Java实现多线程断点下载

JAVA多线程断点下载原理如图：

代码如下：

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; 
 
public class MutileThreadDownload { 
  /** 
   * 线程的数量 
   */ 
  private static int threadCount = 3; 
 
  /** 
   * 每个下载区块的大小 
   */ 
  private static long blocksize; 
 
  /** 
   * 正在运行的线程的数量 
   */ 
  private static int runningThreadCount; 
 
  /** 
   * @param args 
   * @throws Exception 
   */ 
  public static void main(String[] args) throws Exception { 
    // 服务器文件的路径 
    String path = "http://192.168.1.100:8080/ff.exe"; 
    URL url = new URL(path); 
    HttpURLConnection conn = (HttpURLConnection) url.openConnection(); 
    conn.setRequestMethod("GET"); 
    conn.setConnectTimeout(5000); 
    int code = conn.getResponseCode(); 
    if (code == 200) { 
      long size = conn.getContentLength();// 得到服务端返回的文件的大小 
      System.out.println("服务器文件的大小：" + size); 
      blocksize = size / threadCount; 
      // 1.首先在本地创建一个大小跟服务器一模一样的空白文件。 
      File file = new File("temp.exe"); 
      RandomAccessFile raf = new RandomAccessFile(file, "rw"); 
      raf.setLength(size); 
      // 2.开启若干个子线程分别去下载对应的资源。 
      runningThreadCount = threadCount; 
      for (int i = 1; i <= threadCount; i++) { 
        long startIndex = (i - 1) * blocksize; 
        long endIndex = i * blocksize - 1; 
        if (i == threadCount) { 
          // 最后一个线程 
          endIndex = size - 1; 
        } 
        System.out.println("开启线程：" + i + "下载的位置：" + startIndex + "~" 
            + endIndex); 
        new DownloadThread(path, i, startIndex, endIndex).start(); 
      } 
    } 
    conn.disconnect(); 
  } 
 
  private static class DownloadThread extends Thread { 
    private int threadId; 
    private long startIndex; 
    private long endIndex; 
    private String path; 
 
    public DownloadThread(String path, int threadId, long startIndex, 
        long endIndex) { 
      this.path = path; 
      this.threadId = threadId; 
      this.startIndex = startIndex; 
      this.endIndex = endIndex; 
    } 
 
    @Override 
    public void run() { 
      try { 
        // 当前线程下载的总大小 
        int total = 0; 
        File positionFile = new File(threadId + ".txt"); 
        URL url = new URL(path); 
        HttpURLConnection conn = (HttpURLConnection) url 
            .openConnection(); 
        conn.setRequestMethod("GET"); 
        // 接着从上一次的位置继续下载数据 
        if (positionFile.exists() && positionFile.length() > 0) {// 判断是否有记录 
          FileInputStream fis = new FileInputStream(positionFile); 
          BufferedReader br = new BufferedReader( 
              new InputStreamReader(fis)); 
          // 获取当前线程上次下载的总大小是多少 
          String lasttotalstr = br.readLine(); 
          int lastTotal = Integer.valueOf(lasttotalstr); 
          System.out.println("上次线程" + threadId + "下载的总大小：" 
              + lastTotal); 
          startIndex += lastTotal; 
          total += lastTotal;// 加上上次下载的总大小。 
          fis.close(); 
        } 
 
        conn.setRequestProperty("Range", "bytes=" + startIndex + "-" 
            + endIndex); 
        conn.setConnectTimeout(5000); 
        int code = conn.getResponseCode(); 
        System.out.println("code=" + code); 
        InputStream is = conn.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：写文件的开始位置：" 
            + String.valueOf(startIndex)); 
        int len = 0; 
        byte[] buffer = new byte[512]; 
        while ((len = is.read(buffer)) != -1) { 
          RandomAccessFile rf = new RandomAccessFile(positionFile, 
              "rwd"); 
          raf.write(buffer, 0, len); 
          total += len; 
          rf.write(String.valueOf(total).getBytes()); 
          rf.close(); 
        } 
        is.close(); 
        raf.close(); 
 
      } catch (Exception e) { 
        e.printStackTrace(); 
      } finally { 
        // 只有所有的线程都下载完毕后 才可以删除记录文件。 
        synchronized (MutileThreadDownload.class) { 
          System.out.println("线程" + threadId + "下载完毕了"); 
          runningThreadCount--; 
          if (runningThreadCount < 1) { 
            System.out.println("所有的线程都工作完毕了。删除临时记录的文件"); 
            for (int i = 1; i <= threadCount; i++) { 
              File f = new File(i + ".txt"); 
              System.out.println(f.delete()); 
            } 
          } 
        } 
 
      } 
    } 
  } 
} 

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