Package org.springframework.web

Interface WebApplicationInitializer

All Known Implementing Classes:
AbstractAnnotationConfigDispatcherServletInitializer, AbstractContextLoaderInitializer, AbstractDispatcherServletInitializer, AbstractReactiveWebInitializer
public interface WebApplicationInitializer
Interface to be implemented in Servlet environments in order to configure the ServletContext programmatically -- as opposed to (or possibly in conjunction with) the traditional web.xml-based approach.

Implementations of this SPI will be detected automatically by SpringServletContainerInitializer, which itself is bootstrapped automatically by any Servlet container. See its Javadoc for details on this bootstrapping mechanism.

Example

The traditional, XML-based approach

Most Spring users building a web application will need to register Spring's DispatcherServlet. For reference, in WEB-INF/web.xml, this would typically be done as follows: 
 <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>

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

The code-based approach with WebApplicationInitializer

Here is the equivalent DispatcherServlet registration logic, WebApplicationInitializer-style: 
 public class MyWebAppInitializer implements WebApplicationInitializer {

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

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

 }
As an alternative to the above, you can also extend from AbstractDispatcherServletInitializer. As you can see, thanks to the Servlet container's ServletContext.addServlet(java.lang.String, java.lang.String) method we're actually registering an instance of the DispatcherServlet, and this means that the DispatcherServlet can now be treated like any other object -- receiving constructor injection of its application context in this case.

This style is both simpler and more concise. There is no concern for dealing with init-params, etc, just normal JavaBean-style properties and constructor arguments. You are free to create and work with your Spring application contexts as necessary before injecting them into the DispatcherServlet.

Most major Spring Web components have been updated to support this style of registration. You'll find that DispatcherServlet, FrameworkServlet, ContextLoaderListener and DelegatingFilterProxy all now support constructor arguments. Even if a component (e.g. non-Spring, other third party) has not been specifically updated for use within WebApplicationInitializers, they still may be used in any case. The ServletContext API allows for setting init-params, context-params, etc programmatically.

A 100% code-based approach to configuration

In the example above, WEB-INF/web.xml was successfully replaced with code in the form of a WebApplicationInitializer, but the actual dispatcher-config.xml Spring configuration remained XML-based. WebApplicationInitializer is a perfect fit for use with Spring's code-based @Configuration classes. See @Configuration Javadoc for complete details, but the following example demonstrates refactoring to use Spring's AnnotationConfigWebApplicationContext in lieu of XmlWebApplicationContext, and user-defined @Configuration classes AppConfig and DispatcherConfig instead of Spring XML files. This example also goes a bit beyond those above to demonstrate typical configuration of the 'root' application context and registration of the ContextLoaderListener: 
 public class MyWebAppInitializer implements WebApplicationInitializer {

    @Override
    public void onStartup(ServletContext container) {
      // Create the 'root' Spring application context
      AnnotationConfigWebApplicationContext rootContext =
        new AnnotationConfigWebApplicationContext();
      rootContext.register(AppConfig.class);

      // Manage the lifecycle of the root application context
      container.addListener(new ContextLoaderListener(rootContext));

      // Create the dispatcher servlet's Spring application context
      AnnotationConfigWebApplicationContext dispatcherContext =
        new AnnotationConfigWebApplicationContext();
      dispatcherContext.register(DispatcherConfig.class);

      // Register and map the dispatcher servlet
      ServletRegistration.Dynamic dispatcher =
        container.addServlet("dispatcher", new DispatcherServlet(dispatcherContext));
      dispatcher.setLoadOnStartup(1);
      dispatcher.addMapping("/");
    }

 }
As an alternative to the above, you can also extend from AbstractAnnotationConfigDispatcherServletInitializer. Remember that WebApplicationInitializer implementations are detected automatically -- so you are free to package them within your application as you see fit.

Ordering WebApplicationInitializer execution

WebApplicationInitializer implementations may optionally be annotated at the class level with Spring's @Order annotation or may implement Spring's Ordered interface. If so, the initializers will be ordered prior to invocation. This provides a mechanism for users to ensure the order in which servlet container initialization occurs. Use of this feature is expected to be rare, as typical applications will likely centralize all container initialization within a single WebApplicationInitializer.
Since:
3.1
Author:
Chris Beams
See Also:

  • Method Summary

    Modifier and Type
    Method
    Description
    void
    onStartup(ServletContext servletContext)
    Configure the given ServletContext with any servlets, filters, listeners context-params and attributes necessary for initializing this web application.

  • Method Details

    • onStartup

      void onStartup(ServletContext servletContext) throws ServletException
      Configure the given ServletContext with any servlets, filters, listeners context-params and attributes necessary for initializing this web application. See examples above.
      Parameters:
      servletContext - the ServletContext to initialize
      Throws:
      ServletException - if any call against the given ServletContext throws a ServletException