When to use Servlet or @Controller

Question

I need to get a few things cleared up. I have been looking for an answer for this one, but I can\'t seem to find a good answer to my specific questions (eg. this questio

Answer 1

A Servlet and a Spring MVC Controller can be used to do the same thing but they act on different level of a Java Application

The servlet is a part of the J2EE framework and every Java application server (Tomcat, Jetty, etc) is built for running servlets. Servlet are the "low level" layer in the J2EE stack. You don't need a servlet.jar to run you application because it's prepackaged with the application server

A Spring MVC controller is a library built upon the servlet to make things easier. Spring MVC offers more built-in functionalities such as form parameter to controller method parameter mapping, easier handling of binary form submissions (i.e. when your form can upload files). You need to package the required jars to your application in order to run a Spring MVC Controller

You should use a servlet when you need to go "low level", and example could be for performance reason. Spring MVC performs good but if it has some overhead, if you need to squeeze out all you can from your application server (and you have already tuned the other layers such as the db) go with a servlet. You can choose a servlet if you want to understand the foundation of the J2EE web specifications (i.e. for educational purposes)

In all the other cases you can/should choose a web framework. Spring MVC is one of them; with Spring MVC you don't need to reinvent the wheel (i.e binary form management, form parameter to bean conversion, parameter validation and so on). Another plus of Spring MVC is that in one class you can easily manage input from different urls and methods, doing the same in a servlet is possible but the code is more complicated and less readable. My opinion is that Spring MVC is good for building rest services and managing simple applications (i.e web application with simple forms). If you need to manager very complex forms with Ajax, nested forms, and an application with both session and page state my advice is to switch to a component based framework (like apache wicket for example).

Answer 2

If you're a student interested in learning the language then I would stick with servlets for now. It's possible to write a web app using just servlets but in practice you'll probably want to look at JSP's too.

A JSP is a convenient way to write a servlet that allows you to mix html with scripting elements (although it's recommended to avoid Java code in your jsp in favour of tags and el expressions). Under the covers it will be compiled as a servlet but it avoids you having to use lots of messy print statements.

It's important to have at least a basic understanding of servlets and JSP's. Spring MVC is one of many frameworks built on top of servlets to try make the task of writing a web application a bit easier. Basically all requests are mapped to the DispatcherServlet which acts as a front controller.

The DispatcherServlet will then call the controller whose annotations match the incoming request. This is neater than having to write these mappings yourself in the web.xml (although with servlet 3.0 you can annotate servlets now). But you also get many other benefits that can be used like mapping form fields to an object, validating that object with jsr303 annotations, map inputs and outputs to xml or json etc etc. Plus it's tightly integrated with core spring so you can easily wire in your services for the controller to call.

It's worth noting that there are a plethora of competing frameworks built on top of servlets. Spring MVC is one of the most popular so it's not a bad choice to look into.

Answer 3

JSF and JSP aswell as Spring MVC builts upon Servlets. The problem this that servlets are not very "nice" to work with because you have to write direct html.

If you are able to use mordern web technologies I would just use servlets in positions that need direct http output like writing an image from a database to http.

Using SpringMVC or JSF which work with a Dipatcherservlet or FacesServlet is just faster and more fun. They parse your files and send it through a servlet.