深入理解Java多线程中的wait()，notify()和sleep()

大家在学习Java的过程中，势必要进行多线程的系统学习，这部分内容知识对于你在工作中的影响是极大的，并且在面试的过程中，这部分知识也是必然会被问到的。既然多线程的知识如此重要，那么我们就不能浅尝辄止。在这篇文章中，我想通过阅读源码的方式给大家分享一下我自己对于Java中wait()，notify()和sleep()的理解，并且在后面我也做一些内容上的扩展，与大家共同思考。如果在文章中有描述不妥或者逻辑错误的地方，欢迎大家指正，谢谢！

wait()和sleep()

我们先简单的了解一下wait()和sleep()这两个方法：

首先wait()是属于Object类的方法，从源码给出的解释来看，wait()方法可以做到如下几点：

（1）首先，调用了wait()之后会引起当前线程处于等待状状态。

（2）其次，每个线程必须持有该对象的monitor。如果在当前线程中调用wait()方法之后，该线程就会释放monitor的持有对象并让自己处于等待状态。

（3）如果想唤醒一个正在等待的线程，那么需要开启一个线程通过notify()或者notifyAll()方法去通知正在等待的线程获取monitor对象。如此，该线程即可打破等待的状态继续执行代码。

/**
* 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.
*
* @exception  IllegalMonitorStateException  if the current thread is not
*               the owner of the object's monitor.
* @exception  InterruptedException if any thread interrupted the
*             current thread before or while the current thread
*             was waiting for a notification.  The <i>interrupted
*             status</i> of the current thread is cleared when
*             this exception is thrown.
* @see        java.lang.Object#notify()
* @see        java.lang.Object#notifyAll()
*/
public final void wait() throws InterruptedException {
wait(0);
}

代码演示：

public class Main {
public static void main(String[] args) {
Main main = new Main();
main.startThread();
}

/**
* 线程锁
*/
private final Object object = new Object();

/**
* 启动线程
*/
public void startThread() {
Thread t = new Thread(new Runnable() {
@Override
public void run() {
System.out.println("开始执行线程。。。");
System.out.println("进入等待状态。。。");
synchronized (object) {
try {
object.wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
System.out.println("线程结束。。。");
}
});
t.start();
}
}

从代码来看，在执行线程和线程结束之间，我们先让该线程获取object对象作为自己的object's monitor，然后调用了object对象的wait()方法从而让其进入等待状态。那么程序运行的结果如下：



程序在未被唤醒之后，将不再打印“线程结束”，并且程序无法执行完毕一直处于等待状态。

sleep()方法来自于Thread类，从源码给出的解释来看，sleep()方法可以做到如下几点：

（1）首先，调用sleep()之后，会引起当前执行的线程进入暂时中断状态，也即睡眠状态。

（2）其次，虽然当前线程进入了睡眠状态，但是依然持有monitor对象。

（3）在中断完成之后，自动进入唤醒状态从而继续执行代码。

/**
* Causes the currently executing thread to sleep (temporarily cease
* execution) for the specified number of milliseconds, subject to
* the precision and accuracy of system timers and schedulers. The thread
* does not lose ownership of any monitors.
*
* @param  millis
*         the length of time to sleep in milliseconds
*
* @throws  IllegalArgumentException
*          if the value of {@code millis} is negative
*
* @throws  InterruptedException
*          if any thread has interrupted the current thread. The
*          <i>interrupted status</i> of the current thread is
*          cleared when this exception is thrown.
*/
public static native void sleep(long millis) throws InterruptedException;

代码演示：

public class Main {
public static void main(String[] args) {
Main main = new Main();
main.startThread();
}

/**
* 启动线程
*/
public void startThread() {
Thread t = new Thread(new Runnable() {
@Override
public void run() {
System.out.println("开始执行线程。。。");
System.out.println("进入睡眠状态。。。");
try {
Thread.sleep(3000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("线程结束。。。");
}
});
t.start();
}
}

从运行的结果来看，我们可以看出程序虽然在运行过程中中断了3秒，但是在3秒结束之后依然会继续执行代码，直到运行结束。在睡眠的期间内，线程会一直持有monitor对象。



那么从以上的理论和实践来分析，我们能得出如下结论：

（1）在线程的运行过程中，调用该线程持有monitor对象的wait()方法时，该线程首先会进入等待状态，并将自己持有的monitor对象释放。

（2）如果一个线程正处于等待状态时，那么唤醒它的办法就是开启一个新的线程，通过notify()或者notifyAll()的方式去唤醒。当然，需要注意的一点就是，必须
4000
是同一个monitor对象。

（3）sleep()方法虽然会使线程中断，但是不会将自己的monitor对象释放，在中断结束后，依然能够保持代码继续执行。

notify()和notifyAll()

说完了wait()方法之后，我们接下来讨论一下Object类中的另外两个与wait()相关的方法。首先还是通过源码的方式让大家先初步了解一下：

/**
* 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.
*
* @exception  IllegalMonitorStateException  if the current thread is not
*               the owner of this object's monitor.
* @see        java.lang.Object#notifyAll()
* @see        java.lang.Object#wait()
*/
public final native void notify();

