Over a million developers have joined DZone.
{{announcement.body}}
{{announcement.title}}

Spring WebApplicationInitializer and ApplicationContextInitializer confusion

DZone's Guide to

Spring WebApplicationInitializer and ApplicationContextInitializer confusion

· Java Zone ·
Free Resource

Verify, standardize, and correct the Big 4 + more– name, email, phone and global addresses – try our Data Quality APIs now at Melissa Developer Portal!

These are two concepts that I mix up occasionally - a WebApplicationInitializer and an ApplicationContextInitializer, and wanted to describe each of them to clarify them for myself. 

I have previously blogged about WebApplicationInitializerhere and here. It is relevant purely in a Servlet 3.0+ spec compliant servlet container and provides a hook to programmatically configure the servlet context. How does this help - you can have a web application without potentially any web.xml file, typically used in a Spring based web application to describe the root application context and the Spring web front controller called theDispatcherServlet. An example of using WebApplicationInitializer is the following:

public class CustomWebAppInitializer extends AbstractAnnotationConfigDispatcherServletInitializer {
 @Override
 protected Class<?>[] getRootConfigClasses() {
  return new Class<?>[]{RootConfiguration.class};
 }

 @Override
 protected Class<?>[] getServletConfigClasses() {
  return new Class<?>[]{MvcConfiguration.class};
 }

 @Override
 protected String[] getServletMappings() {
  return new String[]{"/"};
 }
}

Now, what is an ApplicationContextInitializer. It is essentially code that gets executed before the Spring application context gets completely created. A good use case for using an ApplicationContextInitializer would be to set a Spring environment profile programmatically, along these lines:

public class DemoApplicationContextInitializer implements ApplicationContextInitializer<ConfigurableApplicationContext> {

 @Override
 public void initialize(ConfigurableApplicationContext ac) {
  ConfigurableEnvironment appEnvironment = ac.getEnvironment();
  appEnvironment.addActiveProfile("demo");

 }
}

If you have a Spring-Boot based application then registering an ApplicationContextInitializer is fairly straightforward:

@Configuration
@EnableAutoConfiguration
@ComponentScan
public class SampleWebApplication {
 
 public static void main(String[] args) {
  new SpringApplicationBuilder(SampleWebApplication.class)
    .initializers(new DemoApplicationContextInitializer())
    .run(args);
 }
}

For a non Spring-Boot Spring application though, it is a little more tricky, if it is a programmatic configuration of web.xml, then the configuration is along these lines:

public class CustomWebAppInitializer implements WebApplicationInitializer {

 @Override
 public void onStartup(ServletContext container) {
  AnnotationConfigWebApplicationContext rootContext = new AnnotationConfigWebApplicationContext();
  rootContext.register(RootConfiguration.class);
  ContextLoaderListener contextLoaderListener = new ContextLoaderListener(rootContext);
  container.addListener(contextLoaderListener);
  container.setInitParameter("contextInitializerClasses", "mvctest.web.DemoApplicationContextInitializer");
  AnnotationConfigWebApplicationContext webContext = new AnnotationConfigWebApplicationContext();
  webContext.register(MvcConfiguration.class);
  DispatcherServlet dispatcherServlet = new DispatcherServlet(webContext);
  ServletRegistration.Dynamic dispatcher = container.addServlet("dispatcher", dispatcherServlet);
  dispatcher.addMapping("/");
 }
}

If it a normal web.xml configuration then the initializer can be specified this way:

<context-param>
<param-name>contextInitializerClasses</param-name>
<param-value>com.myapp.spring.SpringContextProfileInit</param-value>
</context-param>

<listener>
<listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
</listener>

So to conclude, except for the Initializer suffix, both WebApplicationInitializer and ApplicationContextInitializer serve fairly different purposes. Whereas the WebApplicationInitializer is used by a Servlet Container at startup of the web application and provides a way for programmatic creating a web application(replacement for a web.xml file), ApplicationContextInitializer provides a hook to configure the Spring application context before it gets fully created.

Developers! Quickly and easily gain access to the tools and information you need! Explore, test and combine our data quality APIs at Melissa Developer Portal – home to tools that save time and boost revenue. 

Topics:

Published at DZone with permission of

Opinions expressed by DZone contributors are their own.

{{ parent.title || parent.header.title}}

{{ parent.tldr }}

{{ parent.urlSource.name }}