Java线程同步的使用
由 maggie 于 星期二, 2010-01-12 13:04 发表
Synchronized，顾名思义，代表线程同步。那么，在java编程中如何使用它呢？
我们首先来看这样一个情况：对于同一个变量synDemo，我们分别在2个不同的线程中调用synDemo.synMethord1()与synDemo.synMethord2()。
package com.cnblogs.gpcuster;/** * * @author Aaron.Guo * */
public class Tester {public static void main(String[] args) {final SynDemo synDemo = new SynDemo();Thread thread1 = new Thread() {@Overridepublic void run() {// TODO Auto-generated method stub
super.run();synDemo.synMethord1();}};Thread thread2 = new Thread() {@Overridepublic void run() {// TODO Auto-generated method stub
super.run();synDemo.synMethord2();}};thread1.start();thread2.start();while (true) {try {Thread.sleep(1000);System.out.println("main");} catch (InterruptedException e) {// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
}
我们的SynDemo是这样定义的：
package com.cnblogs.gpcuster;/** * * @author Aaron.Guo * */
public class SynDemo{public void synMethord1() {while(true) {try {Thread.sleep(1000);System.out.println("synMethord1");} catch (InterruptedException e) {// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
public void synMethord2() {while(true) {try {Thread.sleep(1000);System.out.println("synMethord2");} catch (InterruptedException e) {// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
}
我们的SynDemo对象没有任何特殊的定义，所以运行起来以后的情况如下：
synMethord2 synMethord1 main synMethord2 synMethord1 main synMethord2 synMethord1
接下来，我们给synMethord1添加上synchronized声明，运行情况还是与上次一样，因为我们只有一个线程在调用synMethord1的方法。
我们给synMethord2也添加上synchronized声明，SynDemo对象的代码修改为：
package com.cnblogs.gpcuster;/** * * @author Aaron.Guo * */
public class SynDemo{public synchronized void synMethord1() {while(true) {try {Thread.sleep(1000);System.out.println("synMethord1");} catch (InterruptedException e) {// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
public synchronized void synMethord2() {while(true) {try {Thread.sleep(1000);System.out.println("synMethord2");} catch (InterruptedException e) {// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
}
这个时候，我们再运行程序，结果就不一样了：
main synMethord1 main synMethord1 main synMethord1
我们发现，对于SynDemo对象，只有synMethord1运行了，而synMethord2却没有运行。这是应为在方法级别的synchronized声明将lock这个类对象的当前实例。所以在synMethord1运行结束unlock之前，当前实例是无法运行synMethord2的。这种方法级别的synchronized声明和以下的做法是等同的：
package com.cnblogs.gpcuster;/** * * @author Aaron.Guo * */
public class SynDemo {public void synMethord1() {synchronized (this) {while (true) {try {Thread.sleep(1000);System.out.println("synMethord1");} catch (InterruptedException e) {// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
}
public synchronized void synMethord2() {synchronized (this) {while (true) {try {Thread.sleep(1000);System.out.println("synMethord2");} catch (InterruptedException e) {// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
}
}
运行程序，结果与上次相同。
如果我们希望分别同步2个方法该如何处理？可以参考这个实现：
package com.cnblogs.gpcuster;/** * * @author Aaron.Guo * */
public class SynDemo {private Object flag1 = new Object();private Object flag2 = new Object();public void synMethord1() {synchronized (flag1) {while (true) {try {Thread.sleep(1000);System.out.println("synMethord1");} catch (InterruptedException e) {// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
}
public void synMethord2() {synchronized (flag2) {while (true) {try {Thread.sleep(1000);System.out.println("synMethord2");} catch (InterruptedException e) {// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
}
}
运行程序，结果如我们预期：
main synMethord2 synMethord2 main synMethord1 main synMethord1 synMethord2
关于Synchronized还有一些其他的话题，如static的问题，继承的问题，与volatile搭配使用等等，在网上都有很详细的说明，这里就不重复介绍了。
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