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Language Features (Java 21+)

Records

public record User(Long id, String name, String email) {}
// Automatic: constructor, equals, hashCode, toString, accessors
// Use with local records for intermediate data transformation

Sealed Classes and Pattern Matching

public sealed interface Payment permits CreditCard, PayPal, Crypto {}
public record CreditCard(String lastFour, String expiry) implements Payment {}
public record PayPal(String email) implements Payment {}
public record Crypto(String walletAddress, String currency) implements Payment {}

public String processPayment(Payment payment) {
    return switch (payment) {
        case CreditCard c when c.expiry().isAfter(LocalDate.now()) -> "credit " + c.lastFour()
        case PayPal p -> "paypal " + p.email()
        case Crypto c -> "crypto " + c.currency()
    };
}

Virtual Threads (Project Loom)

// Platform thread per request -> Virtual thread per request
// Enable: --enable-preview (Java 21), default in Spring Boot 3.4+
try (var executor = Executors.newVirtualThreadPerTaskExecutor()) {
    var futures = tasks.stream()
        .map(task -> executor.submit(() -> processTask(task)))
        .toList();
    for (var future : futures) future.get();
}

Key Language Features

• Records for data carriers, sealed classes for restricted hierarchies, pattern matching for type-safe switching
• Text blocks ("""...""") for multi-line strings (JSON, SQL, XML templates)
• String::formatted (not String.format) for template interpolation
• Stream API with toList() collector, mapMulti for flatMap with control
• Optional for nullable returns (never for fields or parameters)
• CompletionStage / CompletableFuture for async orchestration
• Instant, LocalDate, LocalDateTime, Duration, Period over java.util.Date

Project Structure

Maven: pom.xml

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

• Multi-module projects: parent POM with <modules>, child modules inherit config
• BOM (Bill of Materials) for dependency version management in multi-module projects
• Dependency scopes: compile (default), provided (servlet container), runtime (JDBC), test, import (BOM)

Gradle (Groovy DSL preferred over Kotlin DSL for build simplicity)

java {
    toolchain {
        languageVersion = JavaLanguageVersion.of(21)
    }
}

• Kotlin DSL provides better IDE support but steeper learning curve
• Version catalogs (libs.versions.toml) for centralized dependency management
• Convention plugins for shared build logic across multi-module projects

Spring Boot 3.x Patterns

Project Structure

src/main/java/com/example/app/
  config/        # @Configuration, @Bean definitions
  controller/    # @RestController, @ControllerAdvice
  service/       # @Service, business logic
  repository/    # Spring Data interfaces (@Repository)
  domain/        # Domain models, value objects
  dto/           # Request/response DTOs
  exception/     # Custom exceptions, @ExceptionHandler
  mapper/        # Entity <-> DTO mapping (MapStruct)

Configuration

@ConfigurationProperties(prefix = "app.redis")
public record RedisProperties(String host, int port, Duration timeout) {}

// In application.yml
app.redis.host=localhost
app.redis.port=6379
app.redis.timeout=5s

Rest Controller

@RestController
@RequestMapping("/api/v1/users")
public class UserController {
    private final UserService userService;

    @GetMapping("/{id}")
    public ResponseEntity<UserResponse> getUser(@PathVariable Long id) {
        return ResponseEntity.ok(userService.findById(id));
    }

    @PostMapping
    public ResponseEntity<UserResponse> createUser(@Valid @RequestBody CreateUserRequest request) {
        var user = userService.create(request);
        return ResponseEntity.created(URI.create("/api/v1/users/" + user.id())).body(user);
    }
}

Exception Handling

@RestControllerAdvice
public class GlobalExceptionHandler {
    @ExceptionHandler(ResourceNotFoundException.class)
    public ProblemDetail handleNotFound(ResourceNotFoundException ex) {
        return ProblemDetail.forStatusAndDetail(HttpStatus.NOT_FOUND, ex.getMessage());
    }

    @ExceptionHandler(MethodArgumentNotValidException.class)
    public ProblemDetail handleValidation(MethodArgumentNotValidException ex) {
        var errors = ex.getBindingResult().getFieldErrors().stream()
            .map(e -> new FieldError(e.getField(), e.getDefaultMessage()))
            .toList();
        var problem = ProblemDetail.forStatus(HttpStatus.BAD_REQUEST);
        problem.setProperty("errors", errors);
        return problem;
    }
}

Data Access

Spring Data JPA

• @Entity with @Table, @Id (UUID v7 preferred over auto-increment for distributed)
• @Repository interface extending JpaRepository<T, ID>
• Query methods: findByXxx, findByXxxAndYyy, countByXxx
• @Query with JPQL or native SQL for complex queries
• @EntityGraph for eager loading control (avoid N+1)
• @Lock for pessimistic locking when needed

