Spring Boot Interview Questions
Spring Boot Interview Questions - Detailed Answers
2025-06-12 19:06:51 - Admin Sarwar
1. Spring Boot Auto-Configuration MechanismQuestion: Explain the Spring Boot auto-configuration mechanism. How would you create a custom auto-configuration?Answer:
Auto-Configuration Process:
Spring Boot's auto-configuration works through the following mechanism:
- @EnableAutoConfiguration annotation triggers the auto-configuration process
- Spring Boot scans META-INF/spring.factories files in the classpath
- It loads all classes listed under org.springframework.boot.autoconfigure.EnableAutoConfiguration
- Each auto-configuration class uses conditional annotations to determine if it should be applied
Key Components:
// Example of how Spring Boot determines what to configure
@Configuration
@ConditionalOnClass(DataSource.class)
@ConditionalOnMissingBean(DataSource.class)
@EnableConfigurationProperties(DataSourceProperties.class)
public class DataSourceAutoConfiguration {
@Bean
@ConditionalOnProperty(prefix = "spring.datasource", name = "url")
public DataSource dataSource(DataSourceProperties properties) {
return DataSourceBuilder.create()
.url(properties.getUrl())
.username(properties.getUsername())
.password(properties.getPassword())
.build();
}
}
Creating Custom Auto-Configuration:
- Create Configuration Class:
@Configuration
@ConditionalOnClass(RedisTemplate.class)
@ConditionalOnMissingBean(RedisTemplate.class)
@EnableConfigurationProperties(CustomRedisProperties.class)
public class CustomRedisAutoConfiguration {
@Bean
public RedisTemplate<String, Object> redisTemplate(
RedisConnectionFactory connectionFactory,
CustomRedisProperties properties) {
RedisTemplate<String, Object> template = new RedisTemplate<>();
template.setConnectionFactory(connectionFactory);
// Custom serialization based on properties
if (properties.isUseJsonSerialization()) {
template.setDefaultSerializer(new GenericJackson2JsonRedisSerializer());
}
return template;
}
}
- Create Properties Class:
@ConfigurationProperties(prefix = "custom.redis")
public class CustomRedisProperties {
private boolean useJsonSerialization = true;
private int maxConnections = 10;
private Duration timeout = Duration.ofSeconds(5);
// getters and setters
}
- Register in spring.factories:
# META-INF/spring.factories org.springframework.boot.autoconfigure.EnableAutoConfiguration=\ com.example.autoconfigure.CustomRedisAutoConfiguration
Advanced Conditional Logic:
@Configuration
public class DatabaseAutoConfiguration {
@Configuration
@ConditionalOnProperty(value = "app.database.type", havingValue = "mysql")
static class MySQLConfiguration {
@Bean
public DataSource mysqlDataSource() { /* implementation */ }
}
@Configuration
@ConditionalOnProperty(value = "app.database.type", havingValue = "postgres")
static class PostgreSQLConfiguration {
@Bean
public DataSource postgresDataSource() { /* implementation */ }
}
}
2. Handling Circular DependenciesQuestion: How do you handle circular dependencies in Spring Boot? What are the different strategies?Answer:
Understanding Circular Dependencies:
Circular dependencies occur when Bean A depends on Bean B, and Bean B depends on Bean A, creating a cycle.
