为了体会守护线程的作用，我做了一个下载文件的demo，可以没事用来测测网速什么的，其特性如下
1、一共有三个线程，分别是主线程，下载线程，守护线程
2、主线程启动下载线程和守护线程
3、下载线程连续下载100个文件，如果出现异常自动捕获并进入下一个文件的下载
4、如果下载线程下载某个文件超过了30秒，就认为是超时，而这个超时的检测由守护线程执行
5、如果守护线程发现某个文件下载超时，就停掉下载线程，并想办法另起一个新的下载线程继续下载
首先我们不使用Java设置的守护线程，而是用一个普通线程充当“守护线程”，也就是设置一个计数器，下载成功一个文件就+1，加到100后这个伪“守护线程”就会自我终止，代码如下
package zz.vc.qduedu;
import java.io.BufferedInputStream;
import java.io.BufferedOutputStream;
import java.io.DataOutputStream;
import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.net.HttpURLConnection;
import java.net.URL;
import javax.net.ssl.HttpsURLConnection;
public class DownloadTest {
public static long totalDownloadTime;//全部文件共消耗的下载时间
public static long downloadStartTime;//某个文件下载开始的时间
public static DownloadThread downloadThread;
public static int completedFileCount = 0;//下载成功的文件数
public static final String url = "https://d25hz2nnwuc2tq.cloudfront.net/images/image/cache/data/2016/10/19/1476866804-800x450-1080x608.webp";
public static final int total_times = 100;
public static void main(String[] args) throws IOException {
// TODO Auto-generated method stub
downloadThread = new DownloadThread();
downloadThread.start();
AlxDemonThread demonThread = new AlxDemonThread();
demonThread.start();
}
public static class DownloadThread extends Thread{
public boolean shouldStop = false;//超时后终结此进程
public void run() {
for(int i=completedFileCount;i<total_times;i++){
if(shouldStop)return;
try {
long cost = downloadNow(url, i);
if(shouldStop)return;
System.out.println("第"+i+"次耗时"+cost);
} catch (Exception e) {
// TODO: handle exception
if(shouldStop)return;
System.out.println("下载失败");
e.printStackTrace();
}
if(shouldStop)return;
completedFileCount++;
totalDownloadTime += System.currentTimeMillis() - downloadStartTime;
downloadStartTime = 0;
}
if(!shouldStop)System.out.println("总耗时=="+totalDownloadTime);
}
}
public static class AlxDemonThread extends Thread{
@Override
public void run() {
// TODO Auto-generated method stub
super.run();
while (2>1) {
try {
Thread.sleep(1000);
// System.out.println("守护线程还活着");
} catch (InterruptedException e) {
}
if(completedFileCount == total_times)return;
if(System.currentTimeMillis() - downloadStartTime > 30*1000){
// System.out.println("第"+alreadyTime+"超时了");
System.out.println("the "+completedFileCount+" time out of time");
downloadThread.shouldStop = true;
downloadThread.interrupt();
downloadThread = new DownloadThread();
downloadThread.start();
completedFileCount++;
totalDownloadTime += 30*1000;
}
}
}
}
public static long downloadNow(String strUrl,int times) throws IOException{
downloadStartTime = System.currentTimeMillis();
URL url = new URL(strUrl);
HttpsURLConnection httpURLConnection = (HttpsURLConnection) url.openConnection();
httpURLConnection.setDoInput(true);
httpURLConnection.setUseCaches(false);
httpURLConnection.setRequestMethod("GET");
httpURLConnection.setRequestProperty("Connection", "Keep-Alive");
httpURLConnection.setRequestProperty("Charset", "UTF-8");
httpURLConnection.setRequestProperty("Content-Type","multipart/form-data;boundary=" + "******");
httpURLConnection.connect();
// System.out.println("响应码是::"+httpURLConnection.getResponseCode());
File pictureFile = new File("d:/"+times+".jpg");
BufferedInputStream bis = new BufferedInputStream(httpURLConnection.getInputStream());
FileOutputStream outputStream = new FileOutputStream(pictureFile);
byte[] buffer = new byte[1024];
int len = 0;
BufferedOutputStream out = new BufferedOutputStream(outputStream);
while ((len = bis.read(buffer)) != -1) {
if(System.currentTimeMillis() - downloadStartTime > 30*1000)throw new RuntimeException("超时");
out.write(buffer, 0, len);
}
out.flush();
out.close();
bis.close();
return System.currentTimeMillis()-downloadStartTime;
}
}
运行上面的程序，可以在控制台看见每次文件下载的耗时，如果出现了异常，并不会中断子线程，而是会开始下载下一个文件