import java.io.BufferedReader;
import java.io.File;
import java.io.FileInputStream;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.RandomAccessFile;
import java.net.HttpURLConnection;
import java.net.URL;
/**
* 多线程断点续传的原理
* 1、当多线程突然中断的时候，在缓存文件中保存中断的位置；
* 2、当续传的时候，分别读取缓存文件中的位置，然后作为开始的位置下载。
* @author Sheamus
*
*/
public class MutilHttpDownload {
//完成下载的线程数
static int finishedThread = 0;
static String path = "http://10.31.2.6:8080/demo/QQ.exe";
static int PROCESSNUM = 3;
public static void main(String[] args) {
//请求文件的大小并创建一个一样大小的缓存文件
try {
URL url = new URL(path);
//在项目的目录下创建一个QQ.exe文件
File file = new File("QQ.exe");
RandomAccessFile raf = new RandomAccessFile(file, "rwd");
HttpURLConnection conn = (HttpURLConnection) url.openConnection();
conn.setRequestMethod("GET");
conn.setReadTimeout(5000);
conn.setReadTimeout(5000);
if(conn.getResponseCode() == 200) {
int length = conn.getContentLength();
//设置缓存文件的大小
raf.setLength(length);
raf.close();
//计算每个线程应该下载的大小
int size = length / PROCESSNUM;
//计算每个线程开始和结束的位置
for(int i = 0; i < PROCESSNUM; i++) {
int startIndex = i * size;
int endIndex = (i + 1) * size - 1;
//当时最后一个线程的时候，最后的位置就是总长度减1
if(i == (PROCESSNUM - 1)) {
endIndex = length - 1;
}
//启动线程执行下载
new SingleDownLoadThread(startIndex,endIndex,i).start();
}
}
} catch (Exception e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
class SingleDownLoadThread extends Thread {
private int startIndex;
private int endIndex;
private int currentThreadId;
public SingleDownLoadThread(int startIndex, int endIndex, int currentThreadId) {
super();
this.startIndex = startIndex;
this.endIndex = endIndex;
this.currentThreadId = currentThreadId;
}
@Override
public String toString() {
return "DownLoadThread [startIndex=" + startIndex + ", endIndex="
+ endIndex + ", currentThreadId=" + currentThreadId + "]";
}
@Override
public void run() {
int total = 0;
//下载该线程的要下载的部分
try {
//定义缓存文件用于保存下载的位置
File tempFile = new File("temp" + currentThreadId +".txt");
if(tempFile.exists()) {
//下载的时候，如果有缓存文件,读取缓存中文件，将这个数字替换开始的位置
FileInputStream fis = new FileInputStream(tempFile);
BufferedReader bis = new BufferedReader(new InputStreamReader(fis));
int tempStartIndex = Integer.parseInt(bis.readLine());
startIndex = tempStartIndex;
fis.close();
}
System.out.println("下载的区间是 ：" + startIndex +" -- > " + endIndex +"--------------------------------");
File file = new File("QQ.exe");
RandomAccessFile raf = new RandomAccessFile(file, "rwd");
URL url = new URL(MutilHttpDownload.path);
HttpURLConnection conn = (HttpURLConnection) url.openConnection();
conn.setRequestMethod("GET");
conn.setConnectTimeout(5000);
conn.setReadTimeout(5000);
//设置要下载的部分
conn.setRequestProperty("Range", "bytes=" + startIndex + "-" + endIndex);
//将随机读取读取文件的流的指针指向要读文件的开始位置
raf.seek(startIndex);
//部分文件获取成功的返回码是206
if(conn.getResponseCode() == 206) {
InputStream in = conn.getInputStream();
byte[] b = new byte[1024];
int len = 0;
while((len = in.read(b)) != -1) {
raf.write(b, 0, len);
total += len;
System.out.println("线程 " + currentThreadId + "下载了 " + total);
//创建一个输出流写出当前读到的位置
RandomAccessFile tempRaf = new RandomAccessFile(tempFile, "rwd");
tempRaf.write((total+"").getBytes());
tempRaf.close();
}
System.out.println("线程 " + currentThreadId + "下载完成!");
raf.close();
MutilHttpDownload.finishedThread++;
synchronized (MutilHttpDownload.path) {
if(MutilHttpDownload.finishedThread == MutilHttpDownload.PROCESSNUM) {
for(int i = 0; i < MutilHttpDownload.PROCESSNUM; i++) {
File f = new File("temp" + i + ".txt");
System.out.println("删除" + f.getName() +"文件");
f.delete();
}
//只让一个线程进来删除文件，要不然会出异常
MutilHttpDownload.finishedThread = 0;
}
}
}
} catch (Exception e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}
原文：http://my.oschina.net/Sheamus/blog/493418