Detection:
// This creates a circular dependency
@Service
public class UserService {
private final OrderService orderService;
// UserService needs OrderService
}
@Service
public class OrderService {
private final UserService userService;
// OrderService needs UserService
}
Strategy 1: @Lazy Annotation
@Service
public class UserService {
private final OrderService orderService;
public UserService(@Lazy OrderService orderService) {
this.orderService = orderService;
}
}
@Service
public class OrderService {
private final UserService userService;
public OrderService(UserService userService) {
this.userService = userService;
}
}
Strategy 2: Setter Injection
@Service
public class UserService {
private OrderService orderService;
@Autowired
public void setOrderService(OrderService orderService) {
this.orderService = orderService;
}
}
Strategy 3: ApplicationContextAware
@Service
public class UserService implements ApplicationContextAware {
private ApplicationContext applicationContext;
private OrderService orderService;
@Override
public void setApplicationContext(ApplicationContext applicationContext) {
this.applicationContext = applicationContext;
}
private OrderService getOrderService() {
if (orderService == null) {
orderService = applicationContext.getBean(OrderService.class);
}
return orderService;
}
}
Strategy 4: Restructuring (Best Practice)
// Extract common functionality to a separate service
@Service
public class BusinessLogicService {
public void processUserOrder(User user, Order order) {
// Common logic that both services need
}
}
@Service
public class UserService {
private final BusinessLogicService businessLogicService;
public UserService(BusinessLogicService businessLogicService) {
this.businessLogicService = businessLogicService;
}
}
@Service
public class OrderService {
private final BusinessLogicService businessLogicService;
public OrderService(BusinessLogicService businessLogicService) {
this.businessLogicService = businessLogicService;
}
}
When to Use Each Strategy:
- @Lazy: Quick fix, but can hide design issues
- Setter Injection: Temporary solution, breaks immutability
- ApplicationContextAware: More control, but couples to Spring
- Restructuring: Best long-term solution, improves design
3. Microservices Distributed Transactions (Saga Pattern)Question: Design a distributed transaction strategy for microservices using Spring Boot. Compare Saga pattern vs 2PC.Answer:
Two-Phase Commit (2PC) vs Saga Pattern:
2PC Problems:
- Blocking protocol (coordinator failure blocks all participants)
- Not suitable for microservices (tight coupling)
- Poor performance and availability
Saga Pattern Benefits:
- Non-blocking
- Better fault tolerance
- Maintains service autonomy
Saga Implementation Types:
1. Choreography-Based Saga
// Order Service
@Service
public class OrderService {
@EventListener
public void handlePaymentProcessed(PaymentProcessedEvent event) {
if (event.isSuccessful()) {
// Continue with order processing
confirmOrder(event.getOrderId());
publishEvent(new OrderConfirmedEvent(event.getOrderId()));
} else {
// Compensate
cancelOrder(event.getOrderId());
}
}
private void confirmOrder(String orderId) {
// Update order status
}
private void cancelOrder(String orderId) {
// Compensation logic
}
}
// Payment Service
@Service
public class PaymentService {
@EventListener
public void handleOrderCreated(OrderCreatedEvent event) {
try {
processPayment(event.getOrderId(), event.getAmount());
publishEvent(new PaymentProcessedEvent(event.getOrderId(), true));
} catch (PaymentException e) {
publishEvent(new PaymentProcessedEvent(event.getOrderId(), false));
}
}
}
2. Orchestration-Based Saga
@Component
public class OrderSagaOrchestrator {
private final PaymentService paymentService;
private final InventoryService inventoryService;
private final ShippingService shippingService;
public void processOrder(OrderCreatedEvent event) {
SagaTransaction saga = SagaTransaction.builder()
.transactionId(event.getOrderId())
.addStep(new PaymentStep(paymentService))
.addStep(new InventoryStep(inventoryService))
.addStep(new ShippingStep(shippingService))
.build();
saga.execute();
}
}
