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。

public class downloadwithrange implements runnable
  {
    private string urllocation;

    private string filepath;

    private long start;

    private long end;

    downloadwithrange(string urllocation, string filepath, long start, long end)
    {
      this.urllocation = urllocation;
      this.filepath = filepath;
      this.start = start;
      this.end = end;
    }

    @override
    public void run()
    {
      try
      {
        httpurlconnection conn = gethttp();
        conn.setrequestproperty("range", "bytes=" + start + "-" + end);

        file file = new file(filepath);
        randomaccessfile out = null;
        if (file != null)
        {
          out = new randomaccessfile(file, "rwd");
        }
        out.seek(start);
        inputstream in = conn.getinputstream();
        byte[] b = new byte[1024];
        int len = 0;
        while ((len = in.read(b)) != -1)
        {
          out.write(b, 0, len);
        }
        in.close();
        out.close();
      }
      catch (exception e)
      {
        e.getmessage();
      }

    }

    public httpurlconnection gethttp() throws ioexception
    {
      url url = null;
      if (urllocation != null)
      {
        url = new url(urllocation);
      }
      httpurlconnection conn = (httpurlconnection) url.openconnection();
      conn.setreadtimeout(5000);
      conn.setrequestmethod("get");

      return conn;
    }

  }

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。

public class downloadfilewiththreadpool
{
  public void getfilewiththreadpool(string urllocation,string filepath, int poollength) throws ioexception
  {
    executor threadpool = executors.newfixedthreadpool(poollength);
    
    long len = getcontentlength(urllocation);
    for(int i=0;i<poollength;i++)
    {
      long start=i*len/poollength;
      long end = (i+1)*len/poollength-1;
      if(i==poollength-1)
      {
        end =len;
      }
      downloadwithrange download=new downloadwithrange(urllocation, filepath, start, end);
      threadpool.execute(download);
    }
  }

  public static long getcontentlength(string urllocation) throws ioexception
  {
    url url = null;
    if (urllocation != null)
    {
      url = new url(urllocation);
    }
    httpurlconnection conn = (httpurlconnection) url.openconnection();
    conn.setreadtimeout(5000);
    conn.setrequestmethod("get");
    long len = conn.getcontentlength();

    return len;
  }

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