JNLP File Generated By A Servlet

Question

Based on configuration issues with our system build, I have created a servlet that generates the JNLP file for an app we have created. Once the file is written and contains all the dynamically generated info, it is never written by the servlet again.
I noticed in creating the servlet, initially, that I actually HAD to write the JNLP file. Returning the JNLP file and setting the mime type in the HTTP response was not enough. When Web Start is invoked by the browser, it goes back to the stored JNLP file on the server.
Can anyone explain why this is necessary?? Its seems redundant. If I can create the JNLP dynamically and pass it back to the browser, why must Web Start retrieve the JNLP file AGAIN from the web server after it launches??
John Turner
Software Engineer
Context Media, Inc
[email protected]

Answer 1

There is a limit to what you can do with dynamically generated JNLP files. The combination of codebase and href must exactly specify the JNLP resource that you are retrieving. If you're using a servlet with parameters in a query string to generate the file, it should look like this:
<jnlp spec="1.0+" codebase="http://app.datadevelopment.com/app" href="MyServlet?param1=value1&param2=value2">
As long as the MIME type returned by the servlet is "application/x-java-jnlp-file", JWS will start just fine. Unfortunately, JWS will then bomb badly because it tries to use the href attribute as part of the file name for saving the file, and Windows at least doesn't like question marks in a file name.
A workaround is to replace the query string with path values:
<jnlp spec="1.0+" codebase="http://app.datadevelopment.com/app" href="MyServlet/param1=value1/param2=value2">
I'm not 100% sure that this will work with the '=' sign as my application needed only one parameter so I dispensed with the paramN=valueN and replaced the directory with valueN only. The point is, though, that each and every time the application is launched JWS will go to the web server to get the latest copy of the JNLP and the only way I have found to dynamically generate the content is to append the necessary parameters as path names after the script (JSP in my case, servlet in your case).
If you have multiple parameters and the '=' sign is a problem, strip out the parameter name and require that the parameters be specified in the same order each time. Use one directory delimiter per parameter.
You can use request.getPathInfo() to retrieve the path after your servlet.