Java Filter URL pattern specific to request params

Question

We have a situation where we want to use filter for URL's containing some specific request parameters, e.g:

http://mydomain.com/?id=78&formtype=simple_form&.......    
http://mydomain.com/?id=788&formtype=special_form&.......    

and so on, id are fetched at run time, I want configure filter in web.xml only if formtype=special_form. How should achieve the solution? Can Filter be configured with regex patterns?

Answer 1

As far as I know there is no solution for matching requests to filters by query string directly in web.xml. So you could register the filter in your web.xml using init-params to make the filter configurable and set a pattern via void init(FilterConfig filterConfig) in your javax.servlet.Filter implementation.

package mypackage;

import java.io.IOException;

import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServletRequest;

public class MyFilter implements Filter {

    private String pattern;

    @Override
    public void destroy() {
        // TODO Auto-generated method stub
    }

    @Override
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
            throws IOException, ServletException {
        // check whether we have a httpServletRequest and a pattern
        if (this.pattern != null && request instanceof HttpServletRequest) {
            // resolve the query string from the httpServletRequest
            String queryString = ((HttpServletRequest) request).getQueryString();
            // check whether a query string exists and matches the given pattern
            if (queryString != null && queryString.matches(pattern)) {
                // TODO do someting special
            }
        }
        chain.doFilter(request, response);
    }

    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        this.pattern = filterConfig.getInitParameter("pattern");
    }

}

The configuration would look like this in your web.xml:

<!-- MyFilter -->
<filter>
    <filter-name>myFilter</filter-name>
    <filter-class>mypackage.MyFilter</filter-class>
    <init-param>
        <param-name>pattern</param-name>
        <param-value>{{PATTERN HERE}}</param-value>
    </init-param>
</filter>

<filter-mapping>
    <filter-name>myFilter</filter-name>
    <url-pattern>/*</url-pattern>
</filter-mapping>

Further readings:
http://java.sun.com/javaee/5/docs/api/javax/servlet/Filter.html

Answer 2

You must parse the URL params in the body of the filter, not in web.xml;

I made such a thing, but I used a phase listener, a configuration entry (in a configuration file) that mapped URL params to FQN of classes that handle a particular "action" (by action I mean a URL param from a GET/POST); than, the phase listener would parse all the URL params from each GET/POST and send each of them to a Dispatcher. The Dispatcher's job is to call the right handler (singleton object corresponding to the FQN for that URL param). The handler then just does the specific thing with the URL value he receives.

The same thing can be made by using a filter instead of phase listener.