java SE 文件多线程下载，断点续传 原理

package com.itheima.download;

import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.InputStream;
import java.io.RandomAccessFile;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;
import java.net.URLConnection;

public class TestDownload {
private final static String path = "http://192.168.1.251/web/youdao.exe";

public static void main(String[] args) throws Exception {
URL url = new URL(path);
HttpURLConnection conn = (HttpURLConnection) url.openConnection();
conn.setRequestMethod("GET");
conn.setConnectTimeout(5000);
conn.setRequestProperty("User-Agent",
"Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1)");
int code = conn.getResponseCode();
if (code == 200) {
int len = conn.getContentLength();
RandomAccessFile file = new RandomAccessFile(getFilenName(path),
"rwd");
// 1.设置本地文件大小跟服务器的文件大小一致
file.setLength(len);

// 2 .假设开启3 个线程
int threadnumber = 3;
int blocksize = len / threadnumber;
/**
* 线程1 0~ blocksize 线程2 1*bolocksize ~ 2*blocksize 线程3 2*blocksize ~
* 文件末尾
*/
for (int i = 0; i < threadnumber; i++) {
int startposition = i * blocksize;
int endpositon = (i + 1) * blocksize;
if (i == (threadnumber - 1)) {
// 最后一个线程
endpositon = len;
}

DownLoadTask task = new DownLoadTask(i, path, startposition,
endpositon);
task.start();
}

}
}

public static String getFilenName(String path) {
int start = path.lastIndexOf("/") + 1;
return path.substring(start, path.length());
}

}

class DownLoadTask extends Thread {
public static final String path = "http://192.168.1.251/web/youdao.exe";
int threadid;
String filepath;
int startposition;
int endpositon;

public DownLoadTask(int threadid, String filepath, int startposition,
int endpositon) {
this.threadid = threadid;
this.filepath = filepath;
this.startposition = startposition;
this.endpositon = endpositon;
}

@Override
public void run() {
try {
File postionfile = new File(threadid + ".txt");
URL url = new URL(filepath);
HttpURLConnection conn = (HttpURLConnection) url.openConnection();
System.out.println("线程" + threadid + "正在下载 " + "开始位置 : "
+ startposition + "结束位置 " + endpositon);

if (postionfile.exists()) {
FileInputStream fis = new FileInputStream(postionfile);
byte[] result = StreamTool.getBytes(fis);
int newstartposition = Integer.parseInt(new String(result));
if (newstartposition > startposition) {
startposition = newstartposition;
}
}
conn.setRequestProperty("Range", "bytes=" + startposition + "-"
+ endpositon);
conn.setRequestMethod("GET");
conn.setConnectTimeout(50000);

conn.setRequestProperty("User-Agent",
"Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1)");

InputStream is = conn.getInputStream();
RandomAccessFile file = new RandomAccessFile(getFilenName(path),
"rwd");
// 设置数据从哪个位置开始写
file.seek(startposition);
byte[] buffer = new byte[1024];
int leng = 0;
// 代表当前读到的服务器数据的位置，同时这个值是已经存储的位置
int currentPostition = startposition;
// 创建一个文件对象 记录当前某个文件的下载地址
while ((leng = is.read(buffer)) != -1) {
file.write(buffer, 0, leng);
currentPostition += leng;// 每次写完，当前的位置加上leng 持久化到存储设备里
String positon = currentPostition + "";
FileOutputStream fos = new FileOutputStream(postionfile);
fos.write(positon.getBytes());
fos.flush();
fos.close();
}
file.close();
System.out.println("线程" + threadid + "下载完毕");
System.out.println("线程" + threadid + "开始位置" + startposition
+ "结束位置" + endpositon);
// 当线程下载完毕后，把文件删除掉
if (postionfile.exists()) {
postionfile.delete();
}
} catch (Exception e) {
e.printStackTrace();
}
super.run();
}

public static String getFilenName(String path) {
int start = path.lastIndexOf("/") + 1;
return path.substring(start, path.length());
}

}