Java Spring Boot App

Java Spring Boot Application

Patterns and best practices for building production-ready Spring Boot applications with layered architecture.

Project Structure

src/
├── main/
│   ├── java/com/company/
│   │   ├── Application.java           # Main entry point
│   │   ├── config/                     # Configuration classes
│   │   │   ├── WebConfig.java
│   │   │   └── SecurityConfig.java
│   │   ├── controller/                 # REST controllers (HTTP layer)
│   │   │   └── PersonController.java
│   │   ├── service/                    # Business logic
│   │   │   └── PersonService.java
│   │   ├── repository/                 # Data access (JPA repositories)
│   │   │   └── PersonRepository.java
│   │   ├── domain/                     # JPA entities
│   │   │   └── Person.java
│   │   ├── dto/                        # Data Transfer Objects
│   │   │   ├── PersonCreateRequest.java
│   │   │   ├── PersonUpdateRequest.java
│   │   │   └── PersonResponse.java
│   │   └── validation/                 # Custom validators
│   │       ├── ValidDateRange.java
│   │       └── ValidDateRangeValidator.java
│   └── resources/
│       ├── application.yml             # Configuration
│       └── db/migration/               # Flyway migrations
│           └── V1__initial_schema.sql
└── test/
    └── java/com/company/
        └── controller/
            └── PersonControllerTest.java

Maven Configuration (pom.xml)

Essential dependencies for a Spring Boot project:

<properties>
    <java.version>21</java.version>
    <spring.boot.version>3.2.0</spring.boot.version>
</properties>

<dependencies>
    <!-- Web -->
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-web</artifactId>
    </dependency>
    
    <!-- JPA & Database -->
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-data-jpa</artifactId>
    </dependency>
    <dependency>
        <groupId>org.postgresql</groupId>
        <artifactId>postgresql</artifactId>
        <scope>runtime</scope>
    </dependency>
    
    <!-- Validation -->
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-validation</artifactId>
    </dependency>
    
    <!-- Database Migrations -->
    <dependency>
        <groupId>org.flywaydb</groupId>
        <artifactId>flyway-core</artifactId>
    </dependency>
    
    <!-- API Documentation -->
    <dependency>
        <groupId>org.springdoc</groupId>
        <artifactId>springdoc-openapi-starter-webmvc-ui</artifactId>
        <version>2.6.0</version>
    </dependency>
    
    <!-- Testing -->
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-test</artifactId>
        <scope>test</scope>
    </dependency>
    <dependency>
        <groupId>org.testcontainers</groupId>
        <artifactId>postgresql</artifactId>
        <scope>test</scope>
    </dependency>
</dependencies>

Project Files

.gitignore

# Build output
target/

# IDE files
.idea/
*.iml
.project
.classpath
.settings/
.vscode/
*.swp
*.swo

# Environment and secrets
.env
*.env
application-local.yml

# Logs
*.log
logs/

# OS files
.DS_Store
Thumbs.db

# Package files
*.jar
*.war
*.ear

# Test output
test-output/

.dockerignore

# Build output (rebuilt in container)
target/

# IDE files
.idea/
*.iml
.project
.classpath
.settings/
.vscode/

# Git
.git/
.gitignore

# Documentation
*.md
!README.md

# Environment files
.env*
application-local.yml

# Logs
*.log
logs/

# Test files
src/test/

Dockerfile (Multi-stage build)

# Build stage
FROM maven:3.9-eclipse-temurin-21-alpine AS builder

WORKDIR /app

# Copy pom.xml and download dependencies (cached layer)
COPY pom.xml .
RUN mvn dependency:go-offline -B

# Copy source and build
COPY src ./src
RUN mvn package -DskipTests -B

# Production stage
FROM eclipse-temurin:21-jre-alpine

WORKDIR /app

# Create non-root user for security
RUN addgroup -g 1001 -S spring && \
    adduser -S spring -u 1001 -G spring

# Copy JAR from builder
COPY --from=builder /app/target/*.jar app.jar

# Set ownership
RUN chown -R spring:spring /app

# Switch to non-root user
USER spring

# Expose port
EXPOSE 8080

# JVM tuning for containers
ENV JAVA_OPTS="-XX:+UseContainerSupport -XX:MaxRAMPercentage=75.0"

# Health check
HEALTHCHECK --interval=30s --timeout=3s --start-period=30s --retries=3 \
    CMD wget --no-verbose --tries=1 --spider http://localhost:8080/actuator/health || exit 1

