题目:实现一个Runnable。在run()内部打印一个消息,然后调用yield()。重复这个操作三次,然后从run中返回。在构造器中放置一条启动消息,并且放置一条在任务终止时的关闭消息。使用线程创建大量的这种任务并驱动他们。
package test; public class test { public static void main(String[] args) { // TODO Auto-generated method stub for(int i=1;i<=10;i++){ new Thread(new runabletest()).start();; } } } class runabletest implements Runnable{ public String startstr="start!"; public String stopstr="stop!"; static int num=0; public int id; public runabletest(){ num++; id=num; startstr="#"+id+" "+startstr; stopstr="#"+id+" "+stopstr; System.out.println(startstr); } @Override public void run() { for(int i=1;i<=3;i++){ System.out.println("#"+id+" loop"+i); Thread.yield(); } System.out.println(stopstr); } }
结果是:
#1 start! #2 start! #3 start! #1 loop1 #2 loop1 #2 loop2 #1 loop2 #1 loop3 #1 stop! #2 loop3 #2 stop! #4 start! #3 loop1 #3 loop2 #5 start! #4 loop1 #3 loop3 #4 loop2 #3 stop! #4 loop3 #4 stop! #6 start! #7 start! #5 loop1 #5 loop2 #5 loop3 #5 stop! #7 loop1 #6 loop1 #8 start! #6 loop2 #7 loop2 #7 loop3 #6 loop3 #6 stop! #7 stop! #9 start! #8 loop1 #8 loop2 #8 loop3 #8 stop! #10 start! #9 loop1 #9 loop2 #9 loop3 #9 stop! #10 loop1 #10 loop2 #10 loop3 #10 stop!
如果把yield去掉的话结果是:
#1 start! #2 start! #3 start! #1 loop1 #1 loop2 #1 loop3 #1 stop! #4 start! #2 loop1 #5 start! #2 loop2 #2 loop3 #2 stop! #5 loop1 #5 loop2 #5 loop3 #5 stop! #3 loop1 #3 loop2 #3 loop3 #3 stop! #4 loop1 #4 loop2 #4 loop3 #4 stop! #6 start! #7 start! #8 start! #7 loop1 #7 loop2 #7 loop3 #7 stop! #6 loop1 #6 loop2 #6 loop3 #6 stop! #9 start! #10 start! #9 loop1 #9 loop2 #9 loop3 #9 stop! #10 loop1 #10 loop2 #10 loop3 #10 stop! #8 loop1 #8 loop2 #8 loop3 #8 stop!
结论:我们发现把yield去掉之后每个线程的三次循环基本都一下子输出了,因为任务比较简单可以在一个时间片内完成,所以在中途没有切换上下文。而增加了yield之后,执行完一次循环之后,该线程有很大几率被切换掉。