1、常見的方式：

public HttpServletResponse download(String path, HttpServletResponse response) {
    try {
        // path:下載的檔案的路徑。
        File file = new File(path);
        // 取得檔名。
        String filename = file.getName();
        // 取得檔案的字尾名。
        String ext = filename.substring(filename.lastIndexOf(".") + 1).toUpperCase();
        // 以流的形式下載檔案。
        InputStream fis = new BufferedInputStream(new FileInputStream(path));
        byte[] buffer = new byte[fis.available()];
        fis.read(buffer);
        fis.close();
        // 清空response
        response.reset();
        // 設定response的Header
        response.addHeader("Content-Disposition", "attachment;filename=" + new String(filename.getBytes()));
        response.addHeader("Content-Length", "" + file.length());
        OutputStream toClient = new BufferedOutputStream(response.getOutputStream());
        response.setContentType("application/octet-stream");
        toClient.write(buffer);
        toClient.flush();
        toClient.close();
    } catch (IOException ex) {
        ex.printStackTrace();
    }
    return response;
}

public void downloadLocal(HttpServletResponse response) throws FileNotFoundException {
    // 下載本地檔案
    String fileName = "Operator.doc".toString(); // 檔案的預設儲存名
    // 讀到流中
    InputStream inStream = new FileInputStream("c:/Operator.doc");// 檔案的存放路徑
    // 設定輸出的格式
    response.reset();
    response.setContentType("bin");
    response.addHeader("Content-Disposition", "attachment; filename=\"" + fileName + "\"");
    // 迴圈取出流中的資料
    byte[] b = new byte[100];
    int len;
    try {
        while ((len = inStream.read(b)) > 0)
            response.getOutputStream().write(b, 0, len);
        inStream.close();
    } catch (IOException e) {
        e.printStackTrace();
    }
}

public void downloadNet(HttpServletResponse response) throws MalformedURLException {
    // 下載網路檔案
    int bytesum = 0;
    int byteread = 0;

    URL url = new URL("windine.blogdriver.com/logo.gif");

    try {
        URLConnection conn = url.openConnection();
        InputStream inStream = conn.getInputStream();
        FileOutputStream fs = new FileOutputStream("c:/abc.gif");

        byte[] buffer = new byte[1204];
        int length;
        while ((byteread = inStream.read(buffer)) != -1) {
            bytesum += byteread;
            System.out.println(bytesum);
            fs.write(buffer, 0, byteread);
        }
    } catch (FileNotFoundException e) {
        e.printStackTrace();
    } catch (IOException e) {
        e.printStackTrace();
    }
}

2、支援線上開啟檔案的一種方式：

public void downLoad(String filePath, HttpServletResponse response, boolean isOnLine) throws Exception {
    File f = new File(filePath);
    if (!f.exists()) {
        response.sendError(404, "File not found!");
        return;
    }
    BufferedInputStream br = new BufferedInputStream(new FileInputStream(f));
    byte[] buf = new byte[1024];
    int len = 0;

    response.reset(); // 非常重要
    if (isOnLine) { // 線上開啟方式
        URL u = new URL("file:///" + filePath);
        response.setContentType(u.openConnection().getContentType());
        response.setHeader("Content-Disposition", "inline; filename=" + f.getName());
        // 檔名應該編碼成UTF-8
    } else { // 純下載方式
        response.setContentType("application/x-msdownload");
        response.setHeader("Content-Disposition", "attachment; filename=" + f.getName());
    }
    OutputStream out = response.getOutputStream();
    while ((len = br.read(buf)) > 0)
        out.write(buf, 0, len);
    br.close();
    out.close();
}

3、工作中：

//org.springframework.http.ResponseEntity
public ResponseEntity<byte[]> downloadFile(HttpServletRequest request, String path) {
    try {
        File file = new File(path);
        InputStream inputStream = new FileInputStream(file);
        byte[] bytes = new byte[inputStream.available()];
        inputStream.read(bytes);
        HttpHeaders httpHeaders = new HttpHeaders();
        httpHeaders.add("Content-Disposition", "attchement;filename=" + file.getName());
        HttpStatus statusCode = HttpStatus.OK;
        return new ResponseEntity<byte[]>(bytes, httpHeaders, statusCode);
    } catch (Exception e) {
        return new ResponseEntity("", new HttpHeaders(), HttpStatus.BAD_GATEWAY);
    }
}