Sending and receiving binary data in Servlets

Question

I'm attempting to write a Java Servlet to receive binary data requests and reply to them, using HttpServletRequest.getOutputStream() and HttpServletResponse.getInputStream(). This is for a project which involves having a request sent by a Silverlight client to which this servlet responds to through an HTTP POST connection. For the time being, to test the Servlet I'm implementing a client in Java which I'm more familiar with than Silverlight.

The problem is that in my test project I send the data from a Servlet client as a byte array and expect to receive a byte array with the same length -- only it doesn't, and instead I'm getting a single byte. Therefore I'm posting here the relevant code snippets in the hopes that you might point me where I'm doing wrong and hopefully provide relevant bibliography to help me further.

So here goes.

The client servlet handles POST requests from a very simple HTML page with a form which I use as front-end. I'm not too worried about using JSP etc, instead I'm focused on making the inter-Servlet communication work.

// client HttpServlet invokes this method from doPost(request,response)
private void process(HttpServletRequest request, HttpServletResponse response) 
throws ServletException, IOException {
  String firstName = (String) request.getParameter("firstname");
  String lastName = (String) request.getParameter("lastname");
  String xmlRequest = "<MyRequest><Person><Name Firstname=\""+firstName+"\" Lastname=\""+lastName+"\" /></Person></MyRequest>";

  OutputStream writer = null;
  InputStream reader = null;
  try {
    URL url = new URL("http://localhost:8080/project/Server");
    URLConnection conn = url.openConnection();
    conn.setDoInput(true);
    conn.setDoOutput(true);

    writer = conn.getOutputStream();
    byte[] baXml = xmlRequest.getBytes("UTF-8");
    writer.write(baXml, 0,baXml.length);
    writer.flush();
    // perhaps I should be waiting here? how?

    reader = conn.getInputStream();
    int available = reader.available();
    byte[] data = new byte[available];
    reader.read(data,0,available);
    String xmlResponse = new String(data,"UTF-8");

    PrintWriter print = response.getWriter();
    print.write("<html><body>Response:<br/><pre>");
    print.write(xmlResponse);
    print.write("</pre></body></html>");
    print.close();

  } finally {
    if(writer!=null)
      writer.close();
    if(reader!=null)
      reader.close();
  }
}

The server servlet handles HTTP POST requests. This is done by receiving requests the requests from a client Servlet for testing purposes above, but in the future I intend to use it for clients in other languages (specifically, Silverlight).

// server HttpServlet invokes this method from doPost(request,response)
private void process(HttpServletRequest request, HttpServetResponse response)
throws ServletException, IOException {
  ServletInputStream sis = null;
  try {
    sis = request.getInputStream();
    // maybe I should be using a BufferedInputStream 
    // instead of the InputStream directly?
    int available = sis.available();
    byte[] input = new byte[available];
    int readBytes = sis.read(input,0,available);
    if(readBytes!=available) {
      throw new ServletException("Oops! readBytes!=availableBytes");
    }

    // I ONLY GET 1 BYTE OF DATA !!! 
    // It's the first byte of the client message, a '<'.
    String msg = "Read "+readBytes+" bytes of "
                 +available+" available from request InputStream.";
    System.err.println("Server.process(HttpServletRequest,HttpServletResponse): "+msg);
    String xmlReply = "<Reply><Message>"+msg+"</Message></Reply>";
    byte[] data = xmlReply.getBytes("UTF-8");
    ServletOutputStream sos = response.getOutputStream();
    sos.write(data, 0,data.length);
    sos.flush();
    sos.close();

  } finally {
    if(sis!=null)
      sis.close();
  }
}

I have been sticking to byte arrays instead of using BufferInputStreams so far because I've not decided yet if I'll be using e.g. Base64-encoded strings to transmit data or if I'll be sending binary data as-is.

Thank you in advance.

score 8 · Answer 1 · answered Dec 14 '11 at 15:27

To copy input stream to output stream use the standard way:

InputStream is=request.getInputStream();
OutputStream os=response.getOutputStream();
byte[] buf = new byte[1000];
for (int nChunk = is.read(buf); nChunk!=-1; nChunk = is.read(buf))
{
    os.write(buf, 0, nChunk);
}

score 3 · Accepted Answer · edited May 23 '11 at 10:11

3

The one thing I can think of is that you are reading only request.getInputStream().available() bytes, then deciding that you have had everything. According to the documentation, available() will return the number of bytes that can be read without blocking, but I don't see any mention of whether this is actually guaranteed to be the entire content of the input stream, so am inclined to assume that no such guarantees are made.

I'm not sure how to best find out when there is no more data (maybe Content-Length in the request can help?) without risking blocking indefinitely at EOF, but I would try looping until having read all the data from the input stream. To test that theory, you could always scan the input for a known pattern that occurs further into the stream, maybe a > matching the initial < that you are getting.

edited May 23 '11 at 10:11

jbatista

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answered Mar 03 '11 at 12:13

user

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Just read until `read()` returns `-1`. See also page 1 of every basic Java IO book/tutorial. – BalusC Mar 03 '11 at 12:40
@BalusC: Right. That's what I get for only skimming the headers in the documentation. – user Mar 03 '11 at 12:43
You're right, thanks Michael! I'm using a `ByteArrayOutputStream` to write bytes read from the `ServletInputStream`, and then get a byte array with `ByteArrayOutputStream.toByteArray()`. – jbatista Mar 03 '11 at 12:44

Sending and receiving binary data in Servlets

2 Answers2