How to use a filter in a Struts1 application? - servlet-filters

Question

I have a Struts1 application and am unsuccessfully trying to get a filter to work in order to add headers/etc to the response after the action has completed, but am not able to get it to work.
By the time the struts action is completed and control is returned to my filter, the response is already committed.
web.xml:
<filter>
<filter-name>workflow</filter-name>
<filter-class>webapp.session.WorkflowModifierFilter</filter-class>
</filter>
<filter-mapping>
<filter-name>workflow</filter-name>
<servlet-name>action</servlet-name>
</filter-mapping>
<servlet>
<servlet-name>action</servlet-name>
<servlet-class>org.apache.struts.action.ActionServlet</servlet-class>
<init-param>
<param-name>config</param-name>
<param-value>/WEB-INF/struts-config.xml</param-value>
</init-param>
<init-param>
<param-name>debug</param-name>
<param-value>0</param-value>
</init-param>
<init-param>
<param-name>detail</param-name>
<param-value>2</param-value>
</init-param>
<load-on-startup>1</load-on-startup>
</servlet>
My filter is the following:
public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
if( !servletResponse.isCommitted() )
log.debug("Not Committed! Can modify it!");
filterChain.doFilter(servletRequest, servletResponse);
// depending on the response, I'd like to add headers here
if( servletResponse.getStatus() == 200 )
servletResponse.addIntHeader( "x-success", 1 );
if( servletResponse.isCommitted() )
log.debug("ACCCK! Response Already Committed");
}
However, I noticed that my x-success header was never added. A little digging, and I noticed that my response was already returned/committed by the time the control returned to my filter chain.
What is the proper way to do this in Struts1? Does the filter execution not supposed to wrap the entire servlet? Why is my response being committed prior to my filter finishing? Where else can I add headers based on the response (post action processing) in Struts1?

Answer 1

When you call filterChain.doFilter you pass control from your filter to requested page (ie Struts), which is then free to commit the response if it chooses. Since you want to examine the result of the Struts servlet, you should create a response wrapper class (extends HttpServletRequestWrapper) and pass that to filterChain.doFilter rather than the response parameter that your filter is passed.
HttpServletResponse httpResponse = (HttpServletResponse) servletResponse;
ServletResponse wrapper = new MyHttpServletRequestWrapper(httpResponse);
filter.doChain(servletRequest, wrapper);
The HttpServletRequestWrapper constructor accepts a HttpServletResponse as input and delegates all methods to the wrapped response, but you can override them in your class as necessary. If you want to prevent the response from being committed you'll want to override methods such as flushBuffer, getOutputSteam (the returned output stream can be flushed, committing the response), and getWriter.
But you may not need to do that - try overriding the setStatus method in the wrapper class to check the status code and add the header when it is called:
public class MyHttpServletResponseWrapper extends HttpServletResponseWrapper {
public MyHttpServletResponseWrapper(HttpServletResponse response) {
super(response);
}
#Override
public void setStatus(int sc) {
if(sc == 200) {
addIntHeader("x-success", 1);
}
super.setStatus(sc);
}
}
Although this question was asked in relation to Struts 1, it applies to any web framework; my answer is based on work I did for JSF filters.