Adding Additional Servlets and Filters to Spring MVC (JavaConfig Based) Application

A Javaconfig based Spring MVC Application doesn’t need a deployment descriptor file (web.xml). The initialization is handled by Spring’s WebApplicationInitializer (refer this article for further details). To implement a initializer for our application we extends the AbstractAnnotationConfigDispatcherServletInitializer class which is an implementation of WebApplicationInitializer.

Looking at the way AbstractAnnotationConfigDispatcherServletInitializer is defined, it would only help us to create a DispatcherServlet and a ContextLoaderListener. How do we register additional servlets or filters?

The answer lies in the fact that we have the privilege to define multiple initializer classes in our Javaconfig based Spring MVC application. Thus we can create new initializer classes to register additional components to our web container.

Creating an Initializer class

The simplest way to create an initializer class is by implementing Spring’s WebApplicationInitializer interface. The WebApplicationInitializer interface provides a single method ‘onStartup(ServletContext context)’, the ServletContext passed as the parameter can be used to configure any servlet, filter or listener that are necessary for initialization of the application.

void onStartup(ServletContext servletContext);

Register Additional Servlet

The code below shows how to register an additional servlet to a Spring MVC Application:

public class CustomServletInitializer implements WebApplicationInitializer {

@Override public void onStartup(ServletContext servletContext) throws ServletException { ServletRegistration.Dynamic myServlet =
servletContext.addServlet("myCustomServlet", MyCustomServlet.class); myServlet.addMapping("/customServlet/**"); } }

ServletRegistration.Dynamic is a Servlet 3+ interface. Servlet 3+ specification allows dynamic servlet registration, Spring uses the underlying support to allows configuration using Javaconfig.


Register Additional Filter

Registering an additional filter is very similar to adding an additional servlet as shown below:

public class CustomFilterInitializer implements WebApplicationInitializer {

    @Override
    public void onStartup(ServletContext servletContext) throws ServletException {
	FilterRegistration.Dynamic myFilter = 
            servletContext.addFilter("myCustomFilter", MyCustomFilter.class);
	myFilter.addMappingForUrlPatterns(null, false, "*/customFilter/*");
    }
}

Register and Map filters to DispatcherServlet

To register one or more filters to DispatcherServlet we need to override the getServletFilters() method of the AbstractAnnotationConfigDispatcherServletInitializer. We are not required to define any mappings for these filters, since any filter returned from the getServletFilters() method implicitly maps to DispatcherServlet.

@Override
protected Filter[] getServletFilters() {
    return new Filter[] { new MyCustomFilter() };
}
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