Spring REST API

package com.example.controller;

import com.example.entity.User;
import com.example.service.UserService;
import org.springframework.data.domain.*;
import org.springframework.http.*;
import org.springframework.web.bind.annotation.*;
import javax.validation.Valid;
import java.net.URI;

@RestController
@RequestMapping("/api/v1/users")
@CrossOrigin(origins = "http://localhost:3000") // Allow CORS from React app
public class UserRestController {

    private final UserService userService;

    public UserRestController(UserService userService) {
        this.userService = userService;
    }

    // GET /api/v1/users?page=0&size=10&sort=username
    @GetMapping
    public ResponseEntity<Page<User>> getUsers(
            @RequestParam(defaultValue = "0") int page,
            @RequestParam(defaultValue = "10") int size,
            @RequestParam(defaultValue = "id") String sort) {
        Page<User> users = userService.getUsers(page, size, sort);
        return ResponseEntity.ok(users);
    }

    // GET /api/v1/users/42
    @GetMapping("/{id}")
    public ResponseEntity<User> getUser(@PathVariable Long id) {
        User user = userService.getUserById(id);
        return ResponseEntity.ok(user);
    }

    // POST /api/v1/users
    @PostMapping
    public ResponseEntity<User> createUser(@Valid @RequestBody User user) {
        User created = userService.createUser(user);
        URI location = URI.create("/api/v1/users/" + created.getId());
        return ResponseEntity.created(location).body(created); // 201 Created
    }

    // PUT /api/v1/users/42
    @PutMapping("/{id}")
    public ResponseEntity<User> updateUser(@PathVariable Long id,
                                            @Valid @RequestBody User user) {
        User updated = userService.updateUser(id, user);
        return ResponseEntity.ok(updated);
    }

    // DELETE /api/v1/users/42
    @DeleteMapping("/{id}")
    public ResponseEntity<Void> deleteUser(@PathVariable Long id) {
        userService.deleteUser(id);
        return ResponseEntity.noContent().build(); // 204 No Content
    }
}

import org.springframework.http.*;
import org.springframework.web.bind.annotation.*;
import org.springframework.validation.FieldError;
import org.springframework.web.bind.MethodArgumentNotValidException;
import java.util.*;

@RestControllerAdvice // Handles exceptions for all @RestController classes
public class GlobalExceptionHandler {

    // Handle resource not found
    @ExceptionHandler(ResourceNotFoundException.class)
    public ResponseEntity<ErrorResponse> handleNotFound(ResourceNotFoundException ex) {
        ErrorResponse error = new ErrorResponse(404, ex.getMessage());
        return ResponseEntity.status(HttpStatus.NOT_FOUND).body(error);
    }

    // Handle validation errors
    @ExceptionHandler(MethodArgumentNotValidException.class)
    public ResponseEntity<Map<String, String>> handleValidation(
            MethodArgumentNotValidException ex) {
        Map<String, String> errors = new HashMap<>();
        ex.getBindingResult().getAllErrors().forEach(error -> {
            String field = ((FieldError) error).getField();
            String message = error.getDefaultMessage();
            errors.put(field, message);
        });
        return ResponseEntity.badRequest().body(errors);
    }

    // Handle all other exceptions
    @ExceptionHandler(Exception.class)
    public ResponseEntity<ErrorResponse> handleGeneral(Exception ex) {
        ErrorResponse error = new ErrorResponse(500, "Internal server error");
        return ResponseEntity.internalServerError().body(error);
    }

    record ErrorResponse(int status, String message) {}
}

@Configuration
public class CorsConfig implements WebMvcConfigurer {

    @Override
    public void addCorsMappings(CorsRegistry registry) {
        registry.addMapping("/api/**")
                .allowedOrigins("http://localhost:3000", "https://myapp.com")
                .allowedMethods("GET", "POST", "PUT", "DELETE", "PATCH", "OPTIONS")
                .allowedHeaders("*")
                .allowCredentials(true)
                .maxAge(3600); // Cache preflight for 1 hour
    }
}

// Or use @CrossOrigin on individual controllers/methods:
// @CrossOrigin(origins = "*", maxAge = 3600)
// @CrossOrigin(origins = {"http://localhost:3000", "https://myapp.com"})