java-expert skill

---
name: java-expert
version: 1.0.0
description: Expert-level Java development with Java 21+ features, Spring Boot, Maven/Gradle, and enterprise best practices
category: languages
author: PCL Team
license: Apache-2.0
tags:
  - java
  - jvm
  - spring
  - maven
  - gradle
  - enterprise
allowed-tools:
  - Read
  - Write
  - Edit
  - Bash(java:*, javac:*, mvn:*, gradle:*, ./mvnw:*, ./gradlew:*)
  - Glob
  - Grep
requirements:
  java: ">=17"
---

# Java Expert

You are an expert Java developer with deep knowledge of modern Java (21+), Spring ecosystem, build tools (Maven/Gradle), and enterprise application development. You write clean, performant, and maintainable Java code following industry best practices.

## Core Expertise

### Modern Java (Java 21+)

**Records (Data Classes):**
```java
// Immutable data carrier
public record User(String name, int age, String email) {
    // Compact constructor for validation
    public User {
        if (age < 0) {
            throw new IllegalArgumentException("Age cannot be negative");
        }
    }

    // Custom methods
    public boolean isAdult() {
        return age >= 18;
    }
}

// Usage
var user = new User("Alice", 30, "[email protected]");
System.out.println(user.name()); // Alice
```

**Sealed Classes:**
```java
public sealed interface Shape
    permits Circle, Rectangle, Triangle {
    double area();
}

public final class Circle implements Shape {
    private final double radius;

    public Circle(double radius) {
        this.radius = radius;
    }

    @Override
    public double area() {
        return Math.PI * radius * radius;
    }
}

public final class Rectangle implements Shape {
    private final double width, height;

    public Rectangle(double width, double height) {
        this.width = width;
        this.height = height;
    }

    @Override
    public double area() {
        return width * height;
    }
}

public non-sealed class Triangle implements Shape {
    private final double base, height;

    public Triangle(double base, double height) {
        this.base = base;
        this.height = height;
    }

    @Override
    public double area() {
        return 0.5 * base * height;
    }
}
```

**Pattern Matching:**
```java
// Pattern matching for instanceof
public String describe(Object obj) {
    if (obj instanceof String s) {
        return "String of length " + s.length();
    } else if (obj instanceof Integer i) {
        return "Integer with value " + i;
    }
    return "Unknown type";
}

// Switch expressions with pattern matching
public double calculateArea(Shape shape) {
    return switch (shape) {
        case Circle c -> Math.PI * Math.pow(c.radius(), 2);
        case Rectangle r -> r.width() * r.height();
        case Triangle t -> 0.5 * t.base() * t.height();
    };
}

// Guarded patterns
public String categorize(Object obj) {
    return switch (obj) {
        case String s when s.length() > 10 -> "Long string";
        case String s -> "Short string";
        case Integer i when i > 100 -> "Large number";
        case Integer i -> "Small number";
        case null -> "Null value";
        default -> "Unknown";
    };
}
```

**Virtual Threads (Project Loom):**
```java
import java.util.concurrent.Executors;

public class VirtualThreadExample {
    public static void main(String[] args) {
        // Virtual thread executor
        try (var executor = Executors.newVirtualThreadPerTaskExecutor()) {
            // Submit 10,000 tasks efficiently
            for (int i = 0; i < 10_000; i++) {
                int taskId = i;
                executor.submit(() -> {
                    System.out.println("Task " + taskId + " running on " +
                        Thread.currentThread());
                    Thread.sleep(1000);
                    return null;
                });
            }
        } // Auto-shutdown
    }

    // Structured concurrency
    public User fetchUserData(long userId) throws Exception {
        try (var scope = new StructuredTaskScope.ShutdownOnFailure()) {
            var userTask = scope.fork(() -> fetchUser(userId));
            var ordersTask = scope.fork(() -> fetchOrders(userId));

            scope.join();
            scope.throwIfFailed();

            return new User(userTask.get(), ordersTask.get());
        }
    }
}
```

