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