ENTRYPOINT ["sh", "-c", "java $JAVA_OPTS -jar app.jar"]

docker-compose.yml

version: '3.8'

services:
  app:
    build: .
    ports:
      - "8080:8080"
    environment:
      - SPRING_PROFILES_ACTIVE=prod
      - SPRING_DATASOURCE_URL=jdbc:postgresql://db:5432/myapp
      - SPRING_DATASOURCE_USERNAME=postgres
      - SPRING_DATASOURCE_PASSWORD=postgres
    depends_on:
      db:
        condition: service_healthy
    restart: unless-stopped

  db:
    image: postgres:15-alpine
    environment:
      - POSTGRES_DB=myapp
      - POSTGRES_USER=postgres
      - POSTGRES_PASSWORD=postgres
    volumes:
      - postgres_data:/var/lib/postgresql/data
    healthcheck:
      test: ["CMD-SHELL", "pg_isready -U postgres"]
      interval: 5s
      timeout: 5s
      retries: 5
    ports:
      - "5432:5432"  # Expose for local development

volumes:
  postgres_data:

Dockerfile.layered (Optimized for Spring Boot)

For better caching with Spring Boot's layered JARs:

# Build stage
FROM maven:3.9-eclipse-temurin-21-alpine AS builder

WORKDIR /app
COPY pom.xml .
RUN mvn dependency:go-offline -B

COPY src ./src
RUN mvn package -DskipTests -B && \
    java -Djarmode=layertools -jar target/*.jar extract

# Production stage
FROM eclipse-temurin:21-jre-alpine

WORKDIR /app

RUN addgroup -g 1001 -S spring && \
    adduser -S spring -u 1001 -G spring

# Copy layers in order of change frequency (least → most)
COPY --from=builder /app/dependencies/ ./
COPY --from=builder /app/spring-boot-loader/ ./
COPY --from=builder /app/snapshot-dependencies/ ./
COPY --from=builder /app/application/ ./

RUN chown -R spring:spring /app
USER spring

EXPOSE 8080

ENV JAVA_OPTS="-XX:+UseContainerSupport -XX:MaxRAMPercentage=75.0"

HEALTHCHECK --interval=30s --timeout=3s --start-period=30s --retries=3 \
    CMD wget --no-verbose --tries=1 --spider http://localhost:8080/actuator/health || exit 1

ENTRYPOINT ["sh", "-c", "java $JAVA_OPTS org.springframework.boot.loader.launch.JarLauncher"]

Controller Layer

Controllers handle HTTP concerns only - delegate business logic to services.

package com.company.controller;

import com.company.dto.*;
import com.company.service.PersonService;
import jakarta.validation.Valid;
import java.util.List;
import java.util.UUID;
import org.springframework.http.HttpStatus;
import org.springframework.web.bind.annotation.*;

@RestController
@RequestMapping("/api/v1/people")
public class PersonController {

    private final PersonService personService;

    // Constructor injection (preferred over @Autowired)
    public PersonController(PersonService personService) {
        this.personService = personService;
    }

    @GetMapping
    public List<PersonResponse> list(@RequestParam(value = "q", required = false) String query) {
        return personService.listPeople(query);
    }

    @GetMapping("/{id}")
    public PersonResponse get(@PathVariable UUID id) {
        return personService.getPerson(id);
    }

    @PostMapping
    @ResponseStatus(HttpStatus.CREATED)
    public PersonResponse create(@Valid @RequestBody PersonCreateRequest request) {
        return personService.createPerson(request);
    }

    @PutMapping("/{id}")
    public PersonResponse update(
            @PathVariable UUID id,
            @Valid @RequestBody PersonUpdateRequest request) {
        return personService.updatePerson(id, request);
    }

    @DeleteMapping("/{id}")
    @ResponseStatus(HttpStatus.NO_CONTENT)
    public void delete(@PathVariable UUID id) {
        personService.deletePerson(id);
    }
}

Key patterns:

• Constructor injection (not field @Autowired)
• @Valid for request body validation
• Appropriate HTTP status codes (@ResponseStatus)
• API versioning in path (/api/v1/)
• UUID for entity IDs

Service Layer

Services contain business logic, transactions, and domain orchestration.

package com.company.service;

import com.company.domain.Person;
import com.company.dto.*;
import com.company.repository.PersonRepository;
import java.util.List;
import java.util.UUID;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.http.HttpStatus;
import org.springframework.stereotype.Service;
import org.springframework.transaction.annotation.Transactional;
import org.springframework.web.server.ResponseStatusException;

