java学习_Java实现多线程文件下载的代码示例

Java实现多线程文件下载思路：

1、基本思路是将文件分段切割、分段传输、分段保存。

2、分段切割用到HttpUrlConnection对象的setRequestProperty("Range", "bytes=" + start + "-" + end)方法。

3、分段传输用到HttpUrlConnection对象的getInputStream()方法。

4、分段保存用到RandomAccessFile的seek(int start)方法。

5、创建指定长度的线程池，循环创建线程，执行下载操作。 

 首先，我们要先写一个方法，方法的参数包含URL地址，保存的文件地址，切割后的文件开始位置和结束位置，这样我们就能把分段文件下载到本地。并且这个方法要是run方法，这样我们启动线程时就直接执行该方法。

 代码如下

publicclassDownloadWithRangeimplementsRunnable

  {

    privateString urlLocation;

 

    privateString filePath;

 

    privatelongstart;

 

    privatelongend;

 

    DownloadWithRange(String urlLocation, String filePath,longstart,longend)

    {

      this.urlLocation = urlLocation;

      this.filePath = filePath;

      this.start = start;

      this.end = end;

    }

 

    @Override

    publicvoidrun()

    {

      try

      {

        HttpURLConnection conn = getHttp();

        conn.setRequestProperty("Range","bytes="+ start +"-"+ end);

 

        File file =newFile(filePath);

        RandomAccessFile out =null;

        if(file !=null)

        {

          out =newRandomAccessFile(file,"rwd");

        }

        out.seek(start);

        InputStream in = conn.getInputStream();

        byte[] b =newbyte[1024];

        intlen =0;

        while((len = in.read(b)) != -1)

        {

          out.write(b,0, len);

        }

        in.close();

        out.close();

      }

      catch(Exception e)

      {

        e.getMessage();

      }

 

    }

 

    publicHttpURLConnection getHttp()throwsIOException

    {

      URL url =null;

      if(urlLocation !=null)

      {

        url =newURL(urlLocation);

      }

      HttpURLConnection conn = (HttpURLConnection) url.openConnection();

      conn.setReadTimeout(5000);

      conn.setRequestMethod("GET");

 

      returnconn;

    }

 

  }

然后我们创建线程池，线程池的长度可以自定义，然后循环创建线程来执行请求，每条线程的请求开始位置和结束位置都不同，本地存储的文件开始位置和请求开始位置相同，这样就可以实现多线程下载了。

 代码如下

publicclassDownloadFileWithThreadPool

{

  publicvoidgetFileWithThreadPool(String urlLocation,String filePath,intpoolLength)throwsIOException

  {

    Executor threadPool = Executors.newFixedThreadPool(poolLength);

     

    longlen = getContentLength(urlLocation);

    for(inti=0;i

    {

      longstart=i*len/poolLength;

      longend = (i+1)*len/poolLength-1;

      if(i==poolLength-1)

      {

        end =len;

      }

      DownloadWithRange download=newDownloadWithRange(urlLocation, filePath, start, end);

      threadPool.execute(download);

    }

  }

 

  publicstaticlonggetContentLength(String urlLocation)throwsIOException

  {

    URL url =null;

    if(urlLocation !=null)

    {

      url =newURL(urlLocation);

    }

    HttpURLConnection conn = (HttpURLConnection) url.openConnection();

    conn.setReadTimeout(5000);

    conn.setRequestMethod("GET");

    longlen = conn.getContentLength();

 

    returnlen;

  }

java学习_Java实现多线程文件下载的代码示例

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