Java线程执行顺序小结及线程池终止判定

以下有4种测试情况,分别为test1~4.

import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;import org.junit.Test;public class MyTest {@Testpublic void test1() {// test1:当线程池中所有的线程执行完毕后,才退出主线程.ExecutorService pool = Executors.newCachedThreadPool();for (int i = 0; i < 10; i++) {final int temp = i;pool.execute(new Runnable() {@Overridepublic void run() {// TODO Auto-generated method stubtry {Thread.sleep(3000);} catch (InterruptedException e) {// TODO Auto-generated catch blocke.printStackTrace();}System.out.println(temp + " finished");}});}pool.shutdown();// 停止加入新的线程while (!pool.isTerminated()) {// 如果所有线程执行完成,那么挑出该循环.}System.out.println("main thread is done");}@Testpublic void test2() {// test2:这种情况比较简单,先启动新线程,然后延迟2s,主线程退出.new Thread(new Runnable() {@Overridepublic void run() {// TODO Auto-generated method stubSystem.out.println("new Thread");}}).start();delay(2000);}@Testpublic void test3() {// test3:这种情况就像麻烦了,如果你的程序子线程中操作时间太长,生命周期比主线程还长,// 那么主线程结束了,子线程自然也结束了.得不到子线程中想要的结果.// 测试结果:等待2s后,输出delay is over,并不会输出new Thread.因为主线程已经结束了.// 如果想保持子线程运行,则需要加入守护线程,或者阻塞主线程,等待子线程完成.new Thread(new Runnable() {@Overridepublic void run() {// TODO Auto-generated method stubdelay(3000);System.out.println("new Thread");}}).start();delay(2000);// 延迟函数}@Testpublic void test4() {// test4:先来说一下输出结果:1.先输出delay is over;2.再输出new Thread.//执行到delay时候,由于主线程被阻塞,所以只能等待delay执行完后才轮到new Threaddelay(2000);// 延迟函数new Thread(new Runnable() {@Overridepublic void run() {// TODO Auto-generated method stubSystem.out.println("new Thread");}}).start();}private void delay(long ms) {// TODO Auto-generated method stubtry {Thread.sleep(ms);} catch (InterruptedException e) {// TODO Auto-generated catch blocke.printStackTrace();}System.out.println("delay is over");}}

总结:

1.使用ThreadPool时,先shutDown(不再接受新的线程进入线程池),再通过死循环判断isTerminated(阻塞主线程)  即可判定所有线程是否执行完毕.

2.如果子线程的生命周期比主线程长,应该给子线程加入守护线程,或者阻塞主线程,等待子线程完成(这和Android中的ANR类似..呵呵)

3.切记,不要以为线程在任何地方都是new,start之后就开启的.就如同上述代码的test4部分.所以开启线程时,不要放在不对的位置.