java - 从URL读取InputStream并通过Servlet写入

Question

我正在尝试编写一个 InputStream，它是通过 Servlet 的 doGet 方法从 URL 获取的>。这是我的代码:

public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    String requestedUrl = request.getParameter("url");

    if (StringUtils.isNotBlank(requestedUrl)) {
        ReadableByteChannel inputChannel = null;
        WritableByteChannel outputChannel = null;

        try {
            URL url = new URL(requestedUrl);
            HttpURLConnection httpConnection = (HttpURLConnection) url.openConnection();
            int responseCode = httpConnection.getResponseCode();

            System.out.println(responseCode);

            if (responseCode == HttpURLConnection.HTTP_OK) {
                response.setContentType("image/jpg");
                httpConnection.connect();
                InputStream imageStream = url.openStream();
                OutputStream outputStream = response.getOutputStream();

                inputChannel = Channels.newChannel(imageStream);
                outputChannel = Channels.newChannel(outputStream);
                ByteBuffer buffer = ByteBuffer.allocate(10240);

                while (inputChannel.read(buffer) != -1) {
                    buffer.flip();
                    outputChannel.write(buffer);
                    buffer.clear();
                }
            }
        } catch (Exception ex) {
            Log.error(this, ex.getMessage(), ex);
        } finally {
            if (ObjectUtils.notEqual(outputChannel, null)) {
                try {
                    outputChannel.close();
                } catch (IOException ignore) {
                }
            }

            if (ObjectUtils.notEqual(inputChannel, null)) {
                try {
                    inputChannel.close();
                } catch (IOException ignore) {
                }
            }
        }
    }
}

我可以在控制台中看到 responseCode 为 200，但它没有在页面中写入任何内容。在 Firefox 中我得到:

The image “the_context_root/dam/no-image-aware-servlet?url=http%3A//localhost%3A80/file/MHIS044662&Rendition=164FixedWidth&noSaveAs=1” cannot be displayed because it contains errors.

我找不到我做错了什么。任何指针都会非常有帮助。

Answer 1

我尝试稍微修改一下您的代码，以解决 getResponseCode() 和 connect() 的错误顺序以及其他一些小问题。

特别是如果出现问题(例如在其他服务器上找不到文件、IOException、非法 URL 等)，请确保始终返回错误状态代码(2xx 除外)，否则浏览器始终会收到 200 - 好的，但是没有数据!

public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    String requestedUrl = request.getParameter("url");

    if (StringUtils.isBlank(requestedUrl)) {
        // TODO: send error code 400 - Bad Request
        return;
    }

    try {
        URL url = new URL(requestedUrl);
        HttpURLConnection httpConnection = (HttpURLConnection) url.openConnection();
        response.setContentType("image/jpg");
        httpConnection.connect();

        int responseCode = httpConnection.getResponseCode();

        if (responseCode == HttpURLConnection.HTTP_OK) {
            // TODO: set correct content type to response

            OutputStream outputStream = response.getOutputStream();

            try (InputStream imageStream = url.openStream()) {
                IOUtils.copy(imageStream, outputStream );
            }
        } else {
            // TODO: send error code (depends on responseCode), probably 500
        }
    } catch (Exception ex) {
        Log.error(this, ex.getMessage(), ex);
        // TODO: send error code 400 if malformed url
        // TODO: send error code 404 if image not found (responseCode == 404)
        // TODO: send error code 500 else
    }
    // Don't close the response-OutputStream! You didn't open it either!
}