基本原理：利用HttpsURLConnection获取要下载文件的长度、头部等相关信息，并设置响应的头部信息。并且通过HttpsURLConnection获取输入流，将文件分成指定的块，每一块单独开辟一个线程完成数据的读取、写入。通过输入流读取下载文件的信息，然后将读取的信息用RandomAccessFile随机写入到本地文件中。同时，每个线程写入的数据都文件指针也就是写入数据的长度，需要保存在一个临时文件中。这样当本次下载没有完成的时候，下次下载的时候就从这个文件中读取上一次下载的文件长度，然后继续接着上一次的位置开始下载。并且将本次下载的长度写入到这个文件中。
1 package com.gaolei.download;
2
3 import java.io.File;
4 import java.io.IOException;
5 import java.io.InputStream;
6 import java.io.RandomAccessFile;
7 import java.net.URL;
8
9 import javax.net.ssl.HttpsURLConnection;
10
11 public class MulThreadDownload {
12
13 public static void main(String[] args) throws Exception {
14 String path = "http://192.168.0.1/videonews/QQWubiSetup.exe";
15 new MulThreadDownload().download(path, 3);
16 }
17
18 /**
19 * 下载文件
20 *
21 * @param path网络文件路径
22 * @throws Exception
23 */
24 private void download(String path, int threadsize) throws Exception {
25 URL url = new URL(path);
26 HttpsURLConnection conn = (HttpsURLConnection) url.openConnection();
27 conn.setConnectTimeout(5000);
28 conn.setRequestMethod("GET");
29 if (conn.getResponseCode() == 200) {
30 int length = conn.getContentLength(); // 获取网络文件的长度
31 File file = new File(getFilename(path));
32 RandomAccessFile accessFile = new RandomAccessFile(file, "rwd");// 在本地生成一个长度相等的文件
33 accessFile.setLength(length);
34 accessFile.close();
35
36 // 计算每条线程负责下载的数据量
37 int block = length % threadsize == 0 ? length / threadsize : length
38 / threadsize + 1;
39 for (int threadid = 0; threadid < threadsize; threadid++) {
40 new DownloadThread(threadid, block, url, file).start();
41 }
42 } else {
43 System.out.println("下载失败！");
44 }
45
46 }
47
48 private class DownloadThread extends Thread {
49
50 private int threadid;
51 private int block;
52 private URL url;
53 private File file;
54
55 public DownloadThread(int threadid, int block, URL url, File file) {
56 this.threadid = threadid;
57 this.block = block;
58 this.url = url;
59 this.file = file;
60 }
61
62 @Override
63 public void run() {
64 int start = threadid * block; // 计算该线程从网络文件的什么位置开始下载
65 int end = (threadid + 1) * block - 1; // 下载到网络文件的什么位置结束
66
67 try {
68 RandomAccessFile accessFile = new RandomAccessFile(file, "rwd");// 在本地生成一个长度相等的文件
69 accessFile.seek(start);
70 HttpsURLConnection conn = (HttpsURLConnection) url
71 .openConnection();
72 conn.setConnectTimeout(5000);
73 conn.setRequestMethod("GET");
74 conn.setRequestProperty("Range", "bytes=" + start + "-" + end);
75 if (conn.getResponseCode() == 206) {
76 InputStream inStream = conn.getInputStream();
77 byte[] buffer = new byte[1024];
78 int len = 0;
79 while ((len = inStream.read(buffer)) != -1) {
80 accessFile.write(buffer, 0, len);
81 }
82 accessFile.close();
83 inStream.close();
84 }
85 System.out.println("第" + (threadid + 1) + "条线程已经下载完成！");
86 } catch (IOException e) {
87 e.printStackTrace();
88 }
89
90 }
91
92 }
93
94 private String getFilename(String path) {
95 return path.substring(path.lastIndexOf("/") + 1);
96 }
97 }
98