Volatile的内存语义

1、volatile的特性

可见性：对一个volatile变量的读，总能够看到任意一个线程对这个volatile变量的写入。

原子性：对任意单个volatile变量的读/写具有原子性，但类似于volatile++这种复合操作不具有原子性。

接下来我们用程序验证。


public class OldVolatileFeaturesExample {volatile long v1 = 0L;      // 使用volatile 声明64位的long型变量//long v1 = 0L;public void set(long l){v1 = l;                 //单个volatile 变量的写}public void getAndIncrement(){v1++;                   // 多个volatile 变量的读/写}public long get(){return v1;              //  单个volatile 变量的读}public static void main(String[] args) {final OldVolatileFeaturesExample volatileFeaturesExamlple = new OldVolatileFeaturesExample();Thread thread0 = new Thread(new Runnable() {public void run() {volatileFeaturesExamlple.set(1L);}});thread0.start();Thread thread1 = new Thread(new Runnable() {public void run() {volatileFeaturesExamlple.getAndIncrement();}});thread1.start();Thread thread2 = new Thread(new Runnable() {public void run() {long l = volatileFeaturesExamlple.get();System.out.println("创建的l值-------"+ l);}});thread2.start();/*        for (int i = 0; i < 10; i++) {Thread thread0 = new Thread(new Runnable() {public void run() {volatileFeaturesExamlple.set(1L);}});thread0.start();Thread thread1 = new Thread(new Runnable() {public void run() {volatileFeaturesExamlple.getAndIncrement();}});thread1.start();Thread thread2 = new Thread(new Runnable() {public void run() {long l = volatileFeaturesExamlple.get();System.out.println("创建的l值-------"+ l);}});thread2.start();}*/}}

这里，线程thread0 设置使用volatile修饰的long类型变量 v1 ；线程thread1 进行v1++操作， thread2 获取变量v1的值，并打印结果。那么 v1的值是几呢？

上面这段程序运行结果是：

创建的l值-------2

那么就算不用volatile修饰的v1变量，也执行上述操作，结果会是什么样子呢？没错，还是2。

那么使用volatile修饰的v1变量当使用for 循环呢？也就是多个volatile变量的读写操作的结果：


创建的l值-------2
创建的l值-------2
创建的l值-------1
创建的l值-------1
创建的l值-------1
创建的l值-------2
创建的l值-------3
创建的l值-------1
创建的l值-------2
创建的l值-------3

假设具有原子性，那么v1循环加10次 1,那么它的结果应该是 10，而不是上面的结果。上面的程序等价于：

public class NewVolatileFeaturesExample {long v1 = 0L;public synchronized void set(long l){  //对单个的普通变量的写用同一个锁同步v1 = l;}public void getAndIncrement(){         //普通方法调用long temp = get();                 //调用已同步的读方法temp += 1L;                        //普通写操作set(temp);                         //调用已同步的写方法}public synchronized long get(){         // 对单个的普通变量的读用同一个锁同步return v1;}public static void main(String[] args) {final NewVolatileFeaturesExample newVolatileFeaturesExample = new NewVolatileFeaturesExample();/*        for (int i = 0; i < 10; i++) {Thread thread0 = new Thread(new Runnable() {public void run() {newVolatileFeaturesExample.set(1L);}});thread0.start();Thread thread1 = new Thread(new Runnable() {public void run() {newVolatileFeaturesExample.getAndIncrement();}});thread1.start();Thread thread2 = new Thread(new Runnable() {public void run() {long l = newVolatileFeaturesExample.get();System.out.println("创建的l值-------"+ l);}});thread2.start();}*/Thread thread0 = new Thread(new Runnable() {public void run() {newVolatileFeaturesExample.set(1L);}});thread0.start();Thread thread1 = new Thread(new Runnable() {public void run() {newVolatileFeaturesExample.getAndIncrement();}});thread1.start();Thread thread2 = new Thread(new Runnable() {public void run() {long l = newVolatileFeaturesExample.get();System.out.println("创建的l值-------"+ l);}});thread2.start();}
}