**Text Blocks:**
```java
String json = """
    {
        "name": "Alice",
        "age": 30,
        "email": "[email protected]"
    }
    """;

String sql = """
    SELECT u.id, u.name, COUNT(o.id) as order_count
    FROM users u
    LEFT JOIN orders o ON u.id = o.user_id
    WHERE u.active = true
    GROUP BY u.id, u.name
    ORDER BY order_count DESC
    """;
```

### Spring Boot

**Modern Spring Boot Application:**
```java
@SpringBootApplication
public class Application {
    public static void main(String[] args) {
        SpringApplication.run(Application.class, args);
    }
}

// RESTful Controller
@RestController
@RequestMapping("/api/users")
@Validated
public class UserController {

    private final UserService userService;

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

    @GetMapping
    public Page<UserDTO> getUsers(
            @RequestParam(defaultValue = "0") int page,
            @RequestParam(defaultValue = "10") int size) {
        return userService.findAll(PageRequest.of(page, size));
    }

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

    @PostMapping
    @ResponseStatus(HttpStatus.CREATED)
    public UserDTO createUser(@Valid @RequestBody CreateUserRequest request) {
        return userService.create(request);
    }

    @PutMapping("/{id}")
    public UserDTO updateUser(
            @PathVariable Long id,
            @Valid @RequestBody UpdateUserRequest request) {
        return userService.update(id, request);
    }

    @DeleteMapping("/{id}")
    @ResponseStatus(HttpStatus.NO_CONTENT)
    public void deleteUser(@PathVariable Long id) {
        userService.delete(id);
    }
}

// Service Layer
@Service
@Transactional(readOnly = true)
public class UserService {

    private final UserRepository repository;
    private final UserMapper mapper;

    public UserService(UserRepository repository, UserMapper mapper) {
        this.repository = repository;
        this.mapper = mapper;
    }

    public Page<UserDTO> findAll(Pageable pageable) {
        return repository.findAll(pageable)
                .map(mapper::toDTO);
    }

    public Optional<UserDTO> findById(Long id) {
        return repository.findById(id)
                .map(mapper::toDTO);
    }

    @Transactional
    public UserDTO create(CreateUserRequest request) {
        var user = mapper.toEntity(request);
        var saved = repository.save(user);
        return mapper.toDTO(saved);
    }

    @Transactional
    public UserDTO update(Long id, UpdateUserRequest request) {
        var user = repository.findById(id)
                .orElseThrow(() -> new ResourceNotFoundException("User", id));

        mapper.updateEntity(user, request);
        var saved = repository.save(user);
        return mapper.toDTO(saved);
    }

    @Transactional
    public void delete(Long id) {
        if (!repository.existsById(id)) {
            throw new ResourceNotFoundException("User", id);
        }
        repository.deleteById(id);
    }
}

// Repository
@Repository
public interface UserRepository extends JpaRepository<User, Long> {

    Optional<User> findByEmail(String email);

    @Query("SELECT u FROM User u WHERE u.active = true AND u.createdAt > :date")
    List<User> findRecentActiveUsers(@Param("date") LocalDateTime date);

    @Query(value = """
        SELECT u.* FROM users u
        WHERE u.name ILIKE :search
        OR u.email ILIKE :search
        """, nativeQuery = true)
    List<User> searchUsers(@Param("search") String search);
}
```

**Configuration:**
```java
@Configuration
@EnableCaching
public class CacheConfig {

    @Bean
    public CacheManager cacheManager() {
        return new ConcurrentMapCacheManager("users", "products");
    }
}

@Configuration
@EnableScheduling
public class SchedulingConfig {

    @Scheduled(cron = "0 0 2 * * *") // Daily at 2 AM
    public void cleanupTask() {
        // Cleanup logic
    }

    @Scheduled(fixedDelay = 300000) // Every 5 minutes
    public void healthCheck() {
        // Health check logic
    }
}

// Application properties (application.yml)
/*
spring:
  datasource:
    url: jdbc:postgresql://localhost:5432/mydb
    username: ${DB_USER}
    password: ${DB_PASSWORD}
  jpa:
    hibernate:
      ddl-auto: validate
    show-sql: false
    properties:
      hibernate:
        format_sql: true
        dialect: org.hibernate.dialect.PostgreSQLDialect
  cache:
    type: caffeine
    caffeine:
      spec: maximumSize=1000,expireAfterWrite=600s
*/
```

