java-expert

Java Expert

Virtual Threads (Project Loom)

Lightweight threads that dramatically improve scalability for I/O-bound applications
Use Executors.newVirtualThreadPerTaskExecutor() for thread pools
Perfect for web applications with many concurrent connections
Spring Boot 3.2+ supports virtual threads via configuration

// Enable virtual threads in Spring Boot 3.2+ // application.properties spring.threads.virtual.enabled=true

// Or programmatically @Bean public TomcatProtocolHandlerCustomizer<?> protocolHandlerVirtualThreadExecutorCustomizer() { return protocolHandler -> { protocolHandler.setExecutor(Executors.newVirtualThreadPerTaskExecutor()); }; }

// Using virtual threads directly try (var executor = Executors.newVirtualThreadPerTaskExecutor()) { executor.submit(() -> { // I/O-bound task Thread.sleep(1000); return "result"; }); }

Pattern Matching

Pattern matching for switch (Java 21)
Record patterns
Destructuring with pattern matching

// Pattern matching for switch String result = switch (obj) { case String s -> "String: " + s; case Integer i -> "Integer: " + i; case Long l -> "Long: " + l; case null -> "null"; default -> "Unknown"; };

// Record patterns record Point(int x, int y) {}

if (obj instanceof Point(int x, int y)) { System.out.println("x: " + x + ", y: " + y); }

Records

Immutable data carriers
Automatically generates constructor, getters, equals(), hashCode(), toString()

public record UserDTO(String name, String email, LocalDate birthDate) { // Compact constructor for validation public UserDTO { if (name == null || name.isBlank()) { throw new IllegalArgumentException("Name cannot be blank"); } } }

Sealed Classes

Restrict which classes can extend/implement
Provides exhaustive pattern matching

public sealed interface Result<T> permits Success, Failure { record Success<T>(T value) implements Result<T> {} record Failure<T>(String error) implements Result<T> {} }

Spring Boot 3.x Best Practices (2026)

Framework Setup:

Java 21+ as baseline (virtual threads, pattern matching)
Spring Boot 3.2+ (latest stable)
Spring Framework 6.x
Jakarta EE (not javax.*) - namespace change

Project Structure (Layered Architecture):

src/main/java/com/example/app/ ├── controller/ # REST endpoints (RestController) ├── service/ # Business logic (Service) │ └── impl/ # Service implementations ├── repository/ # Data access (Repository) ├── model/ │ ├── entity/ # JPA entities │ └── dto/ # Data Transfer Objects ├── config/ # Configuration classes ├── exception/ # Custom exceptions and handlers └── util/ # Utility classes

Controller Layer (RestController):

Handle HTTP requests/responses only
Delegate business logic to services
Use DTOs for request/response bodies
Never directly inject repositories

@RestController @RequestMapping("/api/users") @RequiredArgsConstructor public class UserController { private final UserService userService;

@GetMapping("/{id}")
public ResponseEntity&#x3C;UserDTO> getUser(@PathVariable Long id) {
    UserDTO user = userService.findById(id);
    return ResponseEntity.ok(user);
}

@PostMapping
public ResponseEntity&#x3C;UserDTO> createUser(@Valid @RequestBody CreateUserDTO dto) {
    UserDTO created = userService.create(dto);
    return ResponseEntity.status(HttpStatus.CREATED).body(created);
}

}

Service Layer:

Contains business logic
Uses repositories for data access
Converts between entities and DTOs
Annotated with @Service

@Service @RequiredArgsConstructor @Transactional(readOnly = true) public class UserServiceImpl implements UserService { private final UserRepository userRepository; private final ModelMapper modelMapper;

@Override
public UserDTO findById(Long id) {
    User user = userRepository.findById(id)
        .orElseThrow(() -> new UserNotFoundException(id));
    return modelMapper.map(user, UserDTO.class);
}

@Override
@Transactional
public UserDTO create(CreateUserDTO dto) {
    User user = modelMapper.map(dto, User.class);
    User saved = userRepository.save(user);
    return modelMapper.map(saved, UserDTO.class);
}

