文件下载

1.普通单线程下载文件：

　　直接使用URLConnection.openStream()打开网络输入流,然后将流写入到文件中！

public static void downLoad(String path,Context context)throws Exception
{
  URL url = new URL(path);
  InputStream is = url.openStream();
  //截取最后的文件名
  String end = path.substring(path.lastIndexOf("."));
  //打开手机对应的输出流,输出到文件中
  OutputStream os = context.openFileOutput("Cache_"+System.currentTimeMillis()+end, Context.MODE_PRIVATE);
  byte[] buffer = new byte[1024];
  int len = 0;
  //从输入六中读取数据,读到缓冲区中
  while((len = is.read(buffer)) > 0)
  {
    os.write(buffer,0,len);
  }
  //关闭输入输出流
  is.close();
  os.close();
}

2.普通多线程下载：

　　步骤：

• 获取网络连接
• 本地磁盘创建相同大小的空文件
• 计算每条线程需从文件哪个部分开始下载，结束
• 依次创建，启动多条线程来下载网络资源的指定部分

  public class Downloader {
    //添加@Test标记是表示该方法是Junit测试的方法,就可以直接运行该方法了
      @Test
      public void download() throws Exception
      {
        //设置URL的地址和下载后的文件名
        String filename = "meitu.exe";
        String path = "http://10.13.20.32:8080/Test/XiuXiu_Green.exe";
        URL url = new URL(path);
        HttpURLConnection conn = (HttpURLConnection) url.openConnection();
        conn.setConnectTimeout(5000);
        conn.setRequestMethod("GET");
        //获得需要下载的文件的长度(大小)
        int filelength = conn.getContentLength();
        System.out.println("要下载的文件长度"+filelength);
        //生成一个大小相同的本地文件
        RandomAccessFile file = new RandomAccessFile(filename, "rwd");
        file.setLength(filelength);
        file.close();
        conn.disconnect();
        //设置有多少条线程下载
        int threadsize = 3;
        //计算每个线程下载的量
        int threadlength = filelength % 3 == 0 ? filelength/3:filelength+1;
        for(int i = 0;i < threadsize;i++)
        {
          //设置每条线程从哪个位置开始下载
          int startposition = i * threadlength;
          //从文件的什么位置开始写入数据
          RandomAccessFile threadfile = new RandomAccessFile(filename, "rwd");
          threadfile.seek(startposition);
          //启动三条线程分别从startposition位置开始下载文件
          new DownLoadThread(i,startposition,threadfile,threadlength,path).start();
        }
        int quit = System.in.read();
        while('q' != quit)
        {
          Thread.sleep(2000);
        }
      }
    
    
    private class DownLoadThread extends Thread {
      private int threadid;
      private int startposition;
      private RandomAccessFile threadfile;
      private int threadlength;
      private String path;
      public DownLoadThread(int threadid, int startposition,
          RandomAccessFile threadfile, int threadlength, String path) {
        this.threadid = threadid;
        this.startposition = startposition;
        this.threadfile = threadfile;
        this.threadlength = threadlength;
        this.path = path;
      }
      public DownLoadThread() {}
      @Override
      public void run() {
        try
        {
          URL url = new URL(path);
          HttpURLConnection conn = (HttpURLConnection) url.openConnection();
          conn.setConnectTimeout(5000);
          conn.setRequestMethod("GET");
          //指定从什么位置开始下载
          conn.setRequestProperty("Range", "bytes="+startposition+"-");
          //System.out.println(conn.getResponseCode());
          if(conn.getResponseCode() == 206)
          {
            InputStream is = conn.getInputStream();
            byte[] buffer = new byte[1024];
            int len = -1;
            int length = 0;
            while(length < threadlength && (len = is.read(buffer)) != -1)
            {
              threadfile.write(buffer,0,len);
              //计算累计下载的长度
              length += len;
            }
            threadfile.close();
            is.close();
            System.out.println("线程"+(threadid+1) + "已下载完成");
          }
        }catch(Exception ex){System.out.println("线程"+(threadid+1) + "下载出错"+ ex);}
      }
      
    }
  }