What is Dispatcher Servlet in Spring?

java

spring

dispatcher-servlet

request-processing

byAnton Shumikhin·Dec 21, 2024

At its essence, the DispatcherServlet is the central dispatcher in Spring MVC that routes incoming HTTP requests to appropriate handlers (aka controllers). It leverages HandlerMappings to identify the right handler, uses ViewResolvers to resolve views, and employs interceptors for cross-cutting concerns. Basically, it's the Middle Earth's Gandalf of your application, asserting "You shall not pass!" to unchecked requests. Initialize it in your web.xml like so:

<servlet>
    <servlet-name>dispatcher</servlet-name>
    <!-- The key player in the story. Stands tall like a lighthouse in the stormy sea of requests. -->
    <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
    <!-- Ever heard of "The early bird catches the worm"? This is our early bird -->
    <load-on-startup>1</load-on-startup>
</servlet>
<servlet-mapping>
    <servlet-name>dispatcher</servlet-name>
    <!-- "/": Unassuming, yet all-encompassing, like a black hole consuming all paths in its vicinity -->
    <url-pattern>/</url-pattern>
</servlet-mapping>

This elegant code ensures that DispatcherServlet processes all paths (/), creating a seamless conduit between HTTP requests and backend processing.

Deep dive: DispatcherServlet 101

The game of requests and controller mapping

Let's get down to brass tacks - How does the mighty DispatcherServlet decide which controllers should be the hero of a request? It's elementary, my dear Watson! It consults its trusty sidekick, the HandlerMapping, retrieving the controller's identity like a seasoned detective.

Moving from controllers to views, Spring style!

The controller then takes the lead, dealing with the HTTP request and pulling together the ModelAndView object. This object is a dynamic duo--the model (Batman) carrying the data, and the logical view name (his trusted Robin). Back to our hero DispatcherServlet to decode this name using the versatile ViewResolver.

Beyond the regular: SOAP requests, RMI, and more

Did we say DispatcherServlet is a polymath? Yes, while commonly associated with web page requests, it can roll with the punches, effortlessly dealing with Remote Method Invocation (RMI) endpoints and SOAP requests with lucid dexterity.

Playing under the configuration rules

DispatcherServlet is a team player, abiding by the rules defined in its configuration. The settings can vary like a chameleon, ranging from simplified XML configuration up to the full-bodied Java-based configuration in newer versions.

The swiftness of a cheetah: Speedy request processing

One of the unsung virtues of DispatcherServlet is its lightning-fast request processing. By functioning as a central dispatcher, it ensures smooth, fast, and accurate routing of incoming HTTP requests, much like Mercury, the Roman god of communication.

Deconstructing the DispatcherServlet

Calling the shots with handler mappings

HandlerMapping beans act as the oracle guiding the DispatcherServlet, establishing which controllers must rise and shine for a specific HTTP request. Spring provides several inbuilt implementations, each with their special talent.

In the realm of controllers

A controller is your logic superstar--the hero stepping in to deal with the request, wrapping hands with services or liaising with a database, before surrendering control back.

Sail from controllers to views

Here come the ViewResolver beans, taking the logical view name from the controller, and stepping up to resolve the actual view template. Once the resolving is done, your response is now in the expert hands of the view being rendered using the model data received.

The art of response rendering

And the grand finale, the view, which might be a JSP, Thymeleaf, Freemarker, or any other view technology that Spring supports, takes up the baton. It weaves the model attributes into HTML, JSON, or XML output sent back to you, like a seasoned artist painting a masterpiece.

Best practices and pitfalls

The ace of configuration

Ensure your DispatcherServlet is dressed for success - properly configured to handle the right request mappings and view resolutions. Misconfiguration can land you in the spooky alley of 404 errors or incorrect handler mapping.

The version game

Spring's versions bring varying features and changes to DispatcherServlet. Stick with the documentation specific to your version to avoid stepping on the wrong stone.