10.1. Simple Controllers

The first of the MVC elements we’ll work on implementing are the controllers. Recall that controllers are like the traffic cops of our application. They handle the requests made from users interacting with the application’s view and update model data accordingly. Conversely, changes to model data are sent to the view via controller methods.

MVC Flow

MVC flow

10.1.1. Controllers and Static Responses - Video

10.1.2. Controllers and Static Responses - Intro

10.1.2.1. @Controller

In the Spring Boot context, we’ll organize controller code into a controller package. Remember when we mentioned that the framework works by convention over configuration? This is what we mean. It’s not required for a controller to be in a controller package, but it’s generally a good idea.

To designate a given class as a controller within the Spring framework, we use the annotation @Controller. Recall that Java Annotations are like metadata about your code. They help the framework do its work by adding context to your code.

@Controller
public class HelloSpringController {

   // class code here ...

}

10.1.2.2. Controllers Map to Requests

@GetMapping is another critical annotation used on controller methods. @GetMapping designates a controller action with a URL path. For every GET request made to the provided path, the controller method will be called. The path is defined with @GetMapping("pathname"). If the pathname value is null, then the path used is the index path, or /.

@Controller
public class HelloSpringController {

   // responds to get requests at "/hello"
   @GetMapping("hello")
   public String hello() {
      // method code here ...
   }

}

For every controller method that you want to respond to a request, you will want to use a mapping annotation. Not surprisingly, though, @GetMapping only handles GET requests. If you want to write a controller method that takes care of a POST request, you’ll want to use @PostMapping. Of course, there are other annotations for the other request methods, but these are the two we will use in this class.

@Controller
public class HelloSpringController {

   // responds to post requests at "/goodbye"
   @PostMapping("goodbye")
   public String goodbye() {
      // method code here ...
   }

}

If we want to write a controller method that will be used for both GET and POST at the same path, we can label the method with @RequestMapping. @RequestMapping can handle more than one method as such:

@Controller
public class HelloSpringController {

   // responds to get and post requests at "/hellogoodbye"
   @RequestMapping(value="hellogoodbye", method = {RequestMethod.GET, RequestMethod.POST})
   public String hellogoodbye() {
      // method code here ...
   }

}

The default method of @RequestMapping is GET. Another added capability of @RequestMapping is that it can be applied to a whole class, not just a single method. When applied to a whole class, @RequestMapping essentially designates a base path that all methods in the class start with.

10.1.2.3. @ResponseBody

@ResponseBody is yet another annotation used in the Spring controller context to return plain text from a controller method. This annotation we will only need to use for a short while, before we start to work with templates. Spring Boot’s default action when responding to a controller method is to return a template. Since we aren’t doing that yet however, we need to tell the framework to return plain text by adding the @ResponseBody annotation.

Let’s put it all together:

@Controller
public class HelloSpringController {

   // responds to get requests at "/hello"
   @GetMapping("hello")
   @ResponseBody
   public String hello() {
      return "Hello, Spring!";
   }

}

10.1.3. Check Your Understanding

Question

True/False: The @Controller annotation goes above a method to classify it as a controller method.

  1. True
  2. False

Question

Which of the following is true about controllers?

  1. Controllers handle the data storage of an MVC app.
  2. Controllers manage what the user of an MVC application sees.
  3. Controllers relay the messages between data and views in an MVC application.
  4. Controllers determine what information can be submitted in an online form.