java.util.concurrent 范例之一
java.util.concurrent 之一
package test;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
public class TestThreadPool {
public static void main(String args[]) throws InterruptedException{
// only two threads
ExecutorService exec = Executors.newFixedThreadPool(2);
for(int index=0; index < 10; index++){
Runnable run = new Runnable(){
public void run(){
long time = (long)(Math.random() * 1000);
System.out.println("Sleeping " + time + "ms");
try{
Thread.sleep(time);
} catch(InterruptedException e){
}
}
};
exec.execute(run);
} // must shutdown
exec.shutdown();
}
}
可能的运行结果:
Sleeping 543ms
Sleeping 33ms
Sleeping 658ms
Sleeping 740ms
Sleeping 145ms
Sleeping 216ms
Sleeping 890ms
Sleeping 864ms
Sleeping 786ms
Sleeping 644ms
上面是一个简单的例子。使用大小为2的线程池来处理10个线程。
它使用了Executors的静态函数 newFixedThreadPool() 生成一个固定的线程池。顾名思义,
线程池是不会释放的,即使它是Idle。这就会产生性能问题。比如如果线程池的大小为200,当全部使用完毕后,
所有的线程会继续留在池中,相应的内存和线程切换(while(true)+sleep)都会增加。如果要避免这个问题,就
必须直接使用ThreadPoolExecutor()来构造。可以像Tomcat的线程池一样,设置“最大线程数”、“最小线程数”
和“空闲线程keepAlive的时间”。通过这些可以基本上替换Tomcat的线程池实现方案。
需要注意的是线程池必须使用shutdown来显式关闭,否则主线程就无法退出。
java.util.concurrent 范例之二:周期性运行
Java代码
package test;
import static java.util.concurrent.TimeUnit.SECONDS;
import java.util.Date;
import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.ScheduledFuture;
public class TestScheduledThread {
/**
* @param args
*/
public static void main(String[] args) {
final ScheduledExecutorService scheduler = Executors.newScheduledThreadPool(2);
final Runnable beeper = new Runnable() {
int count = 0;
public void run() {
System.out.println(new Date() + " beep " + (++count));
}
};
// 1秒钟后运行,并每隔2秒运行一次
final ScheduledFuture beeperHandle = scheduler.scheduleAtFixedRate(beeper, 1, 2, SECONDS);
// 2秒钟后运行,并每次在上次任务运行完后等待5秒后重新运行
final ScheduledFuture beeperHandle2 = scheduler.scheduleWithFixedDelay(beeper, 2, 5, SECONDS);
// 30秒后结束关闭任务,并且关闭Scheduler
scheduler.schedule(new Runnable() {
public void run() {
beeperHandle.cancel(true);
beeperHandle2.cancel(true);
scheduler.shutdown();
}
}, 30, SECONDS);
}
}
可能的运行结果:
Java代码
Mon Jun 06 21:26:52 CST 2011 beep 1
Mon Jun 06 21:26:53 CST 2011 beep 2
Mon Jun 06 21:26:54 CST 2011 beep 3
Mon Jun 06 21:26:56 CST 2011 beep 4
Mon Jun 06 21:26:58 CST 2011 beep 5
Mon Jun 06 21:26:58 CST 2011 beep 6
Mon Jun 06 21:27:00 CST 2011 beep 7
Mon Jun 06 21:27:02 CST 2011 beep 8
Mon Jun 06 21:27:03 CST 2011 beep 9
Mon Jun 06 21:27:04 CST 2011 beep 10
Mon Jun 06 21:27:06 CST 2011 beep 11
Mon Jun 06 21:27:08 CST 2011 beep 12
Mon Jun 06 21:27:08 CST 2011 beep 13
Mon Jun 06 21:27:10 CST 2011 beep 14
Mon Jun 06 21:27:12 CST 2011 beep 15
Mon Jun 06 21:27:13 CST 2011 beep 16
Mon Jun 06 21:27:14 CST 2011 beep 17
Mon Jun 06 21:27:16 CST 2011 beep 18
Mon Jun 06 21:27:18 CST 2011 beep 19
Mon Jun 06 21:27:18 CST 2011 beep 20
Mon Jun 06 21:27:20 CST 2011 beep 21
