Spring MVC

Why You Need Spring MVC

Spring MVC helps organize a web application by following the principle of separation of concerns, making it easier to develop, test, and maintain code. It provides a framework for creating web pages and handling user requests, as well as for integrating with business logic and databases.

We can integrate it into our project using the pom.xml file, where we will add the dependency within the <dependencies></dependencies> tags.

With it, we can receive requests from the client and process them as needed, and then return a response in the form of HTML or JSON.

Note
We have already included this dependency in the previous chapter, so it is likely already added to your project!

Key Annotations and Their Role

@Controller

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@Controller
public class HomeController {

    @RequestMapping(value = "/", method = RequestMethod.GET)
    public String home() {
        // code
    }
}

In this example, the HomeController class handles requests to the root URL /. This means that if we navigate to example.com/, a request will be sent to the home() method.

The @RequestMapping annotation connects HTTP requests to controller methods. You can use it to specify URL patterns, HTTP methods, and other request parameters. Its attributes allow you to define the URL and the type of request (e.g., GET, POST, DELETE, etc.).

There are annotations that provide more precise mapping for the type of request method, such as @GetMapping, @PostMapping, @DeleteMapping, and so on. In their attributes, you simply specify the URL that they can be invoked with.

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@Controller
public class HomeController {

    @GetMapping("/home")
    public String home() {
        // code 
    }
}

In this example, if we access the URL /home, we will be directed to the home() method ONLY if it was a GET request. If a different type of request method is used, the server will respond with a 404 Not Found error for that URL.

@RequestParam

What are URL request parameters? These are the parameters that appear after the question mark in the URL. For example, if we have the address -> example.com/home?name=Alex, we can directly retrieve the parameter (name) in the welcomeUser() method.

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@Controller
public class WelcomeController {

    @GetMapping("/home")
    public String welcomeUser(@RequestParam(name = "name") String name) {
        // code
    }
}

Here, the parameter name is extracted from the URL (e.g., /home?name=John) and passed to the welcomeUser() method.

Note
You can also extract more than one parameter, not just a single one. All parameters in the URL should be separated by the & symbol. For example: example.com/home?name=Alex&age=20.

@PathVariable

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@Controller
public class UserController {

    @GetMapping("/user/{id}")
    public String getUser(@PathVariable("id") Long userId) {
        // code
    }
}

In this example, id is extracted from the URL (e.g., /user/123) and passed to the getUser() method, where it is assigned to the userId parameter.

@ResponseBody

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@Controller
public class DataController {

    @GetMapping("/data")
    @ResponseBody
    public User getUserData() {
        return new User("John", "Doe"); 
    }
}

Here, the User object will be automatically converted to JSON and sent to the client as the response.

Note
If you want all methods to return JSON, you can use @RestController instead of @Controller on the class level. This way, you won't need to add the @ResponseBody annotation to each method.

These annotations help developers easily configure HTTP request handling, bind data to objects, and manage responses, enhancing code readability and maintainability.

Summary

Spring MVC simplifies web application development by separating request handling, application logic, and view rendering. Controllers, marked with the @Controller or @RestController annotation, handle requests and return data for display.

1. What does the `@Controller` annotation do in Spring MVC?

2. Which annotation method is used to handle `GET requests` in Spring MVC?

Everything was clear?

Section 2. Chapter 4