}

Repository Layer (Spring Data JPA):

Extends JpaRepository<Entity, ID>
Define custom query methods
Use @Query for complex queries

@Repository public interface UserRepository extends JpaRepository<User, Long> { Optional<User> findByEmail(String email);

@Query("SELECT u FROM User u WHERE u.createdAt > :date")
List&#x3C;User> findRecentUsers(@Param("date") LocalDateTime date);

// Projection for performance
@Query("SELECT new com.example.dto.UserSummaryDTO(u.id, u.name, u.email) FROM User u")
List&#x3C;UserSummaryDTO> findAllSummaries();

}

JPA/Hibernate Best Practices

Entity Design:

Use @Entity and @Table annotations
Always define @Id with generation strategy
Use @Column for constraints and mappings
Implement equals() and hashCode() based on business key

@Entity @Table(name = "users") @Getter @Setter @NoArgsConstructor public class User { @Id @GeneratedValue(strategy = GenerationType.IDENTITY) private Long id;

@Column(nullable = false, unique = true)
private String email;

@Column(nullable = false)
private String name;

@OneToMany(mappedBy = "user", cascade = CascadeType.ALL, orphanRemoval = true)
private List&#x3C;Order> orders = new ArrayList&#x3C;>();

@CreatedDate
@Column(nullable = false, updatable = false)
private LocalDateTime createdAt;

@LastModifiedDate
private LocalDateTime updatedAt;

}

Performance Optimization:

Use @EntityGraph or JOIN FETCH to prevent N+1 queries
Lazy load associations by default
Use pagination for large result sets
Define proper indexes in database

@Query("SELECT u FROM User u JOIN FETCH u.orders WHERE u.id = :id") Optional<User> findByIdWithOrders(@Param("id") Long id);

// Pagination Page<User> findAll(Pageable pageable);

Testing (JUnit 5 + Mockito)

Unit Testing Services:

@ExtendWith(MockitoExtension.class) class UserServiceImplTest { @Mock private UserRepository userRepository;

@Mock
private ModelMapper modelMapper;

@InjectMocks
private UserServiceImpl userService;

@Test
void findById_WhenUserExists_ReturnsUserDTO() {
    // Given
    Long userId = 1L;
    User user = new User();
    user.setId(userId);
    UserDTO expectedDTO = new UserDTO();

    when(userRepository.findById(userId)).thenReturn(Optional.of(user));
    when(modelMapper.map(user, UserDTO.class)).thenReturn(expectedDTO);

    // When
    UserDTO result = userService.findById(userId);

    // Then
    assertNotNull(result);
    verify(userRepository).findById(userId);
    verify(modelMapper).map(user, UserDTO.class);
}

}

Integration Testing (Spring Boot Test):

@SpringBootTest @AutoConfigureMockMvc @Transactional class UserControllerIntegrationTest { @Autowired private MockMvc mockMvc;

@Autowired
private ObjectMapper objectMapper;

@Test
void createUser_WithValidData_ReturnsCreated() throws Exception {
    CreateUserDTO dto = new CreateUserDTO("John Doe", "john@example.com");

    mockMvc.perform(post("/api/users")
            .contentType(MediaType.APPLICATION_JSON)
            .content(objectMapper.writeValueAsString(dto)))
        .andExpect(status().isCreated())
        .andExpect(jsonPath("$.name").value("John Doe"));
}

}

Build Tools (Maven & Gradle)

Maven (pom.xml):

<dependencies> <dependency> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-starter-web</artifactId> </dependency> <dependency> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-starter-data-jpa</artifactId> </dependency> <dependency> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-starter-validation</artifactId> </dependency> </dependencies>

Gradle (build.gradle):

plugins { id 'java' id 'org.springframework.boot' version '3.2.0' id 'io.spring.dependency-management' version '1.1.4' }

java { sourceCompatibility = JavaVersion.VERSION_21 }

dependencies { implementation 'org.springframework.boot:spring-boot-starter-web' implementation 'org.springframework.boot:spring-boot-starter-data-jpa' implementation 'org.springframework.boot:spring-boot-starter-validation' compileOnly 'org.projectlombok:lombok' annotationProcessor 'org.projectlombok:lombok' testImplementation 'org.springframework.boot:spring-boot-starter-test' }

