public  void destroy() 
{
        filterConfig = null;
        containerTweaks = null;
}

 public  void doFilter(ServletRequest rq,
    ServletResponse rs,
    FilterChain chain) throws IOException, ServletException 
{
        HttpServletRequest request = (HttpServletRequest) rq;
        HttpServletResponse response = (HttpServletResponse) rs;
        ServletContext servletContext = filterConfig.getServletContext();
        SiteMeshWebAppContext webAppContext = new SiteMeshWebAppContext(request, response, servletContext);
        ContentProcessor contentProcessor = initContentProcessor(webAppContext);
        DecoratorSelector decoratorSelector = initDecoratorSelector(webAppContext);
        if (filterAlreadyAppliedForRequest(request)) {
            // Prior to Servlet 2.4 spec, it was unspecified whether the filter should be called again upon an include().
            chain.doFilter(request, response);
            return;
        }
        if (!contentProcessor.handles(webAppContext)) {
            // Optimization: If the content doesn't need to be processed, bypass SiteMesh.
            chain.doFilter(request, response);
            return;
        }
        if (containerTweaks.shouldAutoCreateSession()) {
            // Some containers (such as Tomcat 4) will not allow sessions to be created in the decorator.
            // (i.e after the response has been committed).
            request.getSession(true);
        }
        try {
            Content content = obtainContent(contentProcessor, webAppContext, request, response, chain);
            if (content == null) {
                return;
            }
            Decorator decorator = decoratorSelector.selectDecorator(content, webAppContext);
            decorator.render(content, webAppContext);
        } catch (IllegalStateException e) {
            // Some containers (such as WebLogic) throw an IllegalStateException when an error page is served.
            // It may be ok to ignore this. However, for safety it is propegated if possible.
            if (!containerTweaks.shouldIgnoreIllegalStateExceptionOnErrorPage()) {
                throw e;
            }
        } catch (RuntimeException e) {
            if (containerTweaks.shouldLogUnhandledExceptions()) {
                // Some containers (such as Tomcat 4) swallow RuntimeExceptions in filters.
                servletContext.log("Unhandled exception occurred whilst decorating page", e);
            }
            throw e;
        }
}

Main method of the Filter.

Checks if the Filter has been applied this request. If not, parses the page and applies com.opensymphony.module.sitemesh.Decorator (if found).

 public  void init(FilterConfig filterConfig) 
{
        this.filterConfig = filterConfig;
        containerTweaks = new ContainerTweaks();
}

 protected ContentProcessor initContentProcessor(SiteMeshWebAppContext webAppContext) 
{
        // TODO: Remove heavy coupling on horrible SM2 Factory
        Factory factory = Factory.getInstance(new Config(filterConfig));
        factory.refresh();
        return new PageParser2ContentProcessor(factory);
}

 protected DecoratorSelector initDecoratorSelector(SiteMeshWebAppContext webAppContext) 
{
        // TODO: Remove heavy coupling on horrible SM2 Factory
        Factory factory = Factory.getInstance(new Config(filterConfig));
        factory.refresh();
        return new DecoratorMapper2DecoratorSelector(factory.getDecoratorMapper());
}