许多长时间运行的应用有时候需要定时运行任务完成一些诸如统计、优化等工作,比如在电信行业中处理用户话单时,需要每隔1分钟处理话单;网站每天凌晨统计用户访问量、用户数;大型超市凌晨3点统计当天销售额、以及最热卖的商品;每周日进行数据库备份;公司每个月的10号计算工资并进行转帐等,这些都是定时任务。通过 java 的并发库concurrent可以轻松的完成这些任务,而且非常的简单。
为了退出进程,上面的代码中加入了关闭Scheduler的操作。而对于24小时运行的应用而言,是没有必要关闭Scheduler的。
java.util.concurrent 之三: 出发--等待--汇合--再出发
在实际应用中,有时候需要多个线程同时工作以完成同一件事情,而且在完成过程中,往往会等待其他线程都完成某一阶段后再执行,等所有线程都到达某一个阶段后再统一执行。
比如有几个旅行团需要途经深圳、广州、韶关、长沙最后到达武汉。旅行团中有自驾游的,有徒步的,有乘坐旅游大巴的;这些旅行团同时出发,并且每到一个目的地,都要等待其他旅行团到达此地后再同时出发,直到都到达终点站武汉。
这时候 CyclicBarrier 就可以派上用场。CyclicBarrier最重要的属性就是参与者个数,另外最要方法是await()。当所有线程都调用了await()后,就表示这些线程都可以继续执行,否则就会等待。
Java代码
package test;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.concurrent.BrokenBarrierException;
import java.util.concurrent.CyclicBarrier;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
public class TestCyclicBarrier{
// 徒步需要的時間
private static int[] timeWalk = { 5, 8, 15, 15, 10 };
// 自駕遊
private static int[] timeSelf = { 1, 3, 4, 4, 5 };
// 旅遊大巴
private static int[] timeBus = { 2,4,6,6,7 };
static String now(){
SimpleDateFormat sdf = new SimpleDateFormat("HH:mm:ss");
return sdf.format(new Date()) + ": ";
}
static class Tour implements Runnable{
private int[] times;
private CyclicBarrier barrier;
private String tourName;
public Tour(CyclicBarrier barrier, String tourName, int[] times){
this.times=times;
this.tourName=tourName;
this.barrier=barrier;
}
public void run(){
try{
Thread.sleep(times[0]*1000);
System.out.println(now() + tourName + " Reached Shenzhen");
barrier.await();
Thread.sleep(times[1]*1000);
System.out.println(now() + tourName + " Reached Guangzhou");
barrier.await();
Thread.sleep(times[2]*1000);
System.out.println(now() + tourName + " Reached Shaoguan");
barrier.await();
Thread.sleep(times[3]*1000);
System.out.println(now() + tourName + " Reached Changsha");
barrier.await();
Thread.sleep(times[4]*1000);
System.out.println(now() + tourName + " Reached Wuhan");
barrier.await();
}catch(InterruptedException e){
}catch(BrokenBarrierException e){
}
}
}
public static void main(String[] args){
// 三個旅行團
CyclicBarrier barrier = new CyclicBarrier(3);
ExecutorService exec = Executors.newFixedThreadPool(3);
exec.submit(new Tour(barrier, "WalkTour", timeWalk));
exec.submit(new Tour(barrier, "SelfTour", timeSelf));
//
exec.submit(new Tour(barrier, "BusTour", timeBus));
exec.shutdown();
}
}
可能的执行结果:
1. 21:47:18: SelfTour Reached Shenzhen
2. 21:47:19: BusTour Reached Shenzhen
3. 21:47:22: WalkTour Reached Shenzhen
4. 21:47:25: SelfTour Reached Guangzhou
5. 21:47:26: BusTour Reached Guangzhou
6. 21:47:30: WalkTour Reached Guangzhou
7. 21:47:34: SelfTour Reached Shaoguan
8. 21:47:36: BusTour Reached Shaoguan
9. 21:47:45: WalkTour Reached Shaoguan
10. 21:47:49: SelfTour Reached Changsha
11. 21:47:51: BusTour Reached Changsha
12. 21:48:00: WalkTour Reached Changsha
13. 21:48:05: SelfTour Reached Wuhan
14. 21:48:07: BusTour Reached Wuhan
- 21:48:10: WalkTour Reached Wuhan
java.util.concurrent 之四:LinkedBlockingQueue
