package com.coderising.download;
import java.io.InputStream;
import java.io.RandomAccessFile;
import java.net.HttpURLConnection;
import java.net.URL;
public class Demo {
public static String path = "https://ss0.bdstatic.com/5aV1bjqh_Q23odCf/static/superman/img/logo/logo_white_fe6da1ec.png";
public static int threadCount = 3;
public static void main(String[] args) throws Exception{
//1.连接服务器,获取一个文件,获取文件的长度,在本地创建一个跟服务器一样大小的临时文件
URL url = new URL(path);
HttpURLConnection conn = (HttpURLConnection)url.openConnection();
conn.setConnectTimeout(5000);
conn.setRequestMethod("GET");
int code = conn.getResponseCode();
if (code == 200) {
//服务器端返回的数据的长度,实际上就是文件的长度
int length = conn.getContentLength();
System.out.println("文件总长度:"+length);
//在客户端本地创建出来一个大小跟服务器端一样大小的临时文件
RandomAccessFile raf = new RandomAccessFile("d:/1.png", "rwd");
//指定创建的这个文件的长度
raf.setLength(length);
raf.close();
//假设是3个线程去下载资源。
//平均每一个线程下载的文件大小.
int blockSize = length / threadCount;
for (int threadId = 1; threadId <= threadCount; threadId++) {
//第一个线程下载的开始位置
int startIndex = (threadId - 1) * blockSize;
int endIndex = threadId * blockSize - 1;
if (threadId == threadCount) {//最后一个线程下载的长度要稍微长一点
endIndex = length;
}
System.out.println("线程:"+threadId+"下载:---"+startIndex+"--->"+endIndex);
new DownLoadThread(path, threadId, startIndex, endIndex).start();
}
}else {
System.out.printf("服务器错误!");
}
}
/**
* 下载文件的子线程 每一个线程下载对应位置的文件
* @author jie
*
*/
public static class DownLoadThread extends Thread{
private int threadId;
private int startIndex;
private int endIndex;
/**
* @param path 下载文件在服务器上的路径
* @param threadId 线程Id
* @param startIndex 线程下载的开始位置
* @param endIndex 线程下载的结束位置
*/
public DownLoadThread(String path, int threadId, int startIndex, int endIndex) {
super();
this.threadId = threadId;
this.startIndex = startIndex;
this.endIndex = endIndex;
}
@Override
public void run() {
try {
URL url = new URL(path);
HttpURLConnection conn = (HttpURLConnection)url.openConnection();
conn.setConnectTimeout(5000);
conn.setRequestMethod("GET");
//重要:请求服务器下载部分文件 指定文件的位置
conn.setRequestProperty("Range", "bytes="+startIndex+"-"+endIndex);
//从服务器请求全部资源返回200 ok如果从服务器请求部分资源 返回 206 ok
int code = conn.getResponseCode();
System.out.println("code:"+code);
InputStream is = conn.getInputStream();//已经设置了请求的位置,返回的是当前位置对应的文件的输入流
RandomAccessFile raf = new RandomAccessFile("d:/1.png", "rwd");
//随机写文件的时候从哪个位置开始写
raf.seek(startIndex);//定位文件
int len = 0;
byte[] buffer = new byte[1024];
while ((len = is.read(buffer)) != -1) {
raf.write(buffer, 0, len);
}
is.close();
raf.close();
System.out.println("线程:"+threadId+"下载完毕");
} catch (Exception e) {
e.printStackTrace();
}
}
}
}