Java Spring Boot Basics Cheat Sheet

Covers the application entry point, REST controllers, service and repository layers, core annotations, and application.properties configuration.

2 PagesIntermediateMar 25, 2026

Application Entry Point

The bootstrap class that starts a Spring Boot application.

java

@SpringBootApplication // combines @Configuration, @EnableAutoConfiguration, @ComponentScanpublic class DemoApplication {    public static void main(String[] args) {        SpringApplication.run(DemoApplication.class, args);    }}

REST Controller

Map HTTP requests to Java methods and return JSON responses.

java

@RestController@RequestMapping("/api/users")public class UserController {    private final UserService userService; // constructor injection (preferred)    public UserController(UserService userService) {        this.userService = userService;    }    @GetMapping("/{id}")    public ResponseEntity<User> getUser(@PathVariable Long id) {        return userService.findById(id)            .map(ResponseEntity::ok)            .orElse(ResponseEntity.notFound().build());    }    @PostMapping    public ResponseEntity<User> create(@RequestBody @Valid User user) {        User saved = userService.save(user);        return ResponseEntity.status(HttpStatus.CREATED).body(saved);    }}

Service & Repository Layers

Separate business logic from data access using Spring Data JPA.

java

@Servicepublic class UserService {    private final UserRepository repo;    public UserService(UserRepository repo) { this.repo = repo; }    public Optional<User> findById(Long id) { return repo.findById(id); }    public User save(User user) { return repo.save(user); }}@Repositorypublic interface UserRepository extends JpaRepository<User, Long> {    Optional<User> findByEmail(String email); // Spring Data derives the query from the method name}

Core Spring Boot Annotations

The annotations you'll see in nearly every Spring Boot project.

@SpringBootApplication- Bootstraps the app; enables component scanning and auto-configuration
@RestController- @Controller + @ResponseBody; return values are serialized directly to the response body (usually JSON)
@Service- Marks a class as a business-logic bean, picked up by component scanning
@Repository- Marks a data-access bean; also translates persistence exceptions into Spring's DataAccessException hierarchy
@Component- Generic stereotype for any Spring-managed bean
@Autowired- Injects a dependency; constructor injection (no annotation needed on the constructor itself) is the recommended style
@Value- Injects a property value from application.properties/.yml: @Value("${server.port}")
@Configuration / @Bean- Defines Java-based configuration classes and manually registered beans

application.properties Configuration

Externalize datasource, port, and JPA settings, with profile support.

properties

# application.propertiesserver.port=8081spring.datasource.url=jdbc:postgresql://localhost:5432/appdbspring.datasource.username=postgresspring.datasource.password=secretspring.jpa.hibernate.ddl-auto=updatespring.jpa.show-sql=true# Environment-specific: application-prod.properties, activated with# spring.profiles.active=prod

Pro Tip

Prefer constructor injection over field injection (@Autowired on a field) - it makes dependencies explicit, allows fields to be final, and makes the class trivially testable without needing a Spring context to instantiate mocks.

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