volatile关键字的主要作用是使变量在多个线程间可见。
jdk1.5之后,每个线程执行的时候线程工作空间会持有主内存的一个数据拷贝。运行的过程中,直接用线程工作空间的拷贝值。
1.可见性的证明
没加volatile
public class RunThread extends Thread {
private boolean isRunning = true;
private void setRunning(boolean running) {
isRunning = running;
}
@Override
public void run() {
System.out.println("进入run方法");
while (isRunning == true) {
//
}
System.out.println("线程停止");
}
public static void main(String[] args) throws InterruptedException {
RunThread rt = new RunThread();
rt.start();
Thread.sleep(3000);
rt.setRunning(false);
System.out.println("isRunning的值已经被设置了false");
Thread.sleep(1000);
System.out.println(rt.isRunning);
}
}
线程没停止,说明正在执行的线程读到的isRunning依然是线程工作空间的值。主内存中isRunning的值没有更新到线程工作空间。
加volatile
public class RunThread extends Thread {
private volatile boolean isRunning = true;
private void setRunning(boolean running) {
isRunning = running;
}
@Override
public void run() {
System.out.println("进入run方法");
while (isRunning == true) {
//
}
System.out.println("线程停止");
}
public static void main(String[] args) throws InterruptedException {
RunThread rt = new RunThread();
rt.start();
Thread.sleep(3000);
rt.setRunning(false);
System.out.println("isRunning的值已经被设置了false");
Thread.sleep(1000);
System.out.println(rt.isRunning);
}
}
isRunning变量如果被volatile修饰,则当变量改变时强制线程执行引擎去主内存里去读取。
2.不能保证原子性
public class VolatileNoAtomic extends Thread {
private static volatile int count;
private static void addCount() {
for (int i = 0; i < 10000; i++) {
count++;
}
System.out.println(count);
}
@Override
public void run() {
addCount();
}
public static void main(String[] args) {
VolatileNoAtomic[] arr = new VolatileNoAtomic[10];
for (int i = 0; i < 10; i++) {
arr[i] = new VolatileNoAtomic();
}
for (int i = 0; i < 10; i++) {
arr[i].start();
}
}
}
最终的结果不是10000
怎么解决?
public class VolatileNoAtomic extends Thread {
private static volatile AtomicInteger count = new AtomicInteger(0);
private static void addCount() {
for (int i = 0; i < 10000; i++) {
count.incrementAndGet();
}
System.out.println(count);
}
@Override
public void run() {
addCount();
}
public static void main(String[] args) {
VolatileNoAtomic[] arr = new VolatileNoAtomic[10];
for (int i = 0; i < 10; i++) {
arr[i] = new VolatileNoAtomic();
}
for (int i = 0; i < 10; i++) {
arr[i].start();
}
}
}
关键字volatile只具有可见性,没有原子性。要实现原子性建议使用atomic类的系列对象,支持原子操作(注意atomic类只保证本身方法原子性,并不保证多次操作的原子性)。