多线程 线程池 守护线程

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<String> urls = new ArrayList<String>();
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