守护线程只是个概念问题，一句话可以总结(不知道总结的对不对^_^);

当所有用户线程都结束的时候，守护线程也就结束了，当有用户线程存在的时候，守护线程就是一个普通线程。

main线程不可以设置成守护线程，应为只有在线程调用start方法前，才可以设置线程为守护线程，main线程是jvm创建的

多线程以及线程池的问题

import java.io.DataInputStream;

import java.io.File;

import java.io.FileOutputStream;

import java.io.IOException;

import java.net.MalformedURLException;

import java.net.URL;

import java.util.ArrayList;

import java.util.List;

import java.util.concurrent.ExecutorService;

import java.util.concurrent.Executors;

public class Down {

ExecutorService executorService = Executors.newFixedThreadPool(3);

public void down() {

boolean a = new File("C:/123/").mkdir();

List urls = new ArrayList();

urls.add("http://img1.cache.netease.com/catchpic/6/66/66F01DF5FA318F96FE695D4A1CA1A698.jpg");

urls.add("http://img1.cache.netease.com/catchpic/C/C5/C5625E5D94AFBBF73DBB8C88D762A634.jpg");

urls.add("http://img1.cache.netease.com/catchpic/C/C5/C5625E5D94AFBBF73DBB8C88D762A634.jpg");

urls.add("http://img1.cache.netease.com/catchpic/C/C5/C5625E5D94AFBBF73DBB8C88D762A634.jpg");

urls.add("http://img1.cache.netease.com/catchpic/C/C5/C5625E5D94AFBBF73DBB8C88D762A634.jpg");

urls.add("http://img1.cache.netease.com/catchpic/C/C5/C5625E5D94AFBBF73DBB8C88D762A634.jpg");

urls.add("http://img1.cache.netease.com/catchpic/C/C5/C5625E5D94AFBBF73DBB8C88D762A634.jpg");

urls.add("http://img1.cache.netease.com/catchpic/C/C5/C5625E5D94AFBBF73DBB8C88D762A634.jpg");

urls.add("http://img1.cache.netease.com/catchpic/C/C5/C5625E5D94AFBBF73DBB8C88D762A634.jpg");

urls.add("http://img1.cache.netease.com/catchpic/C/C5/C5625E5D94AFBBF73DBB8C88D762A634.jpg");

urls.add("http://img1.cache.netease.com/catchpic/C/C5/C5625E5D94AFBBF73DBB8C88D762A634.jpg");

for (int i = 0; i < urls.size(); i++) {

executorService.execute(new DownImg(urls.get(i), i));

}

}

public static void main(String[] args) {

Down down = new Down();

down.down();

// down.executorService.isTerminated()

down.executorService.shutdown();

while (true) {

if (down.executorService.isTerminated()) {

System.out.println("所有的已经下载完毕");

break;

}

try {

Thread.sleep(100);

} catch (InterruptedException e) {

// TODO Auto-generated catch block

e.printStackTrace();

}

}

}

}

class DownImg implements Runnable {

int i;

String imgUrl;

URL url;

int len;

byte[] bytes = new byte[1024];

public DownImg(String imgUrl, int i) {

this.i = i;

this.imgUrl = imgUrl;

try {

url = new URL(imgUrl);

} catch (MalformedURLException e) {

e.printStackTrace();

}

}

@Override

public void run() {

try {

DataInputStream dataInputStream = new DataInputStream(

url.openStream());

FileOutputStream fileOutputStream = new FileOutputStream("C:/123/"

+ i + ".jpg");

while ((len = dataInputStream.read(bytes)) > 0) {

fileOutputStream.write(bytes, 0, len);

}

dataInputStream.close();

fileOutputStream.close();

System.out.println("第" + i + "张下载完毕");

} catch (IOException e) {

// TODO Auto-generated catch block

e.printStackTrace();

}

}

}

down.executorService.shutdown();这个方法，是在线程池中的线程只执行一次任务；

如果没有调用，线程池的线程执行完一次任务后，会一直处于等待状态，还要注意的是这个方法是在for语句执行完毕后，才会执行的