protected  void applyDecorator(Page page,
    Decorator decorator,
    HttpServletRequest request,
    HttpServletResponse response) throws ServletException, IOException 
{
        try {
            request.setAttribute(PAGE, page);
            ServletContext context = filterConfig.getServletContext();
            // see if the URI path (webapp) is set
            if (decorator.getURIPath() != null) {
                // in a security conscious environment, the servlet container
                // may return null for a given URL
                if (context.getContext(decorator.getURIPath()) != null) {
                    context = context.getContext(decorator.getURIPath());
                }
            }
            // get the dispatcher for the decorator
            RequestDispatcher dispatcher = context.getRequestDispatcher(decorator.getPage());
            // create a wrapper around the response
            dispatcher.include(request, response);
            // set the headers specified as decorator init params
            while (decorator.getInitParameterNames().hasNext()) {
                String initParam = (String) decorator.getInitParameterNames().next();
                if (initParam.startsWith("header.")) {
                    response.setHeader(initParam.substring(initParam.indexOf('.')), decorator.getInitParameter(initParam));
                }
            }
            request.removeAttribute(PAGE);
        }
        catch (RuntimeException e) {
            // added a print message here because otherwise Tomcat swallows
            // the error and you never see it = bad!
            if (Container.get() == Container.TOMCAT)
                e.printStackTrace();
            throw e;
        }
}

Apply com.opensymphony.module.sitemesh.Decorator to com.opensymphony.module.sitemesh.Page and write to the response.

 public  void destroy() 
{
        factory = null;
}

Shutdown filter.

 public  void doFilter(ServletRequest rq,
    ServletResponse rs,
    FilterChain chain) throws IOException, ServletException 
{
        HttpServletRequest request = (HttpServletRequest) rq;
        if (rq.getAttribute(FILTER_APPLIED) != null || factory.isPathExcluded(extractRequestPath(request))) {
            // ensure that filter is only applied once per request
            chain.doFilter(rq, rs);
        }
        else {
            request.setAttribute(FILTER_APPLIED, Boolean.TRUE);
            // force creation of the session now because Tomcat 4 had problems with
            // creating sessions after the response had been committed
            if (Container.get() == Container.TOMCAT) {
                request.getSession(true);
            }
            HttpServletResponse response = (HttpServletResponse) rs;
            // parse data into Page object (or continue as normal if Page not parseable)
            Page page = parsePage(request, response, chain);
            if (page != null) {
                page.setRequest(request);
                Decorator decorator = factory.getDecoratorMapper().getDecorator(request, page);
                if (decorator != null && decorator.getPage() != null) {
                    applyDecorator(page, decorator, request, response);
                    page = null;
                    return;
                }
                // if we got here, an exception occured or the decorator was null,
                // what we don't want is an exception printed to the user, so
                // we write the original page
                writeOriginal(request, response, page);
                page = null;
            }
        }
}

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 FilterConfig getFilterConfig() 
{
        return filterConfig;
}

 public  void init(FilterConfig filterConfig) 
{
        if (filterConfig != null) {
            this.filterConfig = filterConfig;
            factory = Factory.getInstance(new Config(filterConfig));
        }
        else {
            destroy();
        }
}

Set FilterConfig, and get instance of com.opensymphony.module.sitemesh.DecoratorMapper .

 protected Page parsePage(HttpServletRequest request,
    HttpServletResponse response,
    FilterChain chain) throws IOException, ServletException 
{
        try {
            PageResponseWrapper pageResponse = new PageResponseWrapper(response, factory);
            chain.doFilter(request, pageResponse);
            // check if another servlet or filter put a page object to the request
            Page result = (Page)request.getAttribute(PAGE);
            if (result == null) {
                // parse the page
                result = pageResponse.getPage();
            }
            request.setAttribute(USING_STREAM, new Boolean(pageResponse.isUsingStream()));
            return result;
        }
        catch (IllegalStateException e) {
            // weblogic throws an IllegalStateException when an error page is served.
            // it's ok to ignore this, however for all other containers it should be thrown
            // properly.
            if (Container.get() != Container.WEBLOGIC) throw e;
            return null;
        }
}

Continue in filter-chain, writing all content to buffer and parsing into returned com.opensymphony.module.sitemesh.Page object. If com.opensymphony.module.sitemesh.Page is not parseable, null is returned.

 public  void setFilterConfig(FilterConfig filterConfig) 
{
        init(filterConfig);
}