Spring Boot application that uses Spring Data JPA to connect to a database and perform basic CRUD

Here is an example of a simple Spring Boot application that uses Spring Data JPA to connect to a database and perform basic CRUD (create, read, update, delete) operations:

First, we will create a simple entity class that represents a user:

import javax.persistence.Entity;

import javax.persistence.GeneratedValue;

import javax.persistence.GenerationType;

import javax.persistence.Id;

@Entity

public class User {

@Id

@GeneratedValue(strategy=GenerationType.AUTO)

private long id;

private String firstName;

private String lastName;

private String email;

// Getters and setters omitted for brevity

}

Next, we will create a repository interface that extends Spring Data JPA's JpaRepository interface and specifies the type of entity and ID that it will be working with:

import org.springframework.data.jpa.repository.JpaRepository;

public interface UserRepository extends JpaRepository<User, Long> {

}

Then, we will create a simple service class that performs basic CRUD operations using the repository:

import org.springframework.stereotype.Service;

@Service

public class UserService {

private final UserRepository userRepository;

public UserService(UserRepository userRepository) {

this.userRepository = userRepository;

}

public List<User> findAll() {

return userRepository.findAll();

}

public User findById(long id) {

return userRepository.findById(id).orElse(null);

}

public User save(User user) {

return userRepository.save(user);

}

public void deleteById(long id) {

userRepository.deleteById(id);

}

}

Finally, we will create a simple controller class that exposes the service's methods as REST endpoints:

import org.springframework.web.bind.annotation.RestController;

@RestController

public class UserController {

private final UserService userService;

public UserController(UserService userService) {

this.userService = userService;

}

@GetMapping("/users")

public List<User> findAll() {

return userService.findAll();

}

@GetMapping("/users/{id}")

public User findById(@PathVariable long id) {

return userService.findById(id);

}

@PostMapping("/users")

public User save(@RequestBody User user) {

return userService.save(user);

}

@DeleteMapping("/users/{id}")

public void deleteById(@PathVariable long id) {

userService.deleteById(id);

}

}

To run this application, you will also need to configure a database and add the necessary dependencies to your pom.xml file.