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;
    }