Understanding of Mavne Structure

@Controller

• Definition: Marks a class as a Spring MVC Controller. It returns a View (like JSP, Thymeleaf) instead of JSON/XML by default.
• Used in MVC pattern for handling web requests.
• If you need REST APIs, you typically use @RestController instead.

@Controller
public class HomeController {
    @RequestMapping("/home")
    public String home() {
        return "home"; // returns view name (home.jsp or home.html from templates)
    }
}

@RestController

• Definition: Shortcut annotation = @Controller + @ResponseBody.
• Automatically serializes returned objects to JSON/XML (using Jackson by default).

@RestController
@RequestMapping("/api")
public class UserController {
    @GetMapping("/hello")
    public String sayHello() {
        return "Hello, World!"; // directly returns JSON response
    }
}

@ResponseBody

• Definition: Tells Spring to serialize method return value into the HTTP response body.
• If not used, Spring assumes return value is a view name.
• Advantages: Converts Java objects → JSON/XML automatically using HttpMessageConverters.

@Controller
public class UserController {
    @RequestMapping("/json")
    @ResponseBody
    public User getUser() {
        return new User("Kishor", "Bhosale"); // returned as JSON
    }
}

@RequestMapping and Derived Annotations

• @RequestMapping: Maps HTTP requests to handler methods/classes.
• Can define: path, method, headers, consumes, produces.
• Derived annotations:
  • @GetMapping
  • @PostMapping
  • @PutMapping
  • @DeleteMapping

@RestController
@RequestMapping("/api/users")
public class UserController {

    @GetMapping("/{id}")
    public String getUser(@PathVariable int id) {
        return "User id: " + id;
    }

    @PostMapping
    public String createUser(@RequestBody User user) {
        return "Created user: " + user.getFirstName();
    }
}

@RequestParam

• Extracts query parameters from URL.
• Supports type conversion: primitive, wrapper, String, Enum, custom objects.
• Example URL: http://localhost:8080/api/fetchUser?firstName=Kishor&lastName=Bhosale

@RestController
@RequestMapping("/api")
public class UserController {

    @GetMapping("/fetchUser")
    public String getUserDetails(
        @RequestParam(required = false) String firstName,
        @RequestParam(required = false) String lastName) {
        return "User: " + firstName + " " + lastName;
    }
}

public class Address {
    private String city;
    private String state;
    // getters & setters
}

@RestController
@RequestMapping("/api")
public class AddressController {

    @InitBinder
    public void initBinder(WebDataBinder binder) {
        binder.registerCustomEditor(Address.class, new PropertyEditorSupport() {
            @Override
            public void setAsText(String text) {
                String[] parts = text.split(",");
                Address addr = new Address();
                addr.setCity(parts[0]);
                addr.setState(parts[1]);
                setValue(addr);
            }
        });
    }

    @GetMapping("/address")
    public Address getAddress(@RequestParam Address address) {
        return address; // e.g. ?address=Pune,MH
    }
}

@PathVariable

• Binds URI template variable to method parameter.
• Example URL: /api/users/101

@RestController
@RequestMapping("/api")
public class UserController {

    @GetMapping("/users/{id}")
    public String getUser(@PathVariable("id") int userId) {
        return "Fetched user id: " + userId;
    }
}

@RequestBody

• Binds HTTP request body → Java object.
• Uses Jackson (default) or Gson for JSON ↔ Object conversion.
• Field mapping is done by matching JSON keys with POJO field names.

public class User {
    private String firstName;
    private String lastName;
    // getters & setters
}

@RestController
@RequestMapping("/api")
public class UserController {

    @PostMapping("/create")
    public User createUser(@RequestBody User user) {
        return user; // JSON → User object
    }
}

@ResponseEntity

• Represents the complete HTTP response: status, headers, body.
• Provides fine-grained control compared to just returning objects.

@RestController
@RequestMapping("/api")
public class UserController {

    @GetMapping("/status")
    public ResponseEntity<String> getStatus() {
        return ResponseEntity
                .status(HttpStatus.OK)
                .header("Custom-Header", "SpringBoot")
                .body("Everything is fine!");
    }
}