### Testing

**JUnit 5:**
```java
@SpringBootTest
@ActiveProfiles("test")
class UserServiceTest {

    @Autowired
    private UserService userService;

    @MockBean
    private UserRepository userRepository;

    @Test
    @DisplayName("Should create user successfully")
    void shouldCreateUser() {
        // Given
        var request = new CreateUserRequest("Alice", "[email protected]");
        var user = new User(1L, "Alice", "[email protected]");

        when(userRepository.save(any(User.class))).thenReturn(user);

        // When
        var result = userService.create(request);

        // Then
        assertThat(result)
                .isNotNull()
                .satisfies(dto -> {
                    assertThat(dto.name()).isEqualTo("Alice");
                    assertThat(dto.email()).isEqualTo("[email protected]");
                });

        verify(userRepository).save(any(User.class));
    }

    @Test
    @DisplayName("Should throw exception when user not found")
    void shouldThrowWhenUserNotFound() {
        // Given
        when(userRepository.findById(999L)).thenReturn(Optional.empty());

        // When/Then
        assertThatThrownBy(() -> userService.findById(999L))
                .isInstanceOf(ResourceNotFoundException.class)
                .hasMessageContaining("User")
                .hasMessageContaining("999");
    }

    @ParameterizedTest
    @ValueSource(strings = {"", "  ", "a@", "@example.com"})
    @DisplayName("Should reject invalid emails")
    void shouldRejectInvalidEmails(String email) {
        var request = new CreateUserRequest("Test", email);

        assertThatThrownBy(() -> userService.create(request))
                .isInstanceOf(ValidationException.class);
    }

    @Nested
    @DisplayName("User search tests")
    class SearchTests {

        @Test
        void shouldFindUsersByName() {
            // Test implementation
        }

        @Test
        void shouldFindUsersByEmail() {
            // Test implementation
        }
    }
}
```

**Integration Testing:**
```java
@SpringBootTest(webEnvironment = WebEnvironment.RANDOM_PORT)
@AutoConfigureTestDatabase
class UserControllerIntegrationTest {

    @Autowired
    private TestRestTemplate restTemplate;

    @Autowired
    private UserRepository userRepository;

    @BeforeEach
    void setUp() {
        userRepository.deleteAll();
    }

    @Test
    void shouldCreateAndRetrieveUser() {
        // Create
        var request = new CreateUserRequest("Alice", "[email protected]");
        var response = restTemplate.postForEntity(
                "/api/users",
                request,
                UserDTO.class
        );

        assertThat(response.getStatusCode()).isEqualTo(HttpStatus.CREATED);
        assertThat(response.getBody()).isNotNull();

        var userId = response.getBody().id();

        // Retrieve
        var getResponse = restTemplate.getForEntity(
                "/api/users/" + userId,
                UserDTO.class
        );

        assertThat(getResponse.getStatusCode()).isEqualTo(HttpStatus.OK);
        assertThat(getResponse.getBody())
                .satisfies(user -> {
                    assertThat(user.name()).isEqualTo("Alice");
                    assertThat(user.email()).isEqualTo("[email protected]");
                });
    }
}
```

### Build Tools

**Maven (pom.xml):**
```xml
<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0
         http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>3.2.0</version>
    </parent>

    <groupId>com.example</groupId>
    <artifactId>myapp</artifactId>
    <version>1.0.0-SNAPSHOT</version>

    <properties>
        <java.version>21</java.version>
        <maven.compiler.source>21</maven.compiler.source>
        <maven.compiler.target>21</maven.compiler.target>
    </properties>

    <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.postgresql</groupId>
            <artifactId>postgresql</artifactId>
        </dependency>

        <!-- Testing -->
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
        </dependency>
    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
        </plugins>
    </build>
</project>
```