并发库中的BlockingQueue是一个比较好玩的类,顾名思义,就是阻塞队列。该类主要提供了两个方法put()和take(),前者将一个对象放到队列尾部,如果队列已经满了,就等待直到有空闲节点;后者从head取一个对象,如果没有对象,就等待直到有可取的对象。
Java代码
package test;
import java.util.concurrent.BlockingQueue;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.LinkedBlockingQueue;
public class MyBlockingQueue extends Thread{
public static BlockingQueue<String> queue=new LinkedBlockingQueue<String>(3);
private int index;
public MyBlockingQueue(int i){
this.index=i;
}
public void run(){
try{
queue.put(String.valueOf(this.index));
System.out.println("put {"+this.index+"} into queue!");
}catch(Exception e){
e.printStackTrace();
}
}
public static void main(String args[]){
ExecutorService service=Executors.newCachedThreadPool();
for( int i=0; i<10; i++){
service.submit(new MyBlockingQueue(i));
}
Thread thread = new Thread(){
public void run(){
try{
while(true){
Thread.sleep((int)(Math.random()*1000));
if(MyBlockingQueue.queue.isEmpty()) break;
String str=MyBlockingQueue.queue.take();
System.out.println("take {" + str+"} out of queue!");
}
}catch(Exception e){
e.printStackTrace();
}
}
};
service.submit(thread);
service.shutdown();
}
}
可能的运行结果:
1. put {0} into queue!
2. put {1} into queue!
3. put {3} into queue!
4. take {0} out of queue!
5. put {2} into queue!
6. take {1} out of queue!
7. put {7} into queue!
8. take {3} out of queue!
9. put {5} into queue!
10. take {2} out of queue!
11. put {4} into queue!
12. take {7} out of queue!
13. put {6} into queue!
14. put {9} into queue!
15. take {5} out of queue!
16. take {4} out of queue!
17. put {8} into queue!
18. take {6} out of queue!
19. take {9} out of queue!
20. take {8} out of queue!
java.util.concurrent 之五:倒计数的锁
从名字可以看出,CountDownLatch是一个倒数计数的锁,当倒数到0时触发事件,也就是开锁,其他人就可以进入了。在一些应用场合中,需要等待某个条件达到要求后才能做后面的事情;同时当线程都完成后也会触发事件,以便进行后面的操作。
CountDownLatch最重要的方法是countDown()和await(),前者主要是倒数一次,后者是等待倒数到0,如果没有到达0,就只有阻塞等待了。
一个CountDownLatch实例是不能重复使用的,也就是说它是一次性的,锁一经被打开就不能再关闭使用了,如果想重复使用,请考虑使用CyclicBarrier。
下面的例子简单的说明了CountDownLatch的使用方法,模拟了100米赛跑,10名选手已经准备就绪,只等裁判一声令下。当所有人都到达终点时,比赛结束。
同样,线程池需要显式shutdown。
Java代码
package test;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
public class TestCountDownLatch{
public static void main(String[] args) throws InterruptedException{
//开始的倒数锁
final CountDownLatch begin=new CountDownLatch(1);
//结束的倒数锁
final CountDownLatch end=new CountDownLatch(10);
//10名选手
final ExecutorService exec=Executors.newFixedThreadPool(10);
for(int index=0; index<10;index++){
final int NO=index + 1;//Cannot refer to a non-final variable NO inside an inner class defined in a different method
Runnable run=new Runnable(){
public void run(){
try{
begin.await();//一直阻塞
Thread.sleep((long)(Math.random() * 10000));
System.out.println("No." + NO + " arrived");
}catch(InterruptedException e){
}finally{
end.countDown();
}
}
};
exec.submit(run);
}
System.out.println("Game Start");
begin.countDown();
