Tutorial :Is it portable to reference the request parameter map after a request-cycle?



Question:

I'd like to know if its conform to the java servlet specification 2.5 to reference/save the return value of request.getParameterMap() between requests.

The final specification only states at page 203:

Returns: an immutable java.util.Map containing parameter names as keys and parameter values as map values. The keys in the parameter map are of type String. The values in the parameter map are of type String array.

But it's not clear to me:

  • if the Map is only immutable to the application, not to the application server

  • if it's allowed that the application server may re-use the instance for another request (for example, if the parameters and their values are the same as in the previous request)

EDIT: The reason for this: I'd like to save the request map of each request and in case of an error to print them out for diagnostic purposes.


Solution:1

if the Map is only immutable to the application, not to the application server

It's immutable in the API, yes. It also makes sense, what's the point of changing the request parameter map? The servletcontainer implementation in turn can of course hold of a mutable map of it "behind the scenes". But you should not worry about the implementation specific details.

if it's allowed that the application server may re-use the instance for another request (for example, if the parameters and their values are the same as in the previous request)

No, the server doesn't do that. It just creates a new one. The parameters are bound to a specific request.

If you want to collect the parameters of every request during a session, then you need to do this yourself. The best place for this would be a Filter.

public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws ServletException, IOException {      HttpServletRequest hsr = (HttpServletRequest) request;      List<Map<String, String[]>> allParameters = (List<Map<String, String[]>>) hsr.getSession().getAttribute("allParameters");      if (allParameters == null) {          allParameters = new ArrayList<Map<String,String[]>>();          hsr.getSession().setAttribute("allParameters", allParameters);      }      allParameters.add(hsr.getParameterMap());      chain.doFilter(request, response);  }  


Solution:2

For all intents and purposes, you cannot modify the contents of the map. However, if you want to keep for future reference, you can always create a new map of your own, copy over all the key/value pairs, and keep it in the Session f.e. (That will cause funkiness if the user has 2 windows open at the same time, but that story is for another time...)


Solution:3

I think you can safely save it for next request. I do this all the time.

Not sure about other implementations. It creates a new HashMap for every requests in Tomcat,

    parameters = new HashMap();      parameters = copyMap(getRequest().getParameterMap());      mergeParameters();      parsedParams = true;  

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