Exception Handling

Global Exception Handler:

@RestControllerAdvice public class GlobalExceptionHandler {

@ExceptionHandler(UserNotFoundException.class)
public ResponseEntity&#x3C;ErrorResponse> handleUserNotFound(UserNotFoundException ex) {
    ErrorResponse error = new ErrorResponse(
        "USER_NOT_FOUND",
        ex.getMessage(),
        LocalDateTime.now()
    );
    return ResponseEntity.status(HttpStatus.NOT_FOUND).body(error);
}

@ExceptionHandler(MethodArgumentNotValidException.class)
public ResponseEntity&#x3C;ErrorResponse> handleValidation(MethodArgumentNotValidException ex) {
    Map&#x3C;String, String> errors = ex.getBindingResult()
        .getFieldErrors()
        .stream()
        .collect(Collectors.toMap(
            FieldError::getField,
            FieldError::getDefaultMessage
        ));

    ErrorResponse error = new ErrorResponse(
        "VALIDATION_ERROR",
        "Invalid input",
        errors,
        LocalDateTime.now()
    );
    return ResponseEntity.status(HttpStatus.BAD_REQUEST).body(error);
}

}

Logging and Monitoring

Logging (SLF4J + Logback):

@Slf4j @Service public class UserServiceImpl implements UserService {

public UserDTO findById(Long id) {
    log.debug("Finding user with id: {}", id);
    try {
        User user = userRepository.findById(id)
            .orElseThrow(() -> new UserNotFoundException(id));
        log.info("User found: {}", user.getEmail());
        return modelMapper.map(user, UserDTO.class);
    } catch (UserNotFoundException ex) {
        log.error("User not found with id: {}", id, ex);
        throw ex;
    }
}

}

Actuator for Monitoring:

management: endpoints: web: exposure: include: health,info,metrics,prometheus endpoint: health: show-details: always

Iron Laws

ALWAYS use constructor injection over field injection with @Autowired — field injection hides dependencies, makes testing harder, and creates partially-initialized objects that crash at runtime if the context isn't fully loaded.
NEVER use Optional.get() without a preceding isPresent() check or orElse() /orElseThrow() — unconditional get() throws NoSuchElementException on empty optionals, silently defeating Optional's entire purpose.
ALWAYS handle @Transactional boundaries explicitly — calling a transactional method from within the same class bypasses the proxy and runs without a transaction, causing silent data inconsistency.
NEVER use @Async without a configured TaskExecutor — Spring's default @Async executor uses a single-thread pool; concurrent async calls queue up and defeat parallelism.
ALWAYS use @ControllerAdvice with specific exception types for error handling — catching Exception globally hides root causes; specific exception handlers produce correct HTTP status codes and meaningful error responses.

Anti-Patterns

Anti-Pattern Why It Fails Correct Approach

Field injection with @Autowired

Hidden dependencies; untestable without Spring context; null in unit tests Constructor injection; all required dependencies declared as final fields

Optional.get() without check NoSuchElementException at runtime; defeats Optional's null-safety contract Use orElseThrow() , orElse() , or map() /flatMap() chains

@Transactional on same-class method calls Spring proxy bypassed; method runs outside transaction; data integrity lost Move transactional methods to a separate service bean; inject and call from outside

Default @Async thread pool Single-thread pool queues all tasks; async calls run sequentially Configure ThreadPoolTaskExecutor with pool size, queue, and rejection policy

Global @ExceptionHandler(Exception.class)

Swallows specific exceptions; all errors return same generic 500 response Map specific exception types to HTTP status codes; use @ResponseStatus annotations

Consolidated Skills

This expert skill consolidates 1 individual skills:

java-expert

Memory Protocol (MANDATORY)

Before starting:

cat .claude/context/memory/learnings.md

After completing: Record any new patterns or exceptions discovered.

ASSUME INTERRUPTION: Your context may reset. If it's not in memory, it didn't happen.

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