通过Java NIO 实现文件下载

private void output(HttpServletResponse response,String filePathAndFileName,String mimeType)
      throws IOException {

    File file = new File(filePathAndFileName);

    // set response headers
    response.setContentType((mimeType != null) ? mimeType : "application/octet-stream");
    response.setContentLength((int) file.length());

    // read and write file
    ServletOutputStream op = response.getOutputStream();
    // 128 KB buffer
    int bufferSize = 131072;
    FileInputStream fileInputStream = new FileInputStream(file);
    FileChannel fileChannel = fileInputStream.getChannel();
    // 6x128 KB = 768KB byte buffer
    ByteBuffer bb = ByteBuffer.allocateDirect(786432);
    byte[] barray = new byte[bufferSize];
    int nRead,nGet;

    try {
      while ((nRead = fileChannel.read(bb)) != -1) {
        if (nRead == 0)
          continue;
        bb.position(0);
        bb.limit(nRead);
        while (bb.hasRemaining()) {
          nGet = Math.min(bb.remaining(),bufferSize);
          // read bytes from disk
          bb.get(barray,nGet);
          // write bytes to output
          op.write(barray);
        }
        bb.clear();
      }
    } catch (IOException e) {
      e.printStackTrace();
    } finally {
      bb.clear();
      fileChannel.close();
      fileInputStream.close();
    }
  }