Java的Object类
public class Object {
public final native void notify();
public final native void notifyAll();
public final native void wait(long timeout) throws InterruptedException;
}
调用这些方法的当前线程必须拥有此对象监视器，否则将会报java.lang.IllegalMonitorStateException exception
wait;
Object的wait方法有三个重载方法，其中一个方法wait() 是无限期(一直)等待，直到其它线程调用notify或notifyAll方法唤醒当前的线程；另外两个方法wait(long timeout) 和wait(long timeout, int nanos)允许传入 当前线程在被唤醒之前需要等待的时间，timeout为毫秒数，nanos为纳秒数。
wait():释放占有的对象锁，线程进入等待池，释放cpu,而其他正在等待的线程即可抢占此锁，获得锁的线程即可运行程序。
而sleep()不同的是，线程调用此方法后，会休眠一段时间，休眠期间，会暂时释放cpu，但并不释放对象锁。也就是说，在休眠期间，其他线程依然无法进入此代码内部。休眠结束，线程重新获得cpu,执行代码。
wait()和sleep()最大的不同在于wait()会释放对象锁，而sleep()不会！
notify:
notify方法只唤醒一个等待（对象的）线程并使该线程开始执行。
调用notify()后，并不会立即释放锁，而是继续执行当前代码，直到synchronized中的代码全部执行完毕，才会释放对象锁。JVM则会在等待的线程中调度一个线程去获得对象锁，执行代码。
需要注意的是，wait()和notify()必须在synchronized代码块中调用。
所以如果有多个线程等待一个对象，这个方法只会唤醒其中一个线程，选择哪个线程取决于操作系统对多线程管理的实现。
notifyAll:
notifyAll 会唤醒所有等待(对象的)线程，尽管哪一个线程将会第一个处理取决于操作系统的实现。
这些方法可以使用于“生产者-消费者”问题，消费者是在队列中等待对象的线程，生产者是在队列中释放对象并通知其他线程的线程。
demo：
public class Main {
public static void main(String[] args) {
Message msg = new Message("process it");
Waiter waiter = new Waiter(msg);
new Thread(waiter,"waiterThread").start();
Waiter waiter1 = new Waiter(msg);
new Thread(waiter1, "waiter1Thread").start();
Notifier notifier = new Notifier(msg);
new Thread(notifier, "notifierThread").start();
System.out.println("All the threads are started");
}
}
class Message {
private String msg;
public Message(String str){
this.msg=str;
}
public String getMsg() {
return msg;
}
public void setMsg(String str) {
this.msg=str;
}
}
class Waiter implements Runnable{
private Message msg;
public Waiter(Message m){
this.msg=m;
}
@Override
public void run() {
String name = Thread.currentThread().getName();
synchronized (msg) {
try{
System.out.println(name+" waiting to get notified at time:"+System.currentTimeMillis());
msg.wait();
}catch(InterruptedException e){
e.printStackTrace();
}
System.out.println(name+" waiter thread got notified at time:"+System.currentTimeMillis());
//process the message now
System.out.println(name+" processed: "+msg.getMsg());
}
}
}
class Notifier implements Runnable {
private Message msg;
public Notifier(Message msg) {
this.msg = msg;
}
@Override
public void run() {
String name = Thread.currentThread().getName();
System.out.println(name+" started");
try {
Thread.sleep(1000);
synchronized (msg) {
msg.setMsg(name+" Notifier work done");
msg.notify();
// msg.notifyAll();
}
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
All the threads are started
waiter1Thread waiting to get notified at time:1480592377675
waiterThread waiting to get notified at time:1480592377677
notifierThread started
waiter1Thread waiter thread got notified at time:1480592378685
waiter1Thread processed: notifierThread Notifier work done
Notifier中的run方法改成msg.notifyAll();
Result:
All the threads are started
waiter1Thread waiting to get notified at time:1480592533780
waiterThread waiting to get notified at time:1480592533780
notifierThread started
waiterThread waiter thread got notified at time:1480592534782
waiterThread processed: notifierThread Notifier work done
waiter1Thread waiter thread got notified at time:1480592534782
waiter1Thread processed: notifierThread Notifier work done
一个通知线程，3个等待线程：
public class Main {
private String flag[] = { "true" };
public static void main(String[] args) {
System.out.println("Main Thread Run!");
Main test = new Main();
System.nanoTime();
NotifyThread notifyThread =test.new NotifyThread("notify01");
WaitThread waitThread01 = test.new WaitThread("waiter01");
WaitThread waitThread02 = test.new WaitThread("waiter02");
WaitThread waitThread03 = test.new WaitThread("waiter03");
notifyThread.start();
waitThread01.start();
waitThread02.start();
waitThread03.start();
}
class NotifyThread extends Thread{
public NotifyThread(String name) {
super(name);
}
public void run() {
try {
sleep(3000);//推迟3秒钟通知
} catch (InterruptedException e) {
e.printStackTrace();
}
synchronized (flag) {
flag[0] = "false";
flag.notifyAll();
}
}
}
class WaitThread extends Thread {
public WaitThread(String name) {
super(name);
}
public void run() {
synchronized (flag) {
while (flag[0] != "false") {
System.out.println(getName() + " begin waiting!");
long waitTime = System.currentTimeMillis();
try {
flag.wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
waitTime = System.currentTimeMillis() - waitTime;
System.out.println("wait time :" + waitTime);
}
System.out.println(getName() + " end waiting!");
}
}
}
}
Main Thread Run!
waiter01 begin waiting!
waiter02 begin waiting!
waiter03 begin waiting!
wait time :2999
waiter03 end waiting!
wait time :2999
waiter02 end waiting!
wait time :2999
waiter01 end waiting!
