web.xml vs Initializer with Spring

1. Overview

In this article we’ll cover three different approaches of configuring a DispatcherServlet available in recent versions of the Spring Framework:

1. We’ll start with an XML configuration and a web.xml file
2. Then we’ll migrate the Servlet declaration from the web.xml file to Java config, but we’ll leave any other configuration in XML
3. Finally in the third and final step of the refactoring, we’ll have a 100% Java-configured project

2. The DispatcherServlet

One of the core concepts of Spring MVC is the DispatcherServlet. The Spring documentation defines it as:

A central dispatcher for HTTP request handlers/controllers, e.g. for web UI controllers or HTTP-based remote service exporters. Dispatches to registered handlers for processing a web request, providing convenient mapping and exception handling facilities.

Basically the DispatcherServlet is the entry point of every Spring MVC application. Its purpose is to intercept HTTP requests and to dispatch them to the right component that will know how to handle it.

3. Configuration With web.xml

If you deal with legacy Spring projects it is very common to find XML configuration and until Spring 3.1 the only way to configure the DispatcherServlet was with the WEB-INF/web.xml file. In this case there are two steps required.

Let’s see an example configuration – the first step is the Servlet declaration:

<servlet>
    <servlet-name>dispatcher</servlet-name>
    <servlet-class>
        org.springframework.web.servlet.DispatcherServlet
    </servlet-class>
    <init-param>
        <param-name>contextConfigLocation</param-name>
        <param-value>/WEB-INF/spring/dispatcher-config.xml</param-value>
    </init-param>
    <load-on-startup>1</load-on-startup>
</servlet>

With this block of XML we are declaring a servlet that:

1. Is named “dispatcher“
2. Is an instance of org.springframework.web.servlet.DispatcherServlet
3. Will be initialized with a parameter named contextConfigLocation which contains the path to the configuration XML

load-on-startup is an integer value that specifies the order for multiple servlets to be loaded. So if you need to declare more than one servlet you can define in which order they will be initialized. Servlets marked with lower integers are loaded before servlets marked with higher integers.

Now our servlet is configured. The second step is declaring a servlet-mapping:

<servlet-mapping>
    <servlet-name>dispatcher</servlet-name>
    <url-pattern>/</url-pattern>
</servlet-mapping>

With the servlet mapping we are bounding it by its name to a URL pattern that specifies what HTTP requests will be handled by it.

4. Hybrid Configuration

With the adoption of the version 3.0 of Servlet APIs, the web.xml file has become optional, and we can now use Java to configure the DispatcherServlet.

We can register a servlet implementing a WebApplicationInitializer. This is the equivalent of the XML configuration above:

public class MyWebAppInitializer implements WebApplicationInitializer {
    @Override
    public void onStartup(ServletContext container) {
        XmlWebApplicationContext context = new XmlWebApplicationContext();
        context.setConfigLocation("/WEB-INF/spring/dispatcher-config.xml");

        ServletRegistration.Dynamic dispatcher = container
          .addServlet("dispatcher", new DispatcherServlet(context));

        dispatcher.setLoadOnStartup(1);
        dispatcher.addMapping("/");
    }
}

In this example we are:

1. Implementing the WebApplicationInitializer interface
2. Overriding the onStartup method we create a new XmlWebApplicationContext configured with the same file passed as contextConfigLocation to the servlet in the XML example
3. Then we are creating an instance of DispatcherServlet with the new context that we just instantiated
4. And finally we are registering the servlet with a mapping URL pattern

So we used Java to declare the servlet and bind it to a URL mapping but we kept the configuration in a separated XML file: dispatcher-config.xml.

5. 100% Java Configuration

With this approach our servlet is declared in Java, but we still need an XML file to configure it. With WebApplicationInitializer you can achieve a 100% Java configuration.

Let’s see how we can refactor the previous example.

The first thing we will need to do is create the application context for the servlet.

This time we will use an annotation based context so that we can use Java and annotations for configuration and remove the need for XML files like dispatcher-config.xml:

AnnotationConfigWebApplicationContext context
  = new AnnotationConfigWebApplicationContext();

This type of context can then be configured registering a configuration class:

context.register(AppConfig.class);

Or setting an entire package that will be scanned for configuration classes:

context.setConfigLocation("com.example.app.config");

Now that our application context is created, we can add a listener to the ServletContext that will load the context:

container.addListener(new ContextLoaderListener(context));

The next step is creating and registering our dispatcher servlet:

ServletRegistration.Dynamic dispatcher = container
  .addServlet("dispatcher", new DispatcherServlet(context));

dispatcher.setLoadOnStartup(1);
dispatcher.addMapping("/");

Now our WebApplicationInitializer should look like this:

public class MyWebAppInitializer implements WebApplicationInitializer {
    @Override
    public void onStartup(ServletContext container) {
        AnnotationConfigWebApplicationContext context
          = new AnnotationConfigWebApplicationContext();
        context.setConfigLocation("com.example.app.config");

        container.addListener(new ContextLoaderListener(context));

        ServletRegistration.Dynamic dispatcher = container
          .addServlet("dispatcher", new DispatcherServlet(context));
        
        dispatcher.setLoadOnStartup(1);
        dispatcher.addMapping("/");
    }
}

Java and annotation configuration offers many advantages. Usually it leads to shorter and more concise configuration and annotations provide more context to declarations, as it’s co-located with the code that they configure.

But this is not always a preferable or even possible way. For example some developers may prefer keeping their code and configuration separated, or you may need to work with third party code that you can’t modify.

6. Conclusion

In this article we covered different ways to configure a DispatcherServlet in Spring 3.2+ and it’s up to you to decide which one to use based on your preferences. Spring will accommodate to your decision whatever you choose.