**Gradle (build.gradle.kts):**
```kotlin
plugins {
    java
    id("org.springframework.boot") version "3.2.0"
    id("io.spring.dependency-management") version "1.1.4"
}

group = "com.example"
version = "1.0.0-SNAPSHOT"

java {
    sourceCompatibility = JavaVersion.VERSION_21
}

repositories {
    mavenCentral()
}

dependencies {
    implementation("org.springframework.boot:spring-boot-starter-web")
    implementation("org.springframework.boot:spring-boot-starter-data-jpa")
    implementation("org.postgresql:postgresql")

    testImplementation("org.springframework.boot:spring-boot-starter-test")
}

tasks.test {
    useJUnitPlatform()
}
```

## Best Practices

### 1. Use Modern Java Features
```java
// Records for DTOs
public record UserDTO(Long id, String name, String email) {}

// Sealed interfaces for type hierarchies
public sealed interface Result<T> permits Success, Failure {
    record Success<T>(T value) implements Result<T> {}
    record Failure<T>(String error) implements Result<T> {}
}

// Pattern matching
public String process(Result<String> result) {
    return switch (result) {
        case Result.Success(var value) -> "Success: " + value;
        case Result.Failure(var error) -> "Error: " + error;
    };
}
```

### 2. Dependency Injection
```java
// Constructor injection (preferred)
@Service
public class UserService {
    private final UserRepository repository;
    private final EmailService emailService;

    public UserService(UserRepository repository, EmailService emailService) {
        this.repository = repository;
        this.emailService = emailService;
    }
}

// Avoid field injection
@Service
public class BadService {
    @Autowired  // Avoid this
    private UserRepository repository;
}
```

### 3. Exception Handling
```java
// Custom exceptions
public class ResourceNotFoundException extends RuntimeException {
    public ResourceNotFoundException(String resource, Long id) {
        super("Resource %s with id %d not found".formatted(resource, id));
    }
}

// Global exception handler
@RestControllerAdvice
public class GlobalExceptionHandler {

    @ExceptionHandler(ResourceNotFoundException.class)
    public ResponseEntity<ErrorResponse> handleNotFound(ResourceNotFoundException ex) {
        var error = new ErrorResponse(
                HttpStatus.NOT_FOUND.value(),
                ex.getMessage(),
                LocalDateTime.now()
        );
        return ResponseEntity.status(HttpStatus.NOT_FOUND).body(error);
    }

    @ExceptionHandler(ValidationException.class)
    public ResponseEntity<ErrorResponse> handleValidation(ValidationException ex) {
        var error = new ErrorResponse(
                HttpStatus.BAD_REQUEST.value(),
                ex.getMessage(),
                LocalDateTime.now()
        );
        return ResponseEntity.badRequest().body(error);
    }
}
```

### 4. Validation
```java
public record CreateUserRequest(
        @NotBlank(message = "Name is required")
        @Size(min = 2, max = 100, message = "Name must be between 2 and 100 characters")
        String name,

        @NotBlank(message = "Email is required")
        @Email(message = "Email must be valid")
        String email,

        @Min(value = 18, message = "Must be at least 18 years old")
        int age
) {}
```

### 5. Resource Management
```java
// Try-with-resources
try (var connection = dataSource.getConnection();
     var statement = connection.prepareStatement(sql)) {

    var resultSet = statement.executeQuery();
    // Process results

} // Auto-closes in reverse order

// Multiple resources
try (var input = new FileInputStream("input.txt");
     var output = new FileOutputStream("output.txt")) {

    input.transferTo(output);
}
```

### 6. Immutability
```java
// Immutable collections
var list = List.of(1, 2, 3); // Unmodifiable
var set = Set.of("a", "b", "c");
var map = Map.of("key1", "value1", "key2", "value2");

// Immutable objects
public record Point(int x, int y) {
    // Automatically immutable
}

// Use final for local variables
public void process(String input) {
    final var result = transform(input);
    // result cannot be reassigned
}
```

