Java多线程总结四:volatile、synchronized示例
Java多线程总结四:volatile、synchronized示例
1、synchronized保证同步
先看一个生成偶数的类
- package demo.thread;
- /**
- *这是一个int生成器的抽象类
- *
- */
- public abstract class IntGenerator {
- private volatile boolean canceled = false;
- public abstract int next();
- public void cancel() {
- canceled = true;
- }
- public boolean isCanceled() {
- return canceled;
- }
- }
- /*
- * 产生偶数
- */
- class EvenGenerator extends IntGenerator {
- private int currentEvenValue = 0;
- String s = "";
- @Override
- public int next() {
- <span style="color:#ff0000;">synchronized </span>(s) {
- ++currentEvenValue;
- ++currentEvenValue;
- return currentEvenValue;
- }
- }
- // //这样也可以
- // public <span style="color:#ff0000;">synchronized </span>int next() {
- // ++currentEvenValue;
- // ++currentEvenValue;
- // return currentEvenValue;
- // }
- }
注意到在产生偶数是要加同步锁,否则可能线程1刚好执行了一句++currentEvenValue;操作,就被线程2抢去了cpu,此时线程2执行return currentEvenValue;这时返回的就是一个奇数。加synchronized 就是两个线程同时只能一个线程执行synchronized 块的代码。
测试代码:
- package demo.thread;
- import java.util.concurrent.ExecutorService;
- import java.util.concurrent.Executors;
- /*
- * 消费数字
- */
- public class EvenChecker implements Runnable {
- private IntGenerator generator;
- private final int id;
- public EvenChecker(IntGenerator g, int ident) {
- generator = g;
- id = ident;
- }
- public void run() {
- while (!generator.isCanceled()) {
- int val = generator.next();
- if (val % 2 != 0) {//如果不是偶数
- System.out.println(val + " not enen!");
- generator.cancel();
- }
- }
- }
- public static void test(IntGenerator gp, int count) {
- ExecutorService exec = Executors.newCachedThreadPool();
- for (int i = 0; i < count; i++)
- exec.execute(new EvenChecker(gp, i));
- exec.shutdown();
- }
- public static void test(IntGenerator gp) {
- test(gp, 10);
- }
- public static void main(String[] args) {
- test(new EvenGenerator());
- }
分析:如果产生偶数的类未加synchronized,那么测试程序将会出现奇数导致退出程序。
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