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