@Service
public class PersonService {

    private static final Logger logger = LoggerFactory.getLogger(PersonService.class);

    private final PersonRepository personRepository;

    public PersonService(PersonRepository personRepository) {
        this.personRepository = personRepository;
    }

    @Transactional(readOnly = true)
    public List<PersonResponse> listPeople(String query) {
        List<Person> people = (query == null || query.isBlank())
            ? personRepository.findAll()
            : personRepository.findByNameContainingIgnoreCase(query);
        return people.stream().map(this::toResponse).toList();
    }

    @Transactional(readOnly = true)
    public PersonResponse getPerson(UUID id) {
        return personRepository.findById(id)
            .map(this::toResponse)
            .orElseThrow(() -> new ResponseStatusException(HttpStatus.NOT_FOUND, "Person not found"));
    }

    @Transactional
    public PersonResponse createPerson(PersonCreateRequest request) {
        // Business validation
        personRepository.findByEmailIgnoreCase(request.email()).ifPresent(existing -> {
            throw new ResponseStatusException(HttpStatus.CONFLICT, "Email already exists");
        });

        Person person = new Person();
        applyRequest(person, request);
        Person saved = personRepository.save(person);
        
        logger.info("person.created id={} email={}", saved.getId(), saved.getEmail());
        return toResponse(saved);
    }

    @Transactional
    public PersonResponse updatePerson(UUID id, PersonUpdateRequest request) {
        Person person = personRepository.findById(id)
            .orElseThrow(() -> new ResponseStatusException(HttpStatus.NOT_FOUND, "Person not found"));

        // Check email uniqueness if changed
        if (!person.getEmail().equalsIgnoreCase(request.email())) {
            personRepository.findByEmailIgnoreCase(request.email()).ifPresent(existing -> {
                throw new ResponseStatusException(HttpStatus.CONFLICT, "Email already exists");
            });
        }

        applyRequest(person, request);
        Person saved = personRepository.save(person);
        
        logger.info("person.updated id={}", saved.getId());
        return toResponse(saved);
    }

    @Transactional
    public void deletePerson(UUID id) {
        if (!personRepository.existsById(id)) {
            throw new ResponseStatusException(HttpStatus.NOT_FOUND, "Person not found");
        }
        personRepository.deleteById(id);
        logger.info("person.deleted id={}", id);
    }

    private void applyRequest(Person person, PersonCreateRequest request) {
        person.setName(request.name());
        person.setEmail(request.email());
        // ... map other fields
    }

    private PersonResponse toResponse(Person person) {
        return new PersonResponse(
            person.getId(),
            person.getName(),
            person.getEmail()
            // ... map other fields
        );
    }
}

Key patterns:

• @Transactional(readOnly = true) for read operations
• @Transactional for write operations
• ResponseStatusException for HTTP error responses
• Structured logging with context (id={})
• Private helper methods for mapping

Entity Layer (JPA)

package com.company.domain;

import jakarta.persistence.*;
import java.time.LocalDate;
import java.util.UUID;

@Entity
@Table(name = "people")
public class Person extends AuditableEntity {

    @Id
    @GeneratedValue
    private UUID id;

    @Column(name = "name", nullable = false)
    private String name;

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

    @Enumerated(EnumType.STRING)
    @Column(name = "status", nullable = false)
    private PersonStatus status;

    @Column(name = "start_date", nullable = false)
    private LocalDate startDate;

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "department_id")
    private Department department;

    @ManyToMany
    @JoinTable(
        name = "person_skills",
        joinColumns = @JoinColumn(name = "person_id"),
        inverseJoinColumns = @JoinColumn(name = "skill_id")
    )
    private Set<Skill> skills = new HashSet<>();

    // Getters and setters
    public UUID getId() { return id; }
    public String getName() { return name; }
    public void setName(String name) { this.name = name; }
    // ...
}

Key patterns:

• Use UUID for IDs (better for distributed systems)
• @Enumerated(EnumType.STRING) for enums (not ORDINAL)
• FetchType.LAZY for relationships
• Explicit @Column names matching snake_case DB convention
• Extend AuditableEntity for created/updated timestamps

Auditable Base Entity

@MappedSuperclass
public abstract class AuditableEntity {

    @Column(name = "created_at", nullable = false, updatable = false)
    private LocalDateTime createdAt;

    @Column(name = "updated_at", nullable = false)
    private LocalDateTime updatedAt;

    @PrePersist
    protected void onCreate() {
        createdAt = LocalDateTime.now();
        updatedAt = LocalDateTime.now();
    }

    