java - 为什么除非我们从服务器读取 POST 响应，否则 HTTP POST 不起作用？

Question

我正在为一个网站编写一个 uploader 类，在上传文件后，我正在读取该网站的上传响应。如果我没有阅读响应，则文件不会上传。我的代码如下，

   String charset = "UTF-8";


        File binaryFile = new File("C:\\TestVideoFile.flv");
        String boundary = Long.toHexString(System.currentTimeMillis());
        System.out.println(boundary);// Just generate some unique random value.
        String CRLF = "\r\n"; // Line separator required by multipart/form-data.
        URLConnection connection = new URL(UPLOAD_URL).openConnection();
        connection.setDoInput(true); 
        connection.setDoOutput(true);
        connection.setRequestProperty("Content-Type", "multipart/form-data; boundary=" + boundary);
        PrintWriter writer = null;
        try {
            OutputStream output = connection.getOutputStream();
            writer = new PrintWriter(new OutputStreamWriter(output, charset), true); // true = autoFlush, important!
            writer.append("--" + boundary).append(CRLF);
            writer.append("Content-Disposition: form-data; name=\"file\"; filename=\"" + binaryFile.getName() + "\"").append(CRLF);
            writer.append("Content-Type: " + URLConnection.guessContentTypeFromName(binaryFile.getName())).append(CRLF);
            writer.append("Content-Transfer-Encoding: binary").append(CRLF);
            writer.append(CRLF).flush();
            InputStream input = null;
            try {
                input = new FileInputStream(binaryFile);
                long filelen = binaryFile.length();
                System.out.println("Length : " + filelen);
                int dataRead = 0;
                byte[] buffer = new byte[1024];
                for (int length = 0; (length = input.read(buffer)) > 0;) {

                    output.write(buffer, 0, length);
                }
                System.out.println("Now only terminating the file write loop");
                output.flush(); // Important! Output cannot be closed. Close of writer will close output as well.
            } catch (Exception e) {
                System.out.println(e);
            } finally {
                if (input != null) {
                    try {
                        input.close();
                    } catch (IOException logOrIgnore) {
                        System.out.println(logOrIgnore);
                    }
                }
            }
            writer.append(CRLF).flush(); // CRLF is important! It indicates end of binary boundary.

            // End of multipart/form-data.
            writer.append("--" + boundary + "--").append(CRLF);

            System.out.println("Sending username");
            // Send normal param.
            writer.append("--" + boundary).append(CRLF);
            writer.append("Content-Disposition: form-data; name=\"user\"").append(CRLF);
            writer.append("Content-Type: text/plain; charset=" + charset).append(CRLF);
            writer.append(CRLF);
            writer.append(username).append(CRLF).flush();
            System.out.println("Sending password");
            writer.append("--" + boundary).append(CRLF);
            writer.append("Content-Disposition: form-data; name=\"password\"").append(CRLF);
            writer.append("Content-Type: text/plain; charset=" + charset).append(CRLF);
            writer.append(CRLF);
            writer.append(password).append(CRLF).flush();


            System.out.println("Reading response from server");

            BufferedReader br = new BufferedReader(new InputStreamReader(connection.getInputStream()));
            String k = "", tmp = "";
            while ((tmp = br.readLine()) != null) {
                System.out.println(tmp);
                k += tmp;
            }
            if (k.contains("Successfully")) {
                System.out.println("File Uploaded successfully into PutLocker :)");
                String downloadLink = parseResponse(k, "<link>", "</link>");
                System.out.println("Download Link : " + downloadLink);
            } else {
                System.out.println("Upload failed :(");
            }

          } catch (Exception e) {
            System.out.println(e);
        } finally {
            if (writer != null) {
                writer.close();
            }
        }

如您所见，我正在以下行中将数据写入服务器，

for (int length = 0; (length = input.read(buffer)) > 0;) {

                        output.write(buffer, 0, length);
                    }

但是在此之后，我必须执行以下操作，

 BufferedReader br = new BufferedReader(new InputStreamReader(connection.getInputStream()));
                String k = "", tmp = "";
                while ((tmp = br.readLine()) != null) {
                    System.out.println(tmp);
                    k += tmp;
                }

为什么我必须读取服务器的响应才能上传成功？

有人能解释一下吗？

提前致谢。

Answer 1

如果您尝试向服务器发布某些内容，并且在完全发送消息之前连接中断，则服务器将停止处理不完整的请求。一旦服务器完成接收请求并且您不等待得到其响应，那么您将永远不知道您的请求是否发送成功。因此，URLConnection 被设计为等待您收到响应。

另一个原因是您可能需要先构建和配置 URLConnection，然后在需要时通过调用 getInputStream 或 getResponseCode 来发送它。您可以更好地控制何时进行交易。

您不需要总是getInputStream，只需调用getResponseCode就足以完成请求。但是，整个输入流仍会发送到您的代码，但它们会被丢弃。