Understanding the Dispatcher servlet configuration

Now we've got a basic idea of how request mapping works. I also mentioned that every web request first comes to the Dispatcher servlet, but the question is how does the Dispatcher servlet know it should handle every incoming request? The answer is we explicitly instructed it to do so through the getServletMappings method of the DispatcherServletInitializer class. Yes, when we return the string array containing only the "/" character, it indicates the DispatcherServlet configuration as the default servlet of the application. So every incoming request will be handled by DispatcherServlet.

Time for action - examining the servlet mapping

Let's observe what will happen when we change the return value of the getServletMappings method.

  1. Open DispatcherServletInitializer and change the return value of the getServletMappings method as return new String[] { "/app/*"}; basically your getServletMappings method should look like the following after your change:
            @Override 
        protected String[] getServletMappings() { 
        return new String[] { "/app/*"}; 
        }
  2. Run your application by right-clicking on your project and choose Run As | Run on Server.
  3. Go to the address bar of the browser and enter the following URL http://localhost:8080/webstore/welcome. You will see the HTTP Status 404 error page in the browser.
  4. Again go to the address bar of the browser and enter the following URL http://localhost:8080/webstore/app/welcome and you will be able to see the same welcome message in the browser.
  5. Now revert the return value of the getServletMappings method to its original value. Basically, your getServletMappings method should look as follows after your change:
            @Override 
        protected String[] getServletMappings() { 
           return new String[] { "/"}; 
        }

What just happened?

As I have already mentioned, we can consider the DispatcherServletInitializer class as equivalent to web.xml as it extends from the AbstractAnnotationConfigDispatcherServletInitializer. If you look close enough, we are overriding three important methods in DispatcherServletInitializer, namely:

  • getRootConfigClasses: This specifies the configuration classes for the root application context
  • getServletConfigClasses: This specifies the configuration classes for the Dispatcher servlet application context
  • getServletMappings: This specifies the servlet mappings for DispatcherServlet

Tip

Typically, the context loaded using the getRootConfigClasses method is the root context, which belongs to the whole application, while the one initialized using the getServletConfigClasses method is actually specific to that Dispatcher servlet. Technically, you can have multiple Dispatcher servlets in an application and so multiple such contexts, each specific for the respective Dispatcher servlet but with the same root context.

In step 1, when we changed the return value of the getServletMappings method, we instructed DispatcherServlet to handle only the web requests that start with a prefix text of /app/. That's why we entered the URL

http://localhost:8080/webstore/welcome. We saw the HTTP Status 404 error page in the browser, since the URL request path didn't start with the /app/ prefix. But when we tried the URL http://localhost:8080/webstore/app/welcome, we were able to see the same welcome message in the browser as the URL started with the /app/ prefix.

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