一：线程通信介绍
线程通信是通过主动放弃对资源的使用，而让给其它线程的过程。合理的安排多个线程对同一资源的使用，即设计线程间的通信，可以完成很多复杂的任务。
二：线程通信实现
1，java.lang.Object超类
a，上图为Object类的常用方法，其中提供的notify()，notifyAll()，wait()，wait(long timeout) 和 wait(long timeout,int nanos)五个方法可以实现线程间通信。
b，五个方法都是被final修饰，所以不能被重写。
2，notify方法
notify()使用介绍：Wakes up a single thread that is waiting for this object's monitor.
notifyAll()使用介绍：Wakes up all threads that are waiting for this object's monitor.
3，wait方法
wait()使用介绍：Causes the current thread to wait until another thread invokes the notify() or the notifyAll() method for this object.
wait(long timeout)使用介绍：Causes the current thread to wait until either another thread invokes the notify() or notifyAll() method for this object,or a specified amount of time has elapsed.
wait(long timeout, int nanos)使用介绍：Causes the current thread to wait until another thread invokes the notify() or the notifyAll() method for this object,or some other thread interrupts the current thread ,or a centain amount of the real time has elapsed.
4，个人总结和附录
a，wait后的线程只能通过notify方法唤醒，从而重新进入就绪状态。
b，notify()方法只能唤醒一个在wait的线程，且是随机的。
c，从使用介绍中的 ‘current thread’,解读wait方法的使用需要使用线程必须拥有目标对象的锁，而notify()中使用介绍是主动唤醒其它线程，所以也必须拥有目标对象的锁才有资格去唤醒其它线程。所以wait()和notify方法的使用线程必须拥有目标对象的锁，即在使用时，wait方法和notify方法必须放在synchronized方法或sysnchronized代码块中。
d，wait方法是主动放弃锁，使线程进入等待锁，发生等待阻塞。而sleep方法并没有让线程放弃对象的锁。
附录
wake up：唤醒
monitor：监视器，即lock,锁。
causer：导致，引起
invokes：调用
specified time：指定的时间
elapsed：逝去的
三：设计案例（1010..案例）
1，基础案例
下述案利用合理设计线程间的通信例，实现了1010..的打印。
1 package com.thread.www; 2 /** 3 * 得到 1010的输出结果 4 * @author xiaojia 5 * 6 */ 7 //目标对象 8 class NumberHolder { 9private int num = 0;10//加数11public synchronized void increaseNum(){12 if(0 != num){13 try {14 wait();15 } catch (InterruptedException e) {16 e.printStackTrace();17 }18 }19 num ++;20 System.out.print(num);21 22 notify();23}24//减数25public synchronized void decreaseNum(){26 if(0 == num){27 try {28 wait();29 } catch (InterruptedException e) {30 e.printStackTrace();31 }32 }33 num --;34 System.out.print(num);35 notify();36}37 }38 39 //加数线程 和 减数线程40 class IncreaseThread extends Thread{41NumberHolder nholder;4243public IncreaseThread(NumberHolder nholder) {44 this.nholder = nholder;45}4647@Override48public void run() {49 50 for (int i = 0; i < 10; i++) {51 nholder.increaseNum();52 }53}54 }55 56 class DecreaseThread extends Thread{57NumberHolder nholder;5859public DecreaseThread(NumberHolder nholder) {60 this.nholder = nholder;61}6263@Override64public void run() {65 66 for (int i = 0; i < 10; i++) {67 nholder.decreaseNum();68 }69}70 }71 72 //启动线程73 public class Test7线程间的通信1 {74 75public static void main(String[] args) {76 NumberHolder nholder = new NumberHolder();77 78 IncreaseThread inTh = new IncreaseThread(nholder);79 DecreaseThread deTh = new DecreaseThread(nholder);80 81 inTh.start();82 deTh.start();83}84 85 }
basics Code
2，问题案例
如果创建多个线程，进行加数和减数，基础案例就得不到1010..有规律的打印了，main方法修改如下：
1 public class Test8线程间的通信2 { 2 3public static void main(String[] args) { 4 NumberHolder numHolder = new NumberHolder(); 56 IncreaseThread inTh = new IncreaseThread(numHolder); 7 DecreaseThread deTh = new DecreaseThread(numHolder); 8 inTh.start(); 9 deTh.start();10 11 IncreaseThread inTh2 = new IncreaseThread(numHolder);12 DecreaseThread deTh2 = new DecreaseThread(numHolder);13 inTh2.start();14 deTh2.start();15}16 }
3，完整案例
问题案例中，由于有四个创建的线程，拥有锁对象的线程唤醒wait的其它三个线程是随机的，因此不能按规律打印1010..。
解决办法：将加数和减数方法进入wait的if判断修改为while条件，这样当线程被唤醒后还是要进行进行零和非零的判断，才能加数或减数。
1 package com.thread.www; 2 3 class NumberHolder3 { 4private int num = 0; 5public synchronized void increaseNum(){ 6 while(num != 0){ 7 try { 8 wait(); 9 } catch (InterruptedException e) {10 e.printStackTrace();11 }12 }13 num ++;14 System.out.print(num+" ");15 16 notify();17}1819public synchronized void decreaseNum(){20 while(num == 0){21 try {22 wait();23 } catch (InterruptedException e) {24 e.printStackTrace();25 }26 }27 num --;28 System.out.print(num+" ");29 30 notify();31}32 }33 34 class IncreaseThread3 extends Thread{35private NumberHolder3 numHolder;36public IncreaseThread3(NumberHolder3 numHolder) {37 this.numHolder = numHolder;38}39@Override40public void run() {41 for (int i = 0; i < 10; i++) {42 try {43 sleep(1000);44 } catch (InterruptedException e) {45 e.printStackTrace();46 }47 numHolder.increaseNum();48 }49}50 }51 52 class DecreaseThread3 extends Thread{53private NumberHolder3 numHolder;54public DecreaseThread3(NumberHolder3 numHolder) {55 this.numHolder = numHolder;56}57@Override58public void run() {59 for (int i = 0; i < 10; i++) {60 try {61 sleep((long)Math.random()*1000);62 } catch (InterruptedException e) {63 e.printStackTrace();64 }65 numHolder.decreaseNum();66 }67}68 }69 70 public class Test9线程间的通信3 {71 72public static void main(String[] args) {73 NumberHolder3 numHolder = new NumberHolder3();74 75 IncreaseThread3 inTh = new IncreaseThread3(numHolder);76 DecreaseThread3 deTh = new DecreaseThread3(numHolder);77 inTh.start();78 deTh.start();79 80 IncreaseThread3 inTh3 = new IncreaseThread3(numHolder);81 DecreaseThread3 deTh3 = new DecreaseThread3(numHolder);82 inTh3.start();83 deTh3.start();84 85}86 87 }
complete code
参考资料：/javase/7/docs/api/java/lang/Object.html#notify()
/mengdd/archive//02/20/2917956.html
原文路径：/xiaojia-new/p/8604606.html