// Saga Transaction Implementation
public class SagaTransaction {
private final List<SagaStep> steps;
private final List<SagaStep> completedSteps;
public void execute() {
try {
for (SagaStep step : steps) {
step.execute();
completedSteps.add(step);
}
} catch (Exception e) {
compensate();
throw new SagaExecutionException("Saga failed", e);
}
}
private void compensate() {
// Execute compensation in reverse order
Collections.reverse(completedSteps);
for (SagaStep step : completedSteps) {
try {
step.compensate();
} catch (Exception e) {
log.error("Compensation failed for step: {}", step.getName(), e);
}
}
}
}
// Abstract Saga Step
public abstract class SagaStep {
public abstract void execute() throws Exception;
public abstract void compensate() throws Exception;
public abstract String getName();
}
// Concrete Implementation
public class PaymentStep extends SagaStep {
private final PaymentService paymentService;
@Override
public void execute() throws Exception {
paymentService.processPayment(getOrderId(), getAmount());
}
@Override
public void compensate() throws Exception {
paymentService.refundPayment(getOrderId());
}
}
Saga State Management:
@Entity
public class SagaState {
@Id
private String sagaId;
private String sagaType;
private SagaStatus status;
private String currentStep;
private String compensationStep;
private LocalDateTime createdAt;
private LocalDateTime updatedAt;
@Convert(converter = JpaConverterJson.class)
private Map<String, Object> sagaData;
}
@Repository
public interface SagaStateRepository extends JpaRepository<SagaState, String> {
List<SagaState> findByStatusAndCreatedAtBefore(SagaStatus status, LocalDateTime dateTime);
}
Compensation Strategies:
- Semantic Rollback: Undo business operations (refund payment)
- Forward Recovery: Continue despite failures
- Retry with Exponential Backoff: Handle transient failures
Implementation Considerations:
- Idempotency: All operations must be idempotent
- Timeout Handling: Implement timeouts for each step
- Monitoring: Track saga execution and failure rates
- Dead Letter Queues: Handle failed compensations
4. Performance Optimization StrategyQuestion: Your Spring Boot application is experiencing high memory usage and slow response times. Walk through your debugging and optimization strategy.Answer:Phase 1: Diagnostics and Monitoring
1. Enable Comprehensive Monitoring:
# application.yml
management:
endpoints:
web:
exposure:
include: "*"
metrics:
export:
prometheus:
enabled: true
endpoint:
health:
show-details: always
2. Memory Analysis Tools:
@RestController
public class DiagnosticsController {
@GetMapping("/diagnostics/memory")
public Map<String, Object> getMemoryInfo() {
Runtime runtime = Runtime.getRuntime();
Map<String, Object> memInfo = new HashMap<>();
memInfo.put("totalMemory", runtime.totalMemory());
memInfo.put("freeMemory", runtime.freeMemory());
memInfo.put("usedMemory", runtime.totalMemory() - runtime.freeMemory());
memInfo.put("maxMemory", runtime.maxMemory());
// GC Information
List<GarbageCollectorMXBean> gcBeans = ManagementFactory.getGarbageCollectorMXBeans();
for (GarbageCollectorMXBean gcBean : gcBeans) {
memInfo.put(gcBean.getName() + "_collections", gcBean.getCollectionCount());
memInfo.put(gcBean.getName() + "_time", gcBean.getCollectionTime());
}
return memInfo;
}
}
Phase 2: Common Performance Issues and Solutions
1. Database Query Optimization:
// Problem: N+1 Query Issue
@Entity
public class User {
@OneToMany(mappedBy = "user", fetch = FetchType.LAZY)
private List<Order> orders;
}
// Solution: Use @EntityGraph or JOIN FETCH
@Repository
public interface UserRepository extends JpaRepository<User, Long> {
@EntityGraph(attributePaths = {"orders"})
@Query("SELECT u FROM User u WHERE u.active = true")
List<User> findActiveUsersWithOrders();
// Or use custom query
@Query("SELECT u FROM User u LEFT JOIN FETCH u.orders WHERE u.active = true")
List<User> findActiveUsersWithOrdersJoinFetch();
}
2. Connection Pool Optimization:
spring:
datasource:
hikari:
maximum-pool-size: 20
minimum-idle: 5
idle-timeout: 300000
max-lifetime: 1200000
connection-timeout: 20000
validation-timeout: 3000
leak-detection-threshold: 60000
3. Caching Strategy:
@Configuration
@EnableCaching
public class CacheConfig {
@Bean
public CacheManager cacheManager() {
CaffeineCacheManager cacheManager = new CaffeineCacheManager();
cacheManager.setCaffeine(Caffeine.newBuilder()
.maximumSize(1000)
.expireAfterWrite(10, TimeUnit.MINUTES)
.recordStats());
return cacheManager;
}
}
@Service
public class UserService {
@Cacheable(value = "users", key = "#id")
public User findById(Long id) {
return userRepository.findById(id).orElse(null);
}
@CacheEvict(value = "users", key = "#user.id")
public User updateUser(User user) {
return userRepository.save(user);
}
}
4. Async Processing:
@Configuration
@EnableAsync
public class AsyncConfig implements AsyncConfigurer {
@Override
public Executor getAsyncExecutor() {
ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
executor.setCorePoolSize(5);
executor.setMaxPoolSize(20);
executor.setQueueCapacity(100);
executor.setThreadNamePrefix("async-");
executor.setRejectedExecutionHandler(new ThreadPoolExecutor.CallerRunsPolicy());
executor.initialize();
return executor;
}
}
@Service
public class NotificationService {
@Async
public CompletableFuture<Void> sendEmailAsync(String email, String message) {
// Long-running email sending process
emailSender.send(email, message);
return CompletableFuture.completedFuture(null);
}
}
Phase 3: JVM Tuning
1. Memory Configuration:
# Production JVM settings
java -Xms2g -Xmx4g \
-XX:+UseG1GC \
-XX:MaxGCPauseMillis=200 \
-XX:+UseStringDeduplication \
-XX:+UseCompressedOops \
-XX:+UseCompressedClassPointers \
-jar application.jar
2. Garbage Collection Monitoring:
@Component
public class GCMonitor {
@EventListener
public void handleContextRefresh(ContextRefreshedEvent event) {
// Register GC notification listener
List<GarbageCollectorMXBean> gcbeans = ManagementFactory.getGarbageCollectorMXBeans();
for (GarbageCollectorMXBean gcbean : gcbeans) {
NotificationEmitter emitter = (NotificationEmitter) gcbean;
emitter.addNotificationListener(this::handleGCNotification, null, null);
}
}
private void handleGCNotification(Notification notification, Object handback) {
if (notification.getType().equals(GarbageCollectionNotificationInfo.GARBAGE_COLLECTION_NOTIFICATION)) {
GarbageCollectionNotificationInfo info =
GarbageCollectionNotificationInfo.from((CompositeData) notification.getUserData());
// Log or alert on long GC pauses
if (info.getGcInfo().getDuration() > 1000) {
log.warn("Long GC pause detected: {} ms", info.getGcInfo().getDuration());
}
}
}
}
Phase 4: Application-Level Optimizations
1. Lazy Loading Configuration:
@Configuration
public class OptimizationConfig {
@Bean
@Lazy
public ExpensiveService expensiveService() {
return new ExpensiveService();
}
}
2. Response Compression:
server:
compression:
enabled: true
mime-types: text/html,text/xml,text/plain,text/css,text/javascript,application/javascript,application/json
min-response-size: 1024
3. Database Batch Operations:
@Service
public class BatchUserService {
@Transactional
public void saveUsers(List<User> users) {
int batchSize = 50;
for (int i = 0; i < users.size(); i += batchSize) {
List<User> batch = users.subList(i, Math.min(i + batchSize, users.size()));
userRepository.saveAll(batch);
// Flush and clear to prevent memory issues
entityManager.flush();
entityManager.clear();
}
}
}
Monitoring and Alerting Setup:
@Component
public class PerformanceMetrics {
private final MeterRegistry meterRegistry;
private final Timer responseTimer;
private final Counter errorCounter;
public PerformanceMetrics(MeterRegistry meterRegistry) {
this.meterRegistry = meterRegistry;
this.responseTimer = Timer.builder("http.request.duration")
.description("HTTP request duration")
.register(meterRegistry);
this.errorCounter = Counter.builder("http.request.errors")
.description("HTTP request errors")
.register(meterRegistry);
}
@EventListener
public void handleHttpRequest(HttpRequestEvent event) {
responseTimer.record(event.getDuration(), TimeUnit.MILLISECONDS);
if (event.isError()) {
errorCounter.increment();
}
}
}
This comprehensive approach addresses both immediate performance issues and establishes long-term monitoring and optimization practices.