end.await();
System.out.println("Game Over");
exec.shutdown();
}
}
可能的结果:
1. Game Start
2. No.4
3. No.6
4. No.8
5. No.5
6. No.10
7. No.3
8. No.2
9. No.7
10. No.1
11. No.9
12. Game Over
java.util.concurrent 之六:使用Future类和Callable类
有时候在实际应用中,某些操作很耗时,但又不是不可或缺的步骤。比如用网页浏览器浏览新闻时,最重要的是显示文字内容,至于与新闻相匹配的图片就没有那么重要的,所以此时首先保证文字信息先显示,而图片信息会后显示,但又不能不显示,由于下载图片是一个耗时的操作,所以必须一开始就得下载。
Java的并发库的
Future类就可以满足这个要求。Future的重要方法包括
get()和cancel(),get()获取数据对象,如果数据没有加载,就会阻塞直到取到数据,而
cancel()是取消数据加载。另外一个get(timeout)操作,表示如果在timeout时间内没有取到就失败返回,而不再阻塞。
下面的Demo简单的说明了
Future的使用方法:一个非常耗时的操作必须一开始启动,但又不能一直等待;其他重要的事情又必须做,等完成后,就可以做不重要的事情。
Java代码
package concurrent;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
public class TestFutureTask {
public static void main(String[] args) throws InterruptedException,ExecutionException{
final ExecutorService exec = Executors.newFixedThreadPool(5);
Callable call = new Callable(){
public String call() throws Exception{
Thread.sleep(1000 * 5);
return "Other less important but longtime things.";
}
};
Future task = exec.submit(call);
//重要的事情
Thread.sleep(1000 * 3);
System.out.println("Let's do important things.");
//其他不重要的事情
String obj = (String)task.get();
System.out.println(obj);
//关闭线程池
exec.shutdown();
}
}
运行结果:
Let's do important things.
Other less important but longtime things.
java.util.concurrent 之七:先下载完成的图片先显示
考虑以下场景:浏览网页时,浏览器用了5个线程下载网页中的图片文件,由于图片大小、网站访问速度等诸多因素的影响,完成图片下载的时间就会有很大的不同。先下载完成的图片就会被先显示到界面上,反之,后下载的图片就后显示。
Java的并发库的CompletionService可以满足这种场景要求。该接口有两个重要方法:submit()和take()。submit用于提交一个runnable或者callable,一般会提交给一个线程池处理;而take就是取出已经执行完毕runnable或者callable实例的Future对象,如果没有满足要求的,就等待了。
CompletionService还有一个对应的方法poll,该方法与take类似,只是不会等待,如果没有满足要求,就返回null对象。
Java代码
package concurrent;
import java.util.concurrent.Callable;
import java.util.concurrent.CompletionService;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorCompletionService;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
public class TestCompletionService {
public static void main(String[] args) throws InterruptedException, ExecutionException{
ExecutorService exec = Executors.newFixedThreadPool(10);
CompletionService serv = new ExecutorCompletionService(exec);
for(int index = 0; index < 5; index++){
final int NO = index;
Callable downImg = new Callable(){
public String call() throws Exception{
Thread.sleep((long)(Math.random() * 10000));
return "Downloaded Image " + NO;
}
};
serv.submit(downImg);
}
Thread.sleep(1000*2);
System.out.println("Show web content");
for(int index=0; index<5; index++){
Future task = serv.take();
String img = (String)task.get();
System.out.println(img);
}
System.out.println("End");
//关闭线程池
exec.shutdown();
}
}
运行结果:
1. Show web content
2. Downloaded Image 2
3. Downloaded Image 3
4. Downloaded Image 0
5. Downloaded Image 4
6. Downloaded Image 1
7. End
标签:java,21,util,concurrent,import,public
From: https://blog.51cto.com/u_16174476/6840276