How Dispatcher servlet gets registered in Spring java based config

Few days ago my colleague ask me about how Spring Dispatcher servlet gets registered in Java based config, as there is no XML used in configuration. One thing I was (actually we both were) sure about that the class AbstractAnnotationConfigDispatcherServletInitializer registers those servlet and all the configuration. But the question was How ?

In order to understand that we will be going back to the servlet world. According to Servlet Java Doc, ServletContainerInitializer is the interface which is used to register servlets, filters and other related stuff.

So any implementation of ServletContainerInitializer will be notified of web application’s startup phase using a callback method

public void onStartup(Set<Class<?>> c, ServletContext ctx) throws ServletException {
    
} 

In above method, you’ll notice there is a parameter c of type Set<Class<?> >. You probably thinking why it’s there. So here is the another part of the magic, there is an annotation called @HandlesTypes used alongside with ServletContainerInitializer. The annotation accepts array of Class as input argument.

public @interface HandlesTypes {
    
    Class<?>[] value();
}

So any class (and its subclasses), which is passed as an argument to that annotation (HandlesTypes) will be scanned by the container and finally the collection of those class will be passed to ServletContainerInitializer#onStartup() during the startup time.

Now you might be thinking, how the implementation of ServletContainerInitializer gets picked by the container? So the answer to that question is:

According to the Java Docs

Implementations of this interface must be declared by a JAR file resource located inside the META-INF/services directory and named for the fully qualified class name of this interface, and will be discovered using the runtime’s service provider lookup mechanism or a container specific mechanism that is semantically equivalent to it. In either case, ServletContainerInitializer services from web fragment JAR files excluded from an absolute ordering must be ignored, and the order in which these services are discovered must follow the application’s classloading delegation model.

Now come back to the Spring Era and check the spring-web*.jar (just extract the jar) and look for the file javax.servlet.ServletContainerInitializer. Inside that file you’ll find a fully qualified class name as org.springframework.web.SpringServletContainerInitializer.

That’s it, you’ve solved the mystery!!

Now check the source code of that class (go to the GitHub), you’ll notice a class WebApplicationInitializer inside @HandlesTypes which is the parent class of most of the Spring’s initializers (AbstractAnnotationConfigDispatcherServletInitializer, AbstractSecurityWebApplicationInitializer).

Now you know how to create your own initializer.

Cheers!!! Happy coding.

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