问题1：Volatile有什么作用？
package com.victor.hello;
import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.TimeUnit;
public class VolatileTest {
private static volatile int volatileCounter = 0;
private static int noneVolatileCounter = 0;
public static void main(String[] args){
final ScheduledExecutorService service = Executors.newScheduledThreadPool(10);
for(int i =0;i<10;i++){
service.scheduleAtFixedRate(new Runnable(){
@Override
public void run() {
String threadName = Thread.currentThread().getName();
volatileCounter++;
sleep();
volatileCounter--;
noneVolatileCounter++;
sleep();
noneVolatileCounter--;
System.out.println(volatileCounter+" "+noneVolatileCounter+" ["+threadName+"]");
}
}, 0, 3, TimeUnit.SECONDS);
}
}
private static void sleep(){
try {
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
为了体验Volatile这个关键字的作用，我写了一个测试方法。两个int类型的变量，分别用volatile和不用volatile修饰。先做一个++的操作，再做一个--的操作。之间休息0.1秒。起十个线程，定时的操作。
有经验的同学一看就知道，这么操作觉得线程不安全。让我们看看执行的结果。
0 9 [pool-1-thread-9]
0 8 [pool-1-thread-7]
0 7 [pool-1-thread-5]
0 6 [pool-1-thread-3]
0 5 [pool-1-thread-1]
0 4 [pool-1-thread-2]
0 3 [pool-1-thread-4]
0 2 [pool-1-thread-8]
0 1 [pool-1-thread-6]
0 0 [pool-1-thread-10]
1 9 [pool-1-thread-3]
1 8 [pool-1-thread-5]
1 7 [pool-1-thread-9]
1 6 [pool-1-thread-7]
1 5 [pool-1-thread-2]
1 4 [pool-1-thread-4]
1 3 [pool-1-thread-1]
1 2 [pool-1-thread-6]
1 1 [pool-1-thread-8]
1 0 [pool-1-thread-10]
1 9 [pool-1-thread-1]
1 8 [pool-1-thread-3]
1 7 [pool-1-thread-5]
1 5 [pool-1-thread-9]
1 5 [pool-1-thread-7]
1 4 [pool-1-thread-4]
1 3 [pool-1-thread-8]
1 2 [pool-1-thread-2]
1 1 [pool-1-thread-6]
1 0 [pool-1-thread-10]
1 9 [pool-1-thread-7]
1 8 [pool-1-thread-5]
1 7 [pool-1-thread-9]
1 6 [pool-1-thread-8]
1 5 [pool-1-thread-4]
1 4 [pool-1-thread-1]
1 3 [pool-1-thread-2]
1 2 [pool-1-thread-3]
1 1 [pool-1-thread-6]
1 0 [pool-1-thread-10]
1 9 [pool-1-thread-3]
1 8 [pool-1-thread-7]
1 7 [pool-1-thread-5]
1 6 [pool-1-thread-4]
1 5 [pool-1-thread-8]
1 4 [pool-1-thread-1]
1 3 [pool-1-thread-9]
1 3 [pool-1-thread-2]
1 1 [pool-1-thread-6]
1 1 [pool-1-thread-10]
1 9 [pool-1-thread-7]
1 8 [pool-1-thread-3]
1 7 [pool-1-thread-9]
1 7 [pool-1-thread-5]
1 6 [pool-1-thread-1]
1 5 [pool-1-thread-4]
1 4 [pool-1-thread-8]
1 3 [pool-1-thread-2]
1 2 [pool-1-thread-6]
1 1 [pool-1-thread-10]
1 10 [pool-1-thread-4]
1 9 [pool-1-thread-7]
1 8 [pool-1-thread-1]
1 7 [pool-1-thread-3]
1 6 [pool-1-thread-8]
1 5 [pool-1-thread-2]
1 4 [pool-1-thread-9]
1 2 [pool-1-thread-5]
1 2 [pool-1-thread-10]
1 1 [pool-1-thread-6]
可见，每次并发的时候。Volatile的修改都能迅速的让其他线程感知到。也就是线程间的可见性。
但几次并发以后，它就忍不住线程不安全了。可见并没有保证线程的安全。
关于Volatile关键字的研究