一、sleep()
sleep()方法源码：
/** * 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; 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)sleep()使当前线程进入停滞状态（阻塞当前线程），让出CUP的使用、目的是不让当前线程独自霸占该进程所获的CPU资源，以留一定时间给其他线程执行的机会;
(2)sleep()是Thread类的Static(静态)的方法；因此他不能改变对象的机锁，所以当在一个Synchronized块中调用Sleep()方法时，线程虽然休眠了，但是对象的机锁并木有被释放，其他线程无法访问这个对象（即使睡着也持有对象锁）。
(3)在sleep()休眠时间期满后，该线程不一定会立即执行，这是因为其它线程可能正在运行而且没有被调度为放弃执行，除非此线程具有更高的优先级。
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(); }
} 开始执行线程。。。
进入睡眠状态。。。
线程结束。。。二、wait()
wait()方法源码：
/** * 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); } 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)wait()方法是Object类里的方法；当一个线程执行到wait()方法时，它就进入到一个和该对象相关的等待池中，同时失去（释放）了对象的机锁（暂时失去机锁，wait(long timeout)超时时间到后还需要返还对象锁）；其他线程可以访问；
(2)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。
(4)wiat()必须放在synchronized block中，否则会在program runtime时扔出”java.lang.IllegalMonitorStateException“异常。
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：
开始执行线程。。。
进入等待状态。。。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。
三、notify()和notifyAll()
说完了sleep()和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(); }线程A开始等待...
线程B开始等待...
线程C开始等待...
唤醒线程开始执行...
线程A结束...
线程B结束...
线程C结束...
所有线程执行完毕...