5. Security Architecture - Zero Trust ImplementationQuestion: Design a zero-trust security architecture for Spring Boot microservices. How would you implement service-to-service authentication without a central auth server?Answer:
Zero Trust Principles:
- Never trust, always verify
- Assume breach has occurred
- Verify explicitly for every transaction
- Use least-privilege access
Implementation Strategy:
1. Mutual TLS (mTLS) for Service Authentication:
@Configuration
@EnableWebSecurity
public class SecurityConfig {
@Bean
public WebSecurityConfigurerAdapter webSecurityConfig() {
return new WebSecurityConfigurerAdapter() {
@Override
protected void configure(HttpSecurity http) throws Exception {
http
.requiresChannel(channel ->
channel.requestMatchers(r -> r.getHeader("X-Forwarded-Proto") != null)
.requiresSecure())
.x509(x509 -> x509
.subjectPrincipalRegex("CN=(.*?)(?:,|$)")
.userDetailsService(customX509UserDetailsService()))
.authorizeRequests(authz -> authz
.requestMatchers("/actuator/health").permitAll()
.requestMatchers("/internal/**").hasRole("SERVICE")
.anyRequest().authenticated())
.sessionManagement(session ->
session.sessionCreationPolicy(SessionCreationPolicy.STATELESS));
}
};
}
@Bean
public X509UserDetailsService customX509UserDetailsService() {
return new X509UserDetailsService() {
@Override
public UserDetails loadUserByUsername(String username) throws UsernameNotFoundException {
// Validate service identity from certificate CN
if (isValidServiceIdentity(username)) {
return User.builder()
.username(username)
.password("")
.roles("SERVICE")
.build();
}
throw new UsernameNotFoundException("Invalid service identity: " + username);
}
};
}
}
2. JWT with Service Identity Claims:
@Component
public class ServiceTokenManager {
private final String privateKey;
private final Map<String, String> trustedServices;
public String generateServiceToken(String serviceId, String targetService) {
Map<String, Object> claims = new HashMap<>();
claims.put("service_id", serviceId);
claims.put("target_service", targetService);
claims.put("permissions", getServicePermissions(serviceId, targetService));
claims.put("issued_at", System.currentTimeMillis());
return Jwts.builder()
.setClaims(claims)
.setIssuer(serviceId)
.setAudience(targetService)
.setExpiration(Date.from(Instant.now().plusSeconds(300))) // 5 minutes
.signWith(SignatureAlgorithm.RS256, getPrivateKey())
.compact();
}
public boolean validateServiceToken(String token, String expectedService) {
try {
Claims claims = Jwts.parser()
.setSigningKey(getPublicKey(getServiceIdFromToken(token)))
.parseClaimsJws(token)
.getBody();
String serviceId = claims.get("service_id", String.class);
String targetService = claims.get("target_service", String.class);
return trustedServices.containsKey(serviceId) &&
expectedService.equals(targetService) &&
hasValidPermissions(claims);
} catch (JwtException e) {
log.warn("Invalid service token: {}", e.getMessage());
return false;
}
}
}
3. Request Signing for Message Integrity:
@Component
public class RequestSigningInterceptor implements ClientHttpRequestInterceptor {
private final String serviceId;
private final PrivateKey privateKey;
@Override
public ClientHttpResponse intercept(
HttpRequest request,
byte[] body,
ClientHttpRequestExecution execution) throws IOException {
// Create signature
String timestamp = String.valueOf(System.currentTimeMillis());
String nonce = UUID.randomUUID().toString();
String stringToSign = createStringToSign(request, body, timestamp, nonce);
String signature = signString(stringToSign);
// Add headers
request.getHeaders().add("X-Service-ID", serviceId);
request.getHeaders().add("X-Timestamp", timestamp);
request.getHeaders().add("X-Nonce", nonce);
request.getHeaders().add("X-Signature", signature);
return execution.execute(request, body);
}
private String createStringToSign(HttpRequest request, byte[] body,
String timestamp, String nonce) {
return String.join("\n",
request.getMethod().name(),
request.getURI().getPath(),
request.getURI().getQuery() != null ? request.getURI().getQuery() : "",
timestamp,
nonce,
DigestUtils.sha256Hex(body)
);
}
}
4. Distributed Authorization with Policy Engine:
@Service
public class PolicyEngine {
private final PolicyRepository policyRepository;
public boolean authorize(ServiceRequest request) {
String serviceId = request.getServiceId();
String resource = request.getResource();
String action = request.getAction();
List<Policy> policies = policyRepository.findByServiceId(serviceId);
for (Policy policy : policies) {
if (policy.matches(resource, action)) {
return evaluatePolicy(policy, request);
}
}
return false; // Deny by default
}
private boolean evaluatePolicy(Policy policy, ServiceRequest request) {
// Implement policy evaluation logic
// Support for RBAC, ABAC, time-based access, etc.
