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## org.springframework.web
Interface WebApplicationInitializer

    public interface WebApplicationInitializer

Interface to be implemented in Servlet 3.0+ 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 3.0 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 you can see, thanks to Servlet 3.0's new `ServletContext#addServlet` 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 componentry has 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 Servlet 3.0 `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("/");
        }

     }

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`.

## Caveats

### web.xml versioning

`WEB-INF/web.xml` and `WebApplicationInitializer` use are not mutually exclusive; for example, web.xml can register one servlet, and a `WebApplicationInitializer` can register another. An initializer can even modify registrations performed in `web.xml` through methods such as `ServletContext#getServletRegistration(String)`. However, if `WEB-INF/web.xml` is present in the application, its `version` attribute must be set to "3.0" or greater, otherwise `ServletContainerInitializer` bootstrapping will be ignored by the servlet container.

### Mapping to '/' under Tomcat

Apache Tomcat maps its internal `DefaultServlet` to "/", and on Tomcat versions <= 7.0.14, this servlet mapping cannot be overridden programmatically. 7.0.15 fixes this issue. Overriding the "/" servlet mapping has also been tested successfully under GlassFish 3.1.

Since:
  3.1
Author:
  Chris Beams
See Also:
  `SpringServletContainerInitializer`

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

Method Detail

### 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`

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