先来看下notify()这个方法，通过阅读源码我们可以总结一下几点：
（1）当一个线程处于wait()状态时，也即等待它之前所持有的object's monitor被释放，通过notify()方法可以让该线程重新处于活动状态，从而去抢夺object's monitor，唤醒该线程。
（2）如果多个线程同时处于等待状态，那么调用notify()方法只能随机唤醒一个线程。
（3）在同一时间内，只有一个线程能够获得object's monitor，执行完毕之后，则再将其释放供其它线程抢占。
当然，如何使线程成为object‘s monitor的持有者，我会在多线程的其他博客中讲解。

接下来，我们再来看看notifyAll()方法：

/**
* 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.
*
* @exception  IllegalMonitorStateException  if the current thread is not
*               the owner of this object's monitor.
* @see        java.lang.Object#notify()
* @see        java.lang.Object#wait()
*/
public final native void notifyAll();

那么顾名思义，notifyAll()就是用来唤醒正在等待状态中的所有线程的，不过也需要注意以下几点：
（1）notifyAll()只会唤醒那些等待抢占指定object's monitor的线程，其他线程则不会被唤醒。
（2）notifyAll()只会一个一个的唤醒，而并非统一唤醒。因为在同一时间内，只有一个线程能够持有object's monitor
（3）notifyAll()只是随机的唤醒线程，并非有序唤醒。
那么如何做到有序唤醒是我们接下来要讨论的问题。

notify()实现有序唤醒的思路和实现

就上节提出的问题，我们在这节中可以进行一下思考和讨论。
首先，简单来说，我们需要去解决的其实就是对于多线程针对object's monitor的有序化。那么根据这一思路，我直接上代码：

public class MyThreadFactory {

// 线程A是否处于等待状态的标志
private boolean isThreadAWaiting;
// 线程B是否处于等待状态的标志
private boolean isThreadBWaiting;
// 线程C是否处于等待状态的标志
private boolean isThreadCWaiting;

public MyThreadFactory() {
isThreadAWaiting = true;
isThreadBWaiting = true;
isThreadCWaiting = true;
}

/**
* 对象锁
*/
private final Object object = new Object();

/**
* 该线程作为一个唤醒线程
*/
public void startWakenThread() {
Thread t = new Thread(new Runnable() {
@Override
public void run() {
synchronized (object) {
System.out.println("唤醒线程开始执行...");
// 首先释放线程A
quitThreadA();
}
}
});
t.start();
}

/**
* 启动线程A
*/
public void startThreadA() {
Thread t = new Thread(new Runnable() {
@Override
public void run() {
synchronized (object) {
System.out.println("线程A开始等待...");
try {
for (; ; ) {
if (!isThreadAWaiting) break;
object.wait();
}
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("线程A结束...");
// 线程A结束后，暂停2秒释放线程B
try {
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
quitThreadB();
}
}
});
t.start();
}

/**
* 启动线程B
*/
public void startThreadB() {
Thread t = new Thread(new Runnable() {
@Override
public void run() {
synchronized (object) {
System.out.println("线程B开始等待...");
try {
for (; ; ) {
if (!isThreadBWaiting) break;
object.wait();
}
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("线程B结束...");
// 线程B结束后，暂停2秒释放线程C
try {
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
quitThreadC();
}
}
});
t.start();
}

/**
* 启动线程C
*/
public void startThreadC() {
Thread t = new Thread(new Runnable() {
@Override
public void run() {
synchronized (object) {
System.out.println("线程C开始等待...");
try {
for (; ; ) {
if (!isThreadCWaiting) break;
object.wait();
}
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("线程C结束...");

try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("所有线程执行完毕！");
}
}
});
t.start();
}

/**
* 线程A退出等待
*/
private void quitThreadA() {
isThreadAWaiting = false;
object.notify();
}

/**
* 线程B退出等待
*/
private void quitThreadB() {
isThreadBWaiting = false;
object.notify();
}

/**
* 线程C退出等待
*/
private void quitThreadC() {
isThreadCWaiting = false;
object.notify();
}
}

在以上代码中，我写了三个线程A，B，C用来作为等待线程，并且最后通过一个唤醒线程来唤醒这三个线程。
我的思路是这样的：
（1）通过notify()方法可以保证每次只唤醒一个线程，但是不能确保唤醒的是哪个线程。
（2）在线程A，B，C中，添加for或者while循环的方式使其进入无限等待的状态。这样能够保证notify()无论如何都不能唤醒线程。
（3）分别给A，B，C线程设置各自的标记，如果要唤醒该线程的话，就改变其状态并且跳出死循环，在最后执行下一个线程。

那么最终调用的main函数如下：

public static void main(String[] args) {
MyThreadFactory factory = new MyThreadFactory();
factory.startThreadA();
factory.startThreadB();
factory.startThreadC();

try {
Thread.sleep(3000);
} catch (InterruptedException e) {
e.printStackTrace();
}
factory.startWakenThread();
}

最后运行的结果如下：

小结

本章是为了通过wait()，sleep()，notify()以及notifyAll()去了解它们是如何使用的，以及它们各自的意义。现在在java.util.concurrent包中，已经有很多针对多线程并发的线程池封装类和接口，大家使用起来会更加灵活和方便，并且能够实现更多并发效果。因此不太推荐大家使用wait()和notify()这种方式。在接下来的博客中，我还会继续深入多线程的知识，给大家带来更多更深入的东西。希望大家给予支持。