是通过锁来实现线程间的通信。这种机制是通过Object类的wait()，notify()方法来实现，下面对这几个方法进行介绍。
二、wait，notify，notifyAll方法介绍
/**
* 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 (<condition does not hold>)
* 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.
*/
public final native void wait() throws InterruptedException;
/**
* 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.
*/
public final native void notify();
/**
* 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.
*/
public final native void notifyAll();
wait，notify，notifyAll方法总结：
- 这几个方法都定义在Object类中
- 都是final方法，不能被重写
- wait是阻塞方法，线程被中断会抛出InterruptedException异常。当前线程必须拥有对象锁，才能调用对象的wait方法，调用wait方法会使当前线程阻塞，当前线程会进入到对象的等待队列，同时释放对象锁，调用wait的伪代码如下：
synchronized (obj) {
while (condition does not hold)
obj.wait();
... // do something
}
wait(timeout) 在wait()的基础上增加了超时时间，超过该时间线程会被重新唤醒
- 调用notify、notifyAll方法的线程必须拥有对象锁，notify会唤醒等待对象监视器的单个线程，被唤醒的线程是随机的，notifyAll方法唤醒等待对象监视器的所有线程，被唤醒的线程(等待线程)不会立即竞争对象锁，只有当前的唤醒线程释放对象锁后，等待线程才能竞争对象锁
三、代码示例
先描述一个场景：
外婆家餐厅一直很火爆，我去过杭州西湖、北京APM、沈阳万象城这三家，味道都差不多，不过西湖边上的新白鹿感觉更胜一筹啊_，扯远了....
我们去外婆家吃饭，如果餐厅有空位，我们可以直接进去用餐，如果餐厅满员，我们需要排队，我们等着叫"XXX号，外婆让你回家吃饭了"，然后我们就可以进入餐厅吃饭，餐厅的容量实际是消费者和服务员之间的共享资源
这个场景抽象一下，可以认为用户是一个线程，而餐厅的服务人员相当于另外一个线程，餐厅是用餐者和服务人员的共享信息
代码如下：
public class WaitNotifyDemo1 {
public static void main(String[] args) {
final Queue queue = new LinkedList<>();
Thread producer = new Producer("producer", queue);
Thread consumer = new Consumer("consumer", queue);
producer.start();
consumer.start();
}
}
public class Producer extends Thread {
private final Queue sharedQueue;//餐厅容量
public Producer(String name, Queue queue) {
super(name);
this.sharedQueue = queue;
}
@Override
public void run() {
//假设餐厅最多容纳10桌客人
for (int i = 0; i <= 10; i++) {
synchronized (sharedQueue) {
while (sharedQueue.size() == 10) {
try {
sharedQueue.wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
sharedQueue.add(i);
System.out.println("producer " + i);
sharedQueue.notify();
}
}
}
}
public class Consumer extends Thread {
private final Queue sharedQueue;
public Consumer(String name, Queue queue) {
super(name);
this.sharedQueue = queue;
}
@Override
public void run() {
//消费者持续不断的来就餐
while (true) {
synchronized (sharedQueue) {
while (sharedQueue.size() == 0) {
try {
sharedQueue.wait();
sleep(300);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
Integer o = (Integer) sharedQueue.poll();
sharedQueue.notify();
if (o == 10) {//只接纳10桌客人
System.out.println("stop service");
break;
}
}
}
}
}
结果输出：
producer 0
producer 1
producer 2
producer 3
producer 4
producer 5
producer 6
producer 7
producer 8
producer 9
producer 10
stop service
结果分析：
- 生产者和消费者共享一个队列Queue
- 核心流程都是先获取对象锁，然后根据条件判断是等待还是唤醒
四、几个问题
（1） 为什么调用wait，notify 方法都必须获得对像锁？
wait方法使当前线程挂起，notify会唤醒在该对象上挂起的线程，也就是说wait，notify一定是成对出现且针对同一个对象的挂起、唤醒操作，我们知道每个对象都有一个监视器对象，synchronized就是通过进入监视器来实现线程间互斥的，基于这个特性，恰好能使wait，notify建立关系，形成协作，所以通过对象锁，既能实现线程间互斥，也能实现线程间协作
（2）为什么 wait，notify，notifyAll都定义在Object类中？
因为wait，notify是依赖 同步锁进行协作的，而每个对象都有且仅有一把对象锁，所以定义在Object类中可以完美解决线程间协作的问题
（3）如果不在同步块或者同步方法中执行wait，notify会怎样？
大家可以自行验证一下，这里就不做实践了
五、总结
（1）等待/通知 机制是通过对象锁来实现的
（2）notify，notifyAll，wait必须在同步块或者同步方法中执行
（3）线程间的交互模型如图所示：
转载请注明作者及出处，并附上链接http://www.jianshu.com/u/ada8c4ee308b