for (PolicyCondition condition : policy.getConditions()) {
if (!condition.evaluate(request.getContext())) {
return false;
}
}
return true;
}
}
@Entity
public class Policy {
@Id
private String id;
private String serviceId;
private String resourcePattern;
private String actionPattern;
private PolicyEffect effect; // ALLOW, DENY
@OneToMany(mappedBy = "policy", cascade = CascadeType.ALL)
private List<PolicyCondition> conditions;
public boolean matches(String resource, String action) {
return resource.matches(resourcePattern) &&
action.matches(actionPattern);
}
}
5. Network Segmentation and Service Mesh Integration:
@Configuration
public class ServiceMeshConfig {
@Bean
public RestTemplate secureRestTemplate() {
RestTemplate restTemplate = new RestTemplate();
// Add interceptors for service mesh integration
restTemplate.getInterceptors().add(new ServiceMeshHeaderInterceptor());
restTemplate.getInterceptors().add(new RequestSigningInterceptor());
restTemplate.getInterceptors().add(new CircuitBreakerInterceptor());
return restTemplate;
}
}
@Component
public class ServiceMeshHeaderInterceptor implements ClientHttpRequestInterceptor {
private final String serviceId = System.getenv("SERVICE_ID");
private final String podId = System.getenv("HOSTNAME");
@Override
public ClientHttpResponse intercept(
HttpRequest request,
byte[] body,
ClientHttpRequestExecution execution) throws IOException {
// Add service mesh headers
request.getHeaders().add("X-Service-Source", serviceId);
request.getHeaders().add("X-Pod-ID", podId);
request.getHeaders().add("X-Trace-ID", getCurrentTraceId());
return execution.execute(request, body);
}
}
6. Runtime Security Monitoring:
@Component
public class SecurityMonitor {
private final MeterRegistry meterRegistry;
private final Counter authFailures;
private final Counter suspiciousActivity;
public SecurityMonitor(MeterRegistry meterRegistry) {
this.meterRegistry = meterRegistry;
this.authFailures = Counter.builder("security.auth.failures")
.description("Authentication failures")
.register(meterRegistry);
this.suspiciousActivity = Counter.builder("security.suspicious.activity")
.description("Suspicious activity detected")
.register(meterRegistry);
}
@EventListener
public void handleAuthenticationFailure(AuthenticationFailureEvent event) {
authFailures.increment(
Tags.of(
"source", event.getSource(),
"reason", event.getException().getClass().getSimpleName()
)
);
// Trigger security response if needed
if (isRepeatedFailure(event.getSource())) {
triggerSecurityResponse(event.getSource());
}
}
private void triggerSecurityResponse(String source) {
// Implement automated response
// - Block IP/service temporarily
// - Alert security team
// - Increase monitoring
}
}
Benefits of this approach:
- No single point of failure: No central auth server dependency
- Defense in depth: Multiple security layers
- Service autonomy: Each service validates independently
- Audit trail: Comprehensive logging and monitoring
- Scalable: Distributed policy evaluation
Challenges and Mitigations:
- Certificate management: Use automated certificate rotation
- Policy consistency: Implement policy validation and testing
- Performance impact: Cache validation results appropriately
- Complexity: Provide clear documentation and tooling
6. Event Sourcing System DesignQuestion: Design an event sourcing system using Spring Boot. How would you handle event versioning, snapshots, and replay mechanisms?Answer:
Event Sourcing Fundamentals: Event sourcing stores all changes to application state as a sequence of events, rather than storing current state directly.
Core Implementation:
1. Event Store Design:
@Entity
@Table(name = "event_store")
public class EventEntity {
@Id
private String eventId;
private String aggregateId;
private String aggregateType;
private Long version;
private String eventType;
private String eventData;
private String metadata;
private LocalDateTime timestamp;
private String userId;
// Optimistic locking
@Version
private Long entityVersion;
}
@Repository
public interface EventRepository extends JpaRepository<EventEntity, String> {
@Query("SELECT e FROM EventEntity e WHERE e.aggregateId = :aggregateId ORDER BY e.version")
List<EventEntity> findByAggregateIdOrderByVersion(@Param("aggregateId") String aggregateId);
@Query("SELECT e FROM EventEntity e WHERE e.aggregateId = :aggregateId AND e.version > :fromVersion ORDER BY e.version")
List<EventEntity> findByAggregateIdAndVersionGreaterThan(@Param("aggregateId") String aggregateId,
@Param("fromVersion") Long fromVersion);
@Query("SELECT e FROM EventEntity e WHERE e.timestamp >= :fromTime ORDER BY e.timestamp")
Stream<EventEntity> findEventsFromTime(@Param("fromTime") LocalDateTime fromTime);
}
2. Aggregate Root with Event Sourcing:
public abstract class AggregateRoot {
private String id;
private Long version = 0L;
private List