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示例：
//将资源封装成对象 class Resour { String name; String gender; } //将线程执行的任务封装成对象 class Input implements Runnable { private Resour r; Input(Resour r) { this.r = r; } public void run() { int x =0; while(true) { if(x==0) { r.name = "Travis"; r.gender = "male"; } else { r.name = "张妙"; r.gender = "女性"; } x = (x+1)%2; } } } class Output implements Runnable { private Resour r; Output(Resour r) { this.r = r; } public void run() { while(true) { System.out.println(r.name+" "+r.gender); } } } class myLock { static Object lockA = new Object(); static Object lockB = new Object(); } class mlpcDemo { public static void main(String[] args) { Resour r = new Resour(); Input in = new Input(r); Output out = new Output(r); Thread th0 = new Thread(in); Thread th1 = new Thread(out); th0.start(); th1.start(); } }
于是，我们给赋值的代码块加上同步。这里要注意必须明确哪些代码是要同步的。
//将资源封装成对象 class Resour { String name; String gender; } //将线程执行的任务封装成对象 class Input implements Runnable { private Resour r; Object obj = new Object(); Input(Resour r) { this.r = r; } public void run() { int x =0; synchronized(obj) {
while(true) { if(x==0) { r.name = "Travis"; r.gender = "male"; } else { r.name = "张妙"; r.gender = "女性"; } x = (x+1)%2; } } } } class Output implements Runnable { private Resour r; Output(Resour r) { this.r = r; } public void run() { while(true) { System.out.println(r.name+" "+r.gender); } } } class myLock { static Object lockA = new Object(); static Object lockB = new Object(); } class mlpcDemo { public static void main(String[] args) { Resour r = new Resour(); Input in = new Input(r); Output out = new Output(r); Thread th0 = new Thread(in); Thread th1 = new Thread(out); th0.start(); th1.start(); } }
表面上看，赋值代码块被加上了同步，但运行的结果依然出现了姓名和性别乱搭配的情况。
于是分析原因，定义同步有两个前提，首先Input和Output这两个线程并未使用同一个锁。
其次，仔细观察下列代码：
Thread th0 = new Thread(in); Thread th1 = new Thread(out);
th0.start();
th1.start();
无论Input还是Output都只有一条线程在执行！
不过，解决这个问题也十分容易。给不同线程的同步函数加上相同的锁。
需注意的错误：
1 public void run() 2 { 3 int x =0; 4 synchronized(r) //这样会使得线程永远走此同步函数内的代码块 5 { 6 while(true) 7 { 8 if(x==0) 9 { 10 r.name = "Travis"; 11 r.gender = "male"; 12 } 13 else 14 { 15 r.name = "张妙"; 16 r.gender = "女性"; 17 } 18 x = (x+1)%2; 19 } 20 } 21 }
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原文地址：http://www.cnblogs.com/tozr/p/4151513.html