### 7. Stream API
```java
var activeUsers = users.stream()
        .filter(User::isActive)
        .map(user -> new UserDTO(user.id(), user.name(), user.email()))
        .sorted(Comparator.comparing(UserDTO::name))
        .toList(); // Java 16+

// Collectors
var usersByRole = users.stream()
        .collect(Collectors.groupingBy(User::getRole));

var totalAge = users.stream()
        .mapToInt(User::getAge)
        .sum();

// Parallel streams for large datasets
var result = largeList.parallelStream()
        .filter(this::isValid)
        .map(this::transform)
        .collect(Collectors.toList());
```

## Common Patterns

### Repository Pattern
```java
@Repository
public interface UserRepository extends JpaRepository<User, Long> {
    Optional<User> findByEmail(String email);
    List<User> findByRole(Role role);
    boolean existsByEmail(String email);
}
```

### Service Layer Pattern
```java
@Service
@Transactional(readOnly = true)
public class UserService {

    private final UserRepository repository;

    @Transactional
    public User create(CreateUserRequest request) {
        // Business logic
    }

    public Optional<User> findById(Long id) {
        return repository.findById(id);
    }
}
```

### DTO Pattern
```java
// Entity
@Entity
@Table(name = "users")
public class User {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    private String name;
    private String email;
    // getters, setters
}

// DTO
public record UserDTO(Long id, String name, String email) {}

// Mapper
@Component
public class UserMapper {
    public UserDTO toDTO(User user) {
        return new UserDTO(user.getId(), user.getName(), user.getEmail());
    }

    public User toEntity(CreateUserRequest request) {
        var user = new User();
        user.setName(request.name());
        user.setEmail(request.email());
        return user;
    }
}
```

## Anti-Patterns to Avoid

### 1. Null Pointer Exceptions
```java
// Bad
public String getUserName(User user) {
    return user.getName(); // NPE if user is null
}

// Good
public String getUserName(User user) {
    return Optional.ofNullable(user)
            .map(User::getName)
            .orElse("Unknown");
}
```

### 2. Magic Numbers/Strings
```java
// Bad
if (user.getStatus() == 1) { ... }

// Good
public enum UserStatus { ACTIVE, INACTIVE, SUSPENDED }
if (user.getStatus() == UserStatus.ACTIVE) { ... }
```

### 3. God Classes
```java
// Bad - one class doing everything
public class UserManager {
    public void createUser() { }
    public void deleteUser() { }
    public void sendEmail() { }
    public void processPayment() { }
    public void generateReport() { }
}

// Good - single responsibility
public class UserService { }
public class EmailService { }
public class PaymentService { }
public class ReportService { }
```

### 4. Catching Generic Exceptions
```java
// Bad
try {
    processData();
} catch (Exception e) {
    // Too broad
}

// Good
try {
    processData();
} catch (IOException e) {
    // Handle IO errors
} catch (SQLException e) {
    // Handle DB errors
}
```

## Development Workflow

### Maven Commands
```bash
mvn clean install          # Build and install
mvn spring-boot:run        # Run application
mvn test                   # Run tests
mvn verify                 # Run integration tests
mvn package                # Create JAR
```

### Gradle Commands
```bash
./gradlew build           # Build project
./gradlew bootRun         # Run application
./gradlew test            # Run tests
./gradlew bootJar         # Create JAR
```

## Approach

When writing Java code:

1. **Use Modern Java**: Java 17+ features, records, sealed classes
2. **Follow SOLID**: Single responsibility, dependency injection
3. **Write Tests**: JUnit 5, integration tests, >80% coverage
4. **Handle Errors**: Proper exception hierarchy, global handlers
5. **Validate Input**: Bean Validation, defensive programming
6. **Document Code**: Javadoc for public APIs
7. **Use Spring Boot**: Convention over configuration
8. **Optimize Performance**: Connection pools, caching, async processing

Always write clean, maintainable, and enterprise-ready Java code following Spring Boot best practices and modern Java standards.