Java实现多线程断点下载（下载过程中可以暂停）

  1 import java.io.BufferedReader;
  2 import java.io.File;
  3 import java.io.FileInputStream;
  4 import java.io.InputStream;
  5 import java.io.InputStreamReader;
  6 import java.io.RandomAccessFile;
  7 import java.net.HttpURLConnection;
  8 import java.net.URL;
  9 public class DownLoad {
 10     // 声明下载路径“红色警戒2”
 11     public static final String PATH = "http://soft3.xzstatic.com/2015/10/hsjj2ghgzh.rar";
 12     public static int threadCount = 0;// 声明线程数量
 13     public static void main(String[] args) {
 14         try {
 15             URL url = new URL(PATH);
 16             // 获取连接
 17             HttpURLConnection conn = (HttpURLConnection) url.openConnection();
 18             // 通过获取连接定义文件名
 19             String[] str = PATH.split("/");
 20             String fileName = str[5];
 21             // 获取下载文件大小
 22             int fileLength = conn.getContentLength();
 23             System.out.println(fileName);
 24             // 在本地创建一个与服务器大小一致的可随机写入文件
 25             RandomAccessFile raf = new RandomAccessFile(fileName, "rwd");
 26             System.out.println(fileLength);// 测试用
 27             raf.setLength(fileLength);
 28             // 自定义线程数量
 29             threadCount = 3;
 30             // 计算每条线程下载数据的大小
 31             int blockSize = fileLength / threadCount;
 32             // 启动线程下载
 33             for (int threadId = 1; threadId <= threadCount; threadId++) {
 34                 // 核心代码，定义每个线程开始以及结束的下载位置
 35                 int startPos = (threadId - 1) * blockSize;// 开始下载的位置
 36                 int endPos = (threadId * blockSize) - 1;// 结束下载的位置（不包含最后一块）
 37                 if (threadCount == threadId) {
 38                     endPos = fileLength;
 39                 }
 40                 new Thread(new DownLoadThread(threadId, startPos, endPos, PATH))
 41                         .start();
 42             }
 43         } catch (Exception e) {
 44             e.printStackTrace();
 45         }
 46     }
 47 
 48     // 实现下载线程
 49     static class DownLoadThread implements Runnable {
 50         private int threadId;
 51         private int startPos;
 52         private int endPos;
 53         private String path;
 54         public DownLoadThread(int threadId, int startPos, int endPos,
 55                 String path) {
 56             super();
 57             this.threadId = threadId;
 58             this.startPos = startPos;
 59             this.endPos = endPos;
 60             this.path = path;
 61         }
 62         public void run() {
 63             try {
 64                 URL url = new URL(path);
 65                 String[] str = PATH.split("/");
 66                 String fileName = str[5];
 67                 HttpURLConnection conn = (HttpURLConnection) url
 68                         .openConnection();
 69                 // 设置URL请求的方法（具体参考API）
 70                 conn.setRequestMethod("GET");
 71                 // 设置500毫秒为超时值
 72                 conn.setReadTimeout(5000);
 73                 File file = new File(threadId + ".txt");
 74                 if (file.exists() && file.length() > 0) {
 75                     BufferedReader br = new BufferedReader(
 76                             new InputStreamReader(new FileInputStream(file)));
 77                     String saveStartPos = br.readLine();
 78                     if (saveStartPos != null && saveStartPos.length() > 0) {
 79                         startPos = Integer.parseInt(saveStartPos);
 80                     }
 81                 }
 82                 // 注意双引号内的格式，不能包含空格（等其他字符），否则报416
 83                 conn.setRequestProperty("Range", "bytes=" + startPos + "-"
 84                         + endPos);
 85                 RandomAccessFile raf = new RandomAccessFile(fileName, "rwd");// 存储下载文件的随机写入文件
 86                 raf.seek(startPos);// 设置开始下载的位置
 87                 System.out.println("线程" + threadId + ":" + startPos + "~~"
 88                         + endPos);
 89                 InputStream is = conn.getInputStream();
 90                 byte[] b = new byte[1024 * 1024 * 10];
 91                 int len = -1;
 92                 int newPos = startPos;
 93                 while ((len = is.read(b)) != -1) {
 94                     RandomAccessFile rr = new RandomAccessFile(file, "rwd");// 存储下载标记的文件
 95                     raf.write(b, 0, len);
 96                     // 将下载标记存入指定文档
 97                     String savaPoint = String.valueOf(newPos += len);
 98                     rr.write(savaPoint.getBytes());
 99                     rr.close();
100                 }
101                 is.close();
102                 raf.close();
103                 System.out.println("下载完成");
104             } catch (Exception e) {
105                 e.printStackTrace();
106             }
107         }
108     }
109 }