Spring Data JDBC

• Simpler than JPA (no lazy loading, no cache, no session)
• Aggregate-oriented: one aggregate root = one repository
• @Column, @Id, @Version for optimistic locking
• @MappedCollection for one-to-many within same aggregate

Flyway Migrations

-- V1__create_users_table.sql
CREATE TABLE users (
    id UUID PRIMARY KEY,
    name VARCHAR(255) NOT NULL,
    email VARCHAR(255) NOT NULL UNIQUE,
    created_at TIMESTAMP WITH TIME ZONE NOT NULL DEFAULT NOW()
);

Testing

JUnit 5 + Mockito

@SpringBootTest
@AutoConfigureMockMvc
class UserControllerTest {
    @Autowired
    private MockMvc mockMvc;

    @MockitoBean
    private UserService userService;

    @Test
    void shouldReturnUser() throws Exception {
        when(userService.findById(1L)).thenReturn(new UserResponse(1L, "Alice", "alice@example.com"));

        mockMvc.perform(get("/api/v1/users/1"))
            .andExpect(status().isOk())
            .andExpect(jsonPath("$.name").value("Alice"));
    }
}

Testcontainers

@Container
static PostgreSQLContainer<?> postgres = new PostgreSQLContainer<>("postgres:16");

@DynamicPropertySource
static void configure(DynamicPropertyRegistry registry) {
    registry.add("spring.datasource.url", postgres::getJdbcUrl);
    registry.add("spring.datasource.username", postgres::getUsername);
    registry.add("spring.datasource.password", postgres::getPassword);
}

Build and Dependency Management

| Tool | Build File | Use Case | |------|-----------|----------| | Maven | pom.xml | Standard, mature, predictable | | Gradle | build.gradle[.kts] | Faster, flexible, Android standard |

Dependency Versions

• Maven BOM: spring-boot-dependencies for curated dependency versions
• Gradle version catalog: libs.versions.toml for centralized management
• Renovate or Dependabot for automated dependency updates
• mvn dependency:tree or gradle dependencies for conflict analysis

Performance and JVM Tuning

JVM Options

java -XX:+UseZGC \                    # Low-pause GC (default in Java 21+ for < 100GB heap)
     -XX:+ZGenerational \             # Generational ZGC (Java 21+)
     -Xms2g -Xmx2g \                  # Heap size (min = max to prevent resize)
     -XX:MaxRAMPercentage=75.0 \       # Container-aware heap sizing
     -Djava.security.egd=file:/dev/./urandom \  # Faster secure random
     -jar application.jar

GC Selection

| Collector | Heap < 4GB | Heap 4-32GB | Heap > 32GB | |-----------|-----------|-------------|-------------| | G1GC (default) | Good | Best | Good | | ZGC | Very good | Very good | Best (< 1ms pause) | | Shenandoah | Best | Very good | Good |

Monitoring

• Micrometer (Spring Boot Actuator) for metrics export (Prometheus, Datadog, Graphite)
• /actuator/health for liveness/readiness probes
• /actuator/metrics for JVM, heap, thread, GC metrics
• /actuator/prometheus for Prometheus scrape endpoint
• Distributed tracing: Micrometer Tracing with Brave/OpenTelemetry

Logging

• SLF4J + Logback for structured JSON logging
• logback-spring.xml for environment-specific configuration
• MDC for correlation ID propagation across async boundaries
• Log levels: ERROR (production failures), WARN (potential issues), INFO (ops), DEBUG (dev), TRACE (rare)
• Lombok @Slf4j for logger boilerplate reduction

<appender name="JSON" class="ch.qos.logback.core.ConsoleAppender">
    <encoder class="net.logstash.logback.encoder.LogstashEncoder"/>
</appender>

Common Libraries

| Category | Library | Purpose | |----------|---------|---------| | HTTP client | RestClient (Spring) | Spring-native HTTP client (replaces RestTemplate) | | JSON | Jackson | Default in Spring Boot, customizable via ObjectMapper | | Validation | Hibernate Validator | Jakarta Bean Validation (JSR 380) | | Mapping | MapStruct | Compile-time DTO mapping (over runtime mappers) | | Caching | Spring Cache + Caffeine | JCache abstraction with Caffeine (in-process) | | Scheduling | Spring Scheduler | @Scheduled, @Async with task executor config | | Retry | Spring Retry | @Retryable, @Recover for transient failures | | Rate limit | Bucket4j | Token bucket rate limiting | | Circuit breaker | Resilience4j | Circuit breaker, retry, rate limiter, bulkhead | | Testing | Testcontainers | Integration tests with disposable containers |

Refer to Java Language Specification and Spring Framework documentation. Use Java 21+ features (records, sealed classes, pattern matching, virtual threads) as the baseline.