分析volatile关键字可以从这三个方面分析，什么是程序的原子性，什么是程序的可见性，什么是程序的有序性
什么是程序的原子性
以下语句那些是原子操作？
public class ThreadCounter implements Runnable {private int count = 0;@Overridepublic void run() {++count;//count++;}public static void main(String[] args) throws InterruptedException {ThreadCounter thread = new ThreadCounter();for(int i = 0; i< 10000; i++){new Thread(thread).start();}Thread.sleep(1000);//确保线程执行完 System.out.println(thread.count);}}
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演示结果：语句一和二是非原子操作，语句三和四是原子操作
执行指令：javap -s -c ThreadCounter run方法的指令码（count++）： count++这行代码分成了4个指令来执行，在多线程的情况下会不一致。
解决方法：
public class ThreadCounter implements Runnable {private int count = 0;@Overridepublic void run() {synchronized (this) {++count;// count++; }}public static void main(String[] args) throws InterruptedException {ThreadCounter thread = new ThreadCounter();for(int i = 0; i< 10000; i++){new Thread(thread).start();}Thread.sleep(1000);//确保线程执行完 System.out.println(thread.count);}}
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什么是程序的可见性？
public class VolatileExample {boolean v =false;private void write(){v =true;}private void read(){while(!v){}System.out.println("程序结束！");}public static void main(String[] args) throws InterruptedException {final VolatileExample example = new VolatileExample();Thread thread1 = new Thread(()->{example.read();});thread1.start();Thread.sleep(1000);Thread thread2 = new Thread(()->{example.write();});thread2.start();}}
View Code 演示结果： 程序没有结束，read方法中的v没有因write方法的修改而退出循环！ 解决方法：为变量v添加volatile关键字
public class VolatileExample {volatile boolean v =false;private void write(){v =true;}private void read(){while(!v){}System.out.println("程序结束！");}public static void main(String[] args) throws InterruptedException {final VolatileExample example = new VolatileExample();Thread thread1 = new Thread(()->{example.read();});thread1.start();Thread.sleep(1000);Thread thread2 = new Thread(()->{example.write();});thread2.start();}}
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什么是程序的有序性？
Volatile应用场景
1.状态标记量public class ThreadTest {private volatile boolean isContinue = false;private class HandleThread extends Thread {@Overridepublic void run() {while (isContinue) {// do something }};}}
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2.double check
总结：
volatile在可见性和有序性可以起到作用，但是不能保证原子性，是一种弱同步。
synchronized可以保证原子性，可见性，一致性，是一种强同步。
