SiteMesh3 configuation example, showing custom decorator selector based on HttpSession.

MyCustomDecoratorSelector.java

public class MyCustomDecoratorSelector extends DecoratorSelector<WebAppContext> {
    @Override
    public String[] selectDecoratorPaths(Content content, WebAppContext context) throws IOException {
        HttpServletRequest request = context.getRequest();
        if (request.getSession().getValue("is_admin") != null) {
            return new String[] {"/decorators/admin-decorator.html"};
        } else {
            return new String[] {"/decorators/normal-decorator.html"};
        }
    }
}

MyCustomSiteMeshFilter.java

public class MyCustomSiteMeshFilter extends ConfigurableSiteMeshFilter {
    @Override
    protected void applyCustomConfiguration(SiteMeshFilterBuilder builder) {
        builder.setCustomDecoratorSelector(new MyCustomDecoratorSelector());
    }
}

web.xml

<web-app>
    <filter>
        <filter-name>sitemesh</filter-name>
        <filter-class>com.blah.MyCustomSiteMeshFilter</filter-class>
    </filter>
    <filter-mapping>
         <filter-name>sitemesh</filter-name>
         <url-pattern>/*</url-pattern>
    </filter-mapping>
</web-app>