Java Servlet & JSP Cookbook
Problem
You want to use a filter to intercept form input and read it. Solution
Use the various getParameter methods of the ServletRequest API to take a look at parameter values in a filter. Discussion
When you develop a filter for a servlet, your filter class has to implement the javax.servlet.Filter interface. This means that your Filter class has to implement the doFilter(request,response) and destroy( ) methods of that interface. The doFilter method contains the hook to the filtered servlet's parameter values. The doFilter 's ServletRequest parameter has the getParameter , getParameterMap , getParameterNames , and getParameterValues methods which allow the filter to peek at a servlet's parameters and values. First, you have to map the Filter you have designed to the servlet. This chunk of web.xml maps a Filter object to a servlet named Viewer . <!-- any context-param elements go here --> <filter> <filter-name>ParamSnoop</filter-name> <filter-class>com.jspservletcookbook.ParamSnoop</filter-class> </filter> <filter-mapping> <filter-name>ParamSnoop</filter-name> <servlet-name>Viewer</servlet-name> </filter-mapping> <!-- web.xml continues --> Place the filter class in the WEB-INF/classes directory of your web application, or inside a JAR file that is placed in WEB-INF/lib . The servlet container creates an instance when the container starts up of each filter that is declared in web.xml . The container then executes the filter (calls its doFilter method) when a user requests any of the servlets the filter is mapped to. So the ParamSnoop filter can inspect a request made to the Viewer servlet before the servlet processes the request.
Example 7-14 gets access to the parameters in the intercepted request by calling ServletRequest.getParameterMap( ) . However, you are free to use other ServletRequest API methods to look at parameters, such as getParameter StringName . The getParameterMap( ) method returns a java.util.Map of parameter names and values, which you extract from the Map using a java.util.Iterator and its next ( ) method.
Example 7-14 also shows how to pull the key/value pairs out of the map and log the values using the ServletContext.log( ) method. Example 7-14. Snooping on parameter values with a servlet
package com.jspservletcookbook; import javax.servlet.*; import javax.servlet.http.*; import java.util.Map; import java.util.Iterator; import java.util.Map.Entry; public class ParamSnoop implements Filter { private FilterConfig config; /** Creates new ParamSnoop */ public ParamSnoop( ) { } public void init(FilterConfig filterConfig) throws ServletException{ this.config = filterConfig; } public void doFilter( ServletRequest request, ServletResponse response, FilterChain chain) throws java.io.IOException, ServletException { Map paramMap = request.getParameterMap( ); ServletContext context = config.getServletContext( ); /* use the ServletContext.log method to log param names/values */ context.log("doFilter called in: " + config.getFilterName( ) + " on " + (new java.util.Date( ))); context.log("Snooping the parameters in request: " + ((HttpServletRequest) request).getRequestURI( )); Iterator iter = paramMap.entrySet( ).iterator( ); while (iter.hasNext( )){ Map.Entry me = (Map.Entry) iter.next( ); context.log((String)me.getKey( ) + ": " + ((String[]) me.getValue( ))[0]); } //continue the request, response to next filter or servlet //destination chain.doFilter(request,response); } public void destroy( ){ /*called before the Filter instance is removed from service by the web container*/ } } The only reason we used the ServletContext.log( ) method was to display the inspection of parameters by the filter. Here is an example of the Tomcat log in <Tomcat-installation-directory>/logs showing the two parameters that were stored in the servlet request ( last , first ). In other words, the web browser request was http://localhost:8080/home/viewer?first=Bruce&last=Perry . 2003-04-13 17:13:33 doFilter called in: ParamSnoop on Sun Apr 13 17:13:33 EDT 2003 2003-04-13 17:13:33 Snooping the parameters in request: /home/viewer 2003-04-13 17:13:33 last: Perry 2003-04-13 17:13:33 first: Bruce See Also
Recipe 7.1 on handling a POST request in a servlet; Recipe 7.7 on using a servlet to add a parameter to a query string; Chapter 19 on filtering requests and responses; Chapter SRV.6 on Filters in the Servlet 2.4 specification. |