How do I get URLs of other servlets from within a servlet

Question

I have two servlets (S1 and S2). S1 renders a HTML-Page which acces S2 via an URL (img src="URL"). I know the servlet name of S2, but not the URL. The URL is configured in the web.xml of course, but how can I access that from S1?

Answer 1

I would guess, that most implementations of the ServletConfig hold that mapping informations (org.apache.catalina.core.StandardWrapper does), but since the ServletConfig-Interface don't provides a getter, you'll have to do some tricks to get it and would bind your application to a specific implementation or application server.

Maybe you just read it from the web.xml. Just select all "servlet-mapping" Elements with the given "servlet-name" and read the "url-pattern". Since this is in the spec, that should work on ever app server out there.

EDIT:

Here is the dirty example. Getting the URL mappings using refelction:

@Override
public void init(ServletConfig config) throws ServletException {
 super.init(config);

 try {

  Class<?> clazz = config.getClass();
  Field configField = clazz.getDeclaredField("config");
  configField.setAccessible(true);
  StandardWrapper standardWrapper = (StandardWrapper) configField.get(config);

  clazz = standardWrapper.getClass();
  Field mappingsField = clazz.getDeclaredField("mappings");
  mappingsField.setAccessible(true);
  List<?> mappings = (List<?>) mappingsField.get(standardWrapper);

  System.out.println(mappings);

 }catch (Exception e) {
  logger.error("", e);
 }
}

That works in my JSF, Tomcat environment. The config Object is "org.apache.catalina.core.StandardWrapperFacade" and has a field called "config" which hold a "org.apache.catalina.core.StandardWrapper", which has a field called "mappings".

But as I said, this is a dirty hack!

Answer 2

Use:

HttpServletResponse.encodeURL(String)

Which in your case should be something like this:

response.encodeURL("/S2");

This method will take care of any URL re-writing that needs to take place to maintain session state and I think will prepend the necessary path info to the URL.

I use JSTL these days so this I'm a little rusty on that last point but if the path info isn't prepended you can get it from the request and add it yourself.

Answer 3

Perhaps you should supply the URL of the second servlet to the first as a servlet parameter. I realise this means encoding the URL twice in the web.xml (which I really abhor), but to avoid problems you can always build the web.xml as part of your build, and populate from a properties file.

A little bit nasty and fiddly, I appreciate, but in the absence of any cross-container API solution, perhaps it's the most pragmatic solution.