如何改善从套接字读取XML的方式

Question

I am working on a Java client that connects to a server via socket and sends/receives XML messages. I know what the schema of the messages will be. I have it working currently, but it feels messy and I haven't tested it yet to see how it handles multiple XMLs sent from the server at once.

Here is the code:

  private static BufferedReader socketIn;
  private static PrintWriter socketOut;
  private static final String SERVICES_FILE = "/etc/services";

  public static void main(String[] args) {
    int port = getServicePortByName("Service1", "tcp");
    try {
      logger.info("Creating socket port with ip: " + InetAddress.getLocalHost() + " and port: " + port);
      Socket socket = new Socket(InetAddress.getLocalHost(), port);
      socketOut = new PrintWriter(socket.getOutputStream(), true);

      String message = createMessageXML("LOGIN");
      socketOut.println(message);

      socketIn = new BufferedReader(new InputStreamReader(socket.getInputStream()));
      StringBuilder inputLine = new StringBuilder();
      String tmp;

      while (true) {
      //loop to keep listening/parsing
        while(socketIn.ready()) {
          tmp = socketIn.readLine();
          inputLine.append(tmp);
        }
        if(inputLine.length() > 0) {
          parseReceivedMessage(inputLine.toString());
        }
        inputLine.replace(0, inputLine.length(), "");
      }

    } catch (UnknownHostException ex) {
      Logger.getLogger(TestClient.class.getName()).log(Level.SEVERE, null, ex);
    } catch (IOException ex) {
      Logger.getLogger(TestClient.class.getName()).log(Level.SEVERE, null, ex);
    }
  }

the parseReceivedMessage method just takes in a String and converts it to a Document object. My question is, is there a better or cleaner way to do this instead of using a StringBuilder? I originally wanted to pass an InputStream to the DOM builder parse method. I saw some examples making use of ByteArray streams, but I had some issues with those. Let me know if anything needs clarification.

Answer 1

ready() is not a valid test for end of message. Get rid of that and the StringBuilder and the readLine() calls altogether and just pass the input stream directly to the DOM parser.

Answer 2

I would strongly suggest you design a framing protocol around your XML messages. Prefix each XML message with a length value that specifies how many bytes are being transmitted, then send the XML itself (in bytes, not characters). The receiver can then read the length, then read the specified number of bytes and pass them to the XML parser, then read the next length, and so on. For example:

  private static DataInputStream socketIn;
  private static DataOutputStream socketOut;
  private static final String SERVICES_FILE = "/etc/services";

  public static void main(String[] args) {
    int port = getServicePortByName("Service1", "tcp");
    try {
      logger.info("Creating socket port with ip: " + InetAddress.getLocalHost() + " and port: " + port);
      Socket socket = new Socket(InetAddress.getLocalHost(), port);
      socketIn = new DataInputStream(new BufferedInputStream(socket.getInputStream()));
      socketOut = new DataOutputStream(new BufferedOutputStream(socket.getOutputStream()));

      byte[] message = createMessageXML("LOGIN");
      socketOut.writeInt(message.length());
      socketOut.write(message, 0, message.length());

      int len = socketIn.readInt();
      if (len > 0)
      {
          message = new byte[len];
          socketIn.readFully(message);
          parseReceivedMessage(message);
      }

    } catch (UnknownHostException ex) {
      Logger.getLogger(TestClient.class.getName()).log(Level.SEVERE, null, ex);
    } catch (IOException ex) {
      Logger.getLogger(TestClient.class.getName()).log(Level.SEVERE, null, ex);
    }
  }