Programmatic instantiation of Components like servlets, filters, and listeners

With programmatic definition, we can instantiate components programmatically and then configure them. Servlet 3.0 Specification supports dynamic instantiation of servlets and filters.

In this example, we will see how you might programmatically instantiate servlets, filters, and listeners.

@WebListener
public class MyServletContextListener implements ServletContextListener {

public void contextInitialized(ServletContextEvent sce) {
LOG.info("Context Listener Initialized");
doProgrammaticRegistration(sce.getServletContext());
}

private void doProgrammaticRegistration(ServletContext sc) {

// servlet definition
ServletRegistration.Dynamic dynaServlet = sc.addServlet(
"MyProgrammaticServlet",
"com.servlets.MyProgrammaticServlet");
dynaServlet.addMapping("/myprogrammaticservlet");

// filter definition
FilterRegistration.Dynamic dynaFilter = sc.addFilter(
"MyProgrammaticFilter",
"com.filters.MyProgrammaticFilter");
EnumSet dispatcherType = EnumSet.of(
DispatcherType.REQUEST, DispatcherType.FORWARD);
dynaFilter.addMappingForServletNames(dispatcherType, true,
"MyProgrammaticServlet");
}
}

As shown, during context initialization, we instantiate a new servlet and returned class ServletRegistration.Dynamic instance is used to configure the appropriate servlet mapping. We do the similar steps with a filter. These steps are similar to what would have happened when a servlet container encountered the <servlet>, <servlet-mapping>, <filter> and <filter-mapping> elements in a web deployment descriptor.

As it can be seen in the next code listings, the servlet and filter are rather straightforward. Since the configuration occurs programmatically, they do not even need any annotations to be used.

//Note: not even an annotation is required
public class MyProgrammaticFilter implements Filter {
private final Logger LOG = Logger.getLogger("MyProgrammaticFilter");

public void destroy() { }

public void doFilter(ServletRequest req, ServletResponse resp,
FilterChain chain) throws ServletException, IOException {

LOG.info("Filtering: " + ((HttpServletRequest)req).getRequestURI());
chain.doFilter(req, resp);
}

public void init(FilterConfig config) throws ServletException {
LOG.info("Filter: " + "Initializing ProgrammaticFilter");
}
}

//Note: not even an annotation is required.
public class MyProgrammaticServlet extends HttpServlet {
private final Logger LOG = Logger.getLogger("MyProgrammaticServlet");

protected void doPost(HttpServletRequest request,
HttpServletResponse response) throws ServletException, IOException {
doGet(request, response);
}

protected void doGet(HttpServletRequest request,
HttpServletResponse response) throws ServletException, IOException {
LOG.info("Within request : " + getServletName());
String html = "Added programmatically (no annotation/web.xml entries): " + new Date();

response.setContentType("text/html");
PrintWriter out = response.getWriter();
out.println(html);
}
}

Hope this will help.