Writing post data from one java servlet to another

Question

I am trying to write a servlet that will send a XML file (xml formatted string) to another servlet via a POST. (Non essential xml generating code replaced with "Hello there")

   StringBuilder sb=  new StringBuilder();
    sb.append("Hello there");

    URL url = new URL("theservlet's URL");
    HttpURLConnection connection = (HttpURLConnection)url.openConnection();                
    connection.setRequestMethod("POST");
    connection.setRequestProperty("Content-Length", "" + sb.length());

    OutputStreamWriter outputWriter = new OutputStreamWriter(connection.getOutputStream());
    outputWriter.write(sb.toString());
    outputWriter.flush();
    outputWriter.close();

This is causing a server error, and the second servlet is never invoked.

Answer 1

This kind of thing is much easier using a library like HttpClient. There's even a post XML code example:

PostMethod post = new PostMethod(url);
RequestEntity entity = new FileRequestEntity(inputFile, "text/xml; charset=ISO-8859-1");
post.setRequestEntity(entity);
HttpClient httpclient = new HttpClient();
int result = httpclient.executeMethod(post);

Answer 2

I recommend using Apache HTTPClient instead, because it's a nicer API.

But to solve this current problem: try calling connection.setDoOutput(true); after you open the connection.

StringBuilder sb=  new StringBuilder();
sb.append("Hello there");

URL url = new URL("theservlet's URL");
HttpURLConnection connection = (HttpURLConnection)url.openConnection();                
connection.setDoOutput(true);
connection.setRequestMethod("POST");
connection.setRequestProperty("Content-Length", "" + sb.length());

OutputStreamWriter outputWriter = new OutputStreamWriter(connection.getOutputStream());
outputWriter.write(sb.toString());
outputWriter.flush();
outputWriter.close();

Answer 3

The contents of an HTTP post upload stream and the mechanics of it don't seem to be what you are expecting them to be. You cannot just write a file as the post content, because POST has very specific RFC standards on how the data included in a POST request is supposed to be sent. It is not just the formatted of the content itself, but it is also the mechanic of how it is "written" to the outputstream. Alot of the time POST is now written in chunks. If you look at the source code of Apache's HTTPClient you will see how it writes the chunks.

There are quirks with the content length as result, because the content length is increased by a small number identifying the chunk and a random small sequence of characters that delimits each chunk as it is written over the stream. Look at some of the other methods described in newer Java versions of the HTTPURLConnection.

http://java.sun.com/javase/6/docs/api/java/net/HttpURLConnection.html#setChunkedStreamingMode(int)

If you don't know what you are doing and don't want to learn it, dealing with adding a dependency like Apache HTTPClient really does end up being much easier because it abstracts all the complexity and just works.

Answer 4

Don't forget to use:

connection.setDoOutput( true)

if you intend on sending output.