volatile修饰符修饰变量，不可以修饰类和方法，volatile不是线程安全的，但是一写多读的多线程情况下是线程安全的。
多线程的特性：
- 可见性 各线程之间的变量会同步到主线程
- 原子性
volatile是可见的，但不是原子的。
synchronized是可见的也是原子的。
一个线程写，多个线程读可以使用volatile，这样的情况下的多线程是安全的。
多个线程写，多个线程读不能使用volatile，这样的情况下的多线程是不安全的。
package crelle.test.juc; /** * 一个线程写，多个线程读 线程安全 */ public class VolatileSingleWriterTest { private static volatile int MY_INT = 0; public static void main(String[] args) { new ChangeListener().start(); new ChangeListener().start(); new ChangeListener().start(); new ChangeListener().start(); new ChangeListener().start(); new ChangeListener().start(); new ChangeMaker().start(); } static class ChangeListener extends Thread { @Override public void run() { int local_value = MY_INT; while ( local_value < 5){ if( local_value!= MY_INT){ System.out.println("Got Change for MY_INT : "+ MY_INT); local_value= MY_INT; } } } } static class ChangeMaker extends Thread{ @Override public void run() { int local_value = MY_INT; while (MY_INT <5){ System.out.println("Incrementing MY_INT to : "+( local_value+1)); MY_INT = ++local_value; try { Thread.sleep(500); } catch (InterruptedException e) { e.printStackTrace(); } } } } }
结果： Incrementing MY_INT to : 1 Got Change for MY_INT : 1 Got Change for MY_INT : 1 Got Change for MY_INT : 1 Got Change for MY_INT : 1 Got Change for MY_INT : 1 Got Change for MY_INT : 1 Incrementing MY_INT to : 2 Got Change for MY_INT : 2 Got Change for MY_INT : 2 Got Change for MY_INT : 2 Got Change for MY_INT : 2 Got Change for MY_INT : 2 Got Change for MY_INT : 2 Incrementing MY_INT to : 3 Got Change for MY_INT : 3 Got Change for MY_INT : 3 Got Change for MY_INT : 3 Got Change for MY_INT : 3 Got Change for MY_INT : 3 Got Change for MY_INT : 3 Incrementing MY_INT to : 4 Got Change for MY_INT : 4 Got Change for MY_INT : 4 Got Change for MY_INT : 4 Got Change for MY_INT : 4 Got Change for MY_INT : 4 Got Change for MY_INT : 4 Incrementing MY_INT to : 5 Got Change for MY_INT : 5 Got Change for MY_INT : 5 Got Change for MY_INT : 5 Got Change for MY_INT : 5 Got Change for MY_INT : 5 Got Change for MY_INT : 5 Process finished with exit code 0
package crelle.test.juc; /** * 多个线程写，多个线程读 线程不安全 */ public class VolatileMultiWriterTest { private static volatile int MY_INT = 0; public static void main(String[] args) { new ChangeListener().start(); new ChangeMaker().start(); new ChangeMaker().start(); new ChangeMaker().start(); new ChangeMaker().start(); new ChangeMaker().start(); new ChangeMaker().start(); } static class ChangeListener extends Thread { @Override public void run() { int local_value = MY_INT; while ( local_value < 5){ if( local_value!= MY_INT){ System.out.println("Got Change for MY_INT : "+ MY_INT); local_value= MY_INT; } } } } static class ChangeMaker extends Thread{ @Override public void run() { int local_value = MY_INT; while (MY_INT <5){ System.out.println("Incrementing MY_INT to : "+( local_value+1)); MY_INT = ++local_value; try { Thread.sleep(500); } catch (InterruptedException e) { e.printStackTrace(); } } } } }
结果：
Incrementing MY_INT to : 1 Incrementing MY_INT to : 1 Incrementing MY_INT to : 1 Got Change for MY_INT : 1 Incrementing MY_INT to : 2 Incrementing MY_INT to : 2 Incrementing MY_INT to : 2 Got Change for MY_INT : 2 Incrementing MY_INT to : 2 Incrementing MY_INT to : 2 Incrementing MY_INT to : 3 Incrementing MY_INT to : 2 Incrementing MY_INT to : 3 Incrementing MY_INT to : 3 Got Change for MY_INT : 3 Incrementing MY_INT to : 3 Incrementing MY_INT to : 3 Incrementing MY_INT to : 4 Incrementing MY_INT to : 3 Incrementing MY_INT to : 4 Incrementing MY_INT to : 4 Got Change for MY_INT : 4 Incrementing MY_INT to : 4 Incrementing MY_INT to : 4 Incrementing MY_INT to : 5 Got Change for MY_INT : 5 Incrementing MY_INT to : 4 Incrementing MY_INT to : 5 Incrementing MY_INT to : 5 Process finished with exit code 0