=====================================================================================
wait和sleep的demo:
public class Main {
public static void main(String[] args) {
new Thread(new Thread1()).start();
try {
Thread.sleep(5000);
} catch (Exception e) {
e.printStackTrace();
}
new Thread(new Thread2()).start();
}
private static class Thread1 implements Runnable{
@Override
public void run(){
synchronized (Main.class) {
System.out.println("enter thread1...");
System.out.println("thread1 is waiting...");
try {
//调用wait()方法，线程会放弃对象锁，进入等待此对象的等待锁定池
Main.class.wait();
} catch (Exception e) {
e.printStackTrace();
}
System.out.println("thread1 is going on ....");
System.out.println("thread1 is over!!!");
}
}
}
private static class Thread2 implements Runnable{
@Override
public void run(){
synchronized (Main.class) {
System.out.println("enter thread2....");
System.out.println("thread2 is sleep....");
//只有针对此对象调用notify()方法后本线程才进入对象锁定池准备获取对象锁进入运行状态。
Main.class.notify();
//==================
//区别
//如果我们把代码：Main.class.notify();给注释掉，即Main.class调用了wait()方法，但是没有调用notify()
//方法，则线程永远处于挂起状态。
try {
//sleep()方法导致了程序暂停执行指定的时间，让出cpu该其他线程，
//但是他的监控状态依然保持者，当指定的时间到了又会自动恢复运行状态。
//在调用sleep()方法的过程中，线程不会释放对象锁。
Thread.sleep(5000);
} catch (Exception e) {
e.printStackTrace();
}
System.out.println("thread2 is going on....");
System.out.println("thread2 is over!!!");
}
}
}
}
enter thread1...
thread1 is waiting...
enter thread2....
thread2 is sleep....
thread2 is going on....
thread2 is over!!!
thread1 is going on ....
thread1 is over!!!
wait,notify的使用，线程同步唤醒
package com.qhong; /**
* Created by Administrator on 2017/7/4 0004.
*/
public class MyThreadPrinter2 implements Runnable { private String name;
private Object prev;
private Object self; private MyThreadPrinter2(String name, Object prev, Object self) {
this.name = name;
this.prev = prev;
this.self = self;
} @Override
public void run() {
int count = ;
while (count > ) {
synchronized (prev) {
synchronized (self) {
System.out.print(name);
count--; self.notify();
}
try {
prev.wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
} }
} public static void main(String[] args) throws Exception {
Object a = new Object();
Object b = new Object();
Object c = new Object();
MyThreadPrinter2 pa = new MyThreadPrinter2("A", c, a);
MyThreadPrinter2 pb = new MyThreadPrinter2("B", a, b);
MyThreadPrinter2 pc = new MyThreadPrinter2("C", b, c); new Thread(pa).start();
Thread.sleep(); //确保按顺序A、B、C执行
new Thread(pb).start();
Thread.sleep();
new Thread(pc).start();
Thread.sleep();
}
}
Output:
ABCABCABCABCABCABCABCABCABCABC
Thread.sleep(100)是保证ABC的执行顺序
在这个例子中，可以深刻的体现出wait不仅是阻塞线程，更体现wait释放锁对象。
比如是4轮，第一轮的abc是有Thread.sleep来控制的，这个时候只有锁对象c是被释放的，锁对象a阻塞线程B，锁对象b阻塞线程C
第二轮，a的prev是c，所以是可以运行的，并且在这一步释放锁对象a,锁对象c注释线程A
................
就这样一轮一轮的控制，虽然想通了，但是让我写还是想不出来的，挺繁琐！
有是一个demo
package com.qhong.thread;
public class MyWaitNotify{
MonitorObject myMonitorObject = new MonitorObject();
boolean wasSignalled = false;
public void doWait(){
synchronized(myMonitorObject){
while(!wasSignalled){
try{
myMonitorObject.wait();
} catch(InterruptedException e){...}
}
//clear signal and continue running.
wasSignalled = false;
}
}
public void doNotify(){
synchronized(myMonitorObject){
wasSignalled = true;
myMonitorObject.notify();
}
}
}
class MonitorObject{}
代码比较简单，但是要注意下面的说明，有好几个概念以前没注意:
1、不管是等待线程还是唤醒线程都在同步块里调用wait()和notify()。这是强制性的！一个线程如果没有持有对象锁，将不能调用wait()，notify()或者notifyAll()。否则，会抛出IllegalMonitorStateException异常。
2、一旦线程调用了wait()方法，它就释放了所持有的监视器对象上的锁。这将允许其他线程也可以调用wait()或者notify()。
3、为了避免丢失信号，必须把它们保存在信号类里。如上面的wasSignalled变量。
4、假唤醒：由于莫名其妙的原因，线程有可能在没有调用过notify()和notifyAll()的情况下醒来。这就是所谓的假唤醒（spurious wakeups）。为了防止假唤醒，保存信号的成员变量将在一个while循环里接受检查，而不是在if表达式里。这样的一个while循环叫做自旋锁。
5、不要在字符串常量或全局对象中调用wait()。即上面MonitorObject不能是字符串常量或是全局对象。每一个MyWaitNotify的实例都拥有一个属于自己的监视器对象，而不是在空字符串上调用wait()/notify()。
http://www.cnphp6.com/archives/62258
http://blog.csdn.net/luoweifu/article/details/46664809
http://developer.51cto.com/art/201508/487488.htm