public class MutiThreadDownload {
    public void download(String url, String filePath){  
        int threadCount = 5; // 下载线程数
        long blocksize; // 每线程下载区块大小
        int runningThreadCount; // 正运行线程数
    
        HttpURLConnection conn = (HttpURLConnection) new URL(url).openConnection();
        int code = conn.getResponseCode();
        if (code == 200) {
            long size = conn.getContentLength();// 文件大小 
            blocksize = size / threadCount;
            // 1.创建RandomAccessFile文件(可以随机访问)       
            RandomAccessFile raf = new RandomAccessFile(new File(filePath), "rw");
            raf.setLength(size);
            // 2.多线程下载对应区块
            runningThreadCount = threadCount;
            for (int i = 1; i <= threadCount; i++) {
                long startIndex = (i - 1) * blocksize;
                long endIndex = i * blocksize - 1;
                if (i == threadCount) {             
                    endIndex = size - 1;
                }           
                new DownloadThread(i, url, startIndex, endIndex).start();
            }
        }
        conn.disconnect();
    }

    private class DownloadThread extends Thread {
        private int id;
        private String url;
        private long startIndex;
        private long endIndex;

        public DownloadThread(int id, String url, String filePath, long startIndex, long endIndex) {
            this.id = id;
            this.url = url;         
            this.startIndex = startIndex;
            this.endIndex = endIndex;
        }

        @Override
        public void run() {
            try {                       
                HttpURLConnection conn = (HttpURLConnection) new URL(url).openConnection();
                conn.setRequestMethod("GET");
                
                // 1.读取文件断点位置curIndex
                int curIndex = 0;
                File indexFile = new File("indexFile_"+id);
                if (indexFile.exists() && indexFile.length() > 0) {
                    FileInputStream fis = new FileInputStream(indexFile);
                    BufferedReader br = new BufferedReader(new InputStreamReader(fis));                 
                    curIndex = Integer.valueOf(br.readLine());          
                    startIndex += curIndex;             
                    fis.close();
                }
                    
                // 2.从startIndex位置下载文件, 并从startIndex位置写入raf文件
                conn.setRequestProperty("Range", "bytes=" + startIndex + "-" + endIndex);           
                InputStream is = conn.getInputStream();             
                RandomAccessFile raf = new RandomAccessFile(new File(filePath), "rw");                          
                raf.seek(startIndex);
                int len = 0;
                byte[] buffer = new byte[1024*1024];                
                while ((len = is.read(buffer)) != -1) {                 
                    raf.write(buffer, 0, len);
                    // 更新断点位置
                    RandomAccessFile indexRaf = new RandomAccessFile(indexFile, "rwd");
                    curIndex += len;
                    indexRaf.write(String.valueOf(curIndex).getBytes());
                    indexRaf.close();
                }
                is.close();
                raf.close();                
            } catch (Exception e) {
                e.printStackTrace();
            } finally {
                // 3.所有线程下载完,删除断点位置文件indexFile
                synchronized (MutiThreadDownload.class){
                    if (--runningThreadCount == 0) {
                        for (int i = 1; i <= threadCount; i++) new File("indexFile_"+i).delete();                       
                    }
                }
            }
        }
    }
}

简书: http://www.jianshu.com/p/cb8fab446acd
CSDN博客: http://blog.csdn.net/qq_32115439/article/details/70180205
GitHub博客：http://lioil.win/2017/04/14/Java-MutiThreadDown.html
Coding博客：http://c.lioil.win/2017/04/14/Java-MutiThreadDown.html