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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