Spring Cheat Sheet

@RestController
@RequestMapping("/api/posts")
public class PostController {
    @GetMapping
    public List<Post> list() { return postService.findAll(); }

    @GetMapping("/{id}")
    public Post get(@PathVariable Long id) { return postService.findById(id); }

    @PostMapping
    @ResponseStatus(HttpStatus.CREATED)
    public Post create(@Valid @RequestBody PostDTO dto) {
        return postService.create(dto);
    }
}

@GetMapping("/{id}")
public ResponseEntity<Post> get(@PathVariable Long id) {
    return postService.findById(id)
        .map(ResponseEntity::ok)
        .orElse(ResponseEntity.notFound().build());
}

@GetMapping("/search")
public Page<Post> search(
    @RequestParam(defaultValue = "") String q,
    @RequestParam(defaultValue = "0") int page,
    @RequestParam(defaultValue = "20") int size
) {
    return postService.search(q, PageRequest.of(page, size));
}

@Service
public class PostService {
    private final PostRepository repo;
    private final EventPublisher events;

    public PostService(PostRepository repo, EventPublisher events) {
        this.repo = repo;
        this.events = events;
    }
}

@Component     // Generic bean
@Service       // Business logic
@Repository    // Data access
@Controller    // MVC controller
@RestController // @Controller + @ResponseBody
@Configuration // Bean definitions

@Configuration
public class AppConfig {
    @Bean
    public ObjectMapper objectMapper() {
        return new ObjectMapper()
            .registerModule(new JavaTimeModule());
    }
}

@Entity
@Table(name = "posts")
public class Post {
    @Id @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @Column(nullable = false)
    private String title;

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "author_id")
    private User author;

    @OneToMany(mappedBy = "post", cascade = CascadeType.ALL)
    private List<Comment> comments;
}

public interface PostRepository extends JpaRepository<Post, Long> {
    List<Post> findByPublishedTrue();
    Page<Post> findByAuthorId(Long authorId, Pageable pageable);

    @Query("SELECT p FROM Post p WHERE p.title LIKE %:q%")
    List<Post> search(@Param("q") String query);
}

Pageable pageable = PageRequest.of(0, 20, Sort.by("createdAt").descending());
Page<Post> page = postRepository.findAll(pageable);
page.getContent();    // List<Post>
page.getTotalPages(); // int
page.getTotalElements(); // long

@Configuration
@EnableWebSecurity
public class SecurityConfig {
    @Bean
    public SecurityFilterChain filterChain(HttpSecurity http) throws Exception {
        return http
            .csrf(csrf -> csrf.disable())
            .authorizeHttpRequests(auth -> auth
                .requestMatchers("/api/public/**").permitAll()
                .requestMatchers("/api/admin/**").hasRole("ADMIN")
                .anyRequest().authenticated()
            )
            .oauth2ResourceServer(oauth2 -> oauth2.jwt(Customizer.withDefaults()))
            .build();
    }
}

@PreAuthorize("hasRole('ADMIN')")
public void deletePost(Long id) { ... }

@PreAuthorize("#post.author.id == authentication.principal.id")
public void updatePost(Post post) { ... }

# application.yml
spring:
  datasource:
    url: jdbc:postgresql://localhost:5432/mydb
    username: ${DB_USER}
    password: ${DB_PASS}
  jpa:
    hibernate.ddl-auto: validate
    show-sql: false

@Value("${app.name:DefaultApp}")
private String appName;

@Value("${server.port}")
private int port;

@ConfigurationProperties(prefix = "app")
public record AppConfig(
    String name,
    int maxRetries,
    Duration timeout
) {}

# application-dev.yml
spring.jpa.show-sql: true
# application-prod.yml
spring.jpa.show-sql: false

# Activate: --spring.profiles.active=dev
# Or: SPRING_PROFILES_ACTIVE=prod

@SpringBootTest
@AutoConfigureMockMvc
class PostControllerTest {
    @Autowired MockMvc mockMvc;

    @Test
    void listPosts() throws Exception {
        mockMvc.perform(get("/api/posts"))
            .andExpect(status().isOk())
            .andExpect(jsonPath("$", hasSize(3)));
    }
}

@SpringBootTest
class PostServiceTest {
    @MockBean PostRepository repo;
    @Autowired PostService service;

    @Test
    void findById() {
        when(repo.findById(1L)).thenReturn(Optional.of(post));
        assertEquals("Title", service.findById(1L).getTitle());
    }
}