What is Dispatcher Servlet in Spring?

The job of the DispatcherServlet is to take an incoming URI and find the right combination of handlers (generally methods on Controller classes) and views (generally JSPs) that combine to form the page or resource that's supposed to be found at that location.

I might have

• a file /WEB-INF/jsp/pages/Home.jsp

• and a method on a class

  @RequestMapping(value="/pages/Home.html")
  private ModelMap buildHome() {
      return somestuff;
  }
  

The Dispatcher servlet is the bit that "knows" to call that method when a browser requests the page, and to combine its results with the matching JSP file to make an html document.

How it accomplishes this varies widely with configuration and Spring version.

There's also no reason the end result has to be web pages. It can do the same thing to locate RMI end points, handle SOAP requests, anything that can come into a servlet.

In Spring MVC, all incoming requests go through a single servlet. This servlet - DispatcherServlet - is the front controller. Front controller is a typical design pattern in the web applications development. In this case, a single servlet receives all requests and transfers them to all other components of the application.

The task of the DispatcherServlet is to send request to the specific Spring MVC controller.

Usually we have a lot of controllers and DispatcherServlet refers to one of the following mappers in order to determine the target controller:

• BeanNameUrlHandlerMapping;
• ControllerBeanNameHandlerMapping;
• ControllerClassNameHandlerMapping;
• DefaultAnnotationHandlerMapping;
• SimpleUrlHandlerMapping.

If no configuration is performed, the DispatcherServlet uses BeanNameUrlHandlerMapping and DefaultAnnotationHandlerMapping by default.

When the target controller is identified, the DispatcherServlet sends request to it. The controller performs some work according to the request (or delegate it to the other objects), and returns back to the DispatcherServlet with the Model and the name of the View.

The name of the View is only a logical name. This logical name is then used to search for the actual View (to avoid coupling with the controller and specific View). Then DispatcherServlet refers to the ViewResolver and maps the logical name of the View to the specific implementation of the View.

Some possible Implementations of the ViewResolver are:

• BeanNameViewResolver;
• ContentNegotiatingViewResolver;
• FreeMarkerViewResolver;
• InternalResourceViewResolver;
• JasperReportsViewResolver;
• ResourceBundleViewResolver;
• TilesViewResolver;
• UrlBasedViewResolver;
• VelocityLayoutViewResolver;
• VelocityViewResolver;
• XmlViewResolver;
• XsltViewResolver.

When the DispatcherServlet determines the view that will display the results it will be rendered as the response.

Finally, the DispatcherServlet returns the Response object back to the client.