Android菜鸟之路-多线程下载

1. 在Android客户端首先创建一个空文件（空白文件），大小跟服务器的一模一样。
2. 开启若干线程，分别去下载服务器上对应的资源。
3. 如果所有的线程都下载完毕，服务器的资源就下载完毕。

Android客户端下载逻辑

1. 获取到服务器端资源文件的大小(connection.getContentLength())

   String path = "https://localhost:8080/ff.exe";  URL url = new URL(path);  // 获取链接 HttpURLConnection connection = (HttpURLConnection) url.openConnection();  // 参数 connection.setRequestMethod("GET"); connection.setConnectTimeout(5000);  // 响应码 int responseCode = connection.getResponseCode();  if (responseCode == 200) {      // 得到服务端返回的文件大小     long contentLength = connection.getContentLength();     System.out.println("服务器文件大小:"+contentLength); }  // 断开连接 connection.disconnect(); 

2. 在本地创建一个大小和服务器一样的的空白文件

   File file = new File("temp.ext"); RandomAccessFile raf= new RandomAccessFile(file, "rw"); raf.setLength(contentLength); 

3. 根据开启线程的数量，把服务器资源给等分成若干份（threadCount：线程数量）

   假设文件大小是10个byte。（长度：length） 假设3个线程（线程数量：threadCount）。  int blockSize = length / threadCount; 线程1 下载3个byte    1-3     0 * blockSize + 1 ~ 1 * blockSize 线程2 下载3个byte    4-6     1 * blockSize + 1 ~ 2 * blockSize  线程3 下载4个byte    7-10    2 * blockSize + 1 ~ 3 * blockSize(length)  // 开启若干个子线程，分别去下载对应的资源 long threadCount = 3; long blockSize = contentLength / threadCount;  for (int i = 1; i <= threadCount; i++) {     // 计算下载起始、结束位置     long startIndex = (i - 1) * blockSize + 0;// 服务器也是从0开始     long endIndex = i * blockSize - 1; //     // 最后一个线程     if (i == threadCount) {         endIndex = contentLength - 1;     }     System.out.println("开启线程:" + i + "，下载的位置：(" + startIndex + "~"             + endIndex + ")");     new DownloadThread(i, startIndex, endIndex, path).start(); } 

4. 创建线程类

   static class DownloadThread extends Thread {     private int threadId;     private long startIndex;     private long endIndex;     private String path;      public DownloadThread(int threadId, long startIndex, long endIndex,             String path) {         super();         this.threadId = threadId;         this.startIndex = startIndex;         this.endIndex = endIndex;         this.path = path;     }      public void run() {         URL url;         try {             url = new URL(path);              // 获取链接             HttpURLConnection connection = (HttpURLConnection) url                     .openConnection();              // 参数             connection.setRequestMethod("GET");             connection.setConnectTimeout(5000);             // 告诉服务器下载的参数             connection.setRequestProperty("Range", "bytes=" + startIndex                     + "-" + endIndex);              // 响应码             int responseCode = connection.getResponseCode();              System.out.println("服务器状态码:" + responseCode);             // 下载服务器文件，返回的是206             if (responseCode == 206) {                 InputStream inputStream = connection.getInputStream();                  File file = new File("temp.exe");                 RandomAccessFile raf = new RandomAccessFile(file, "rw");                 // 指定文件开始写的位置                 raf.seek(startIndex);                 System.out.println("第" + threadId + "个线程，写文件的开始、结束位置("                         + (startIndex) + "," + (endIndex) + ")");                  int len = 0;                  byte[] buf = new byte[1024];                 while ((len = inputStream.read(buf)) != -1) {                     raf.write(buf, 0, len);                 }                  inputStream.close();                 raf.close();                 System.out.println("第" + threadId + "个线程下载完毕了");              }         } catch (Exception e) {             // TODO Auto-generated catch block             e.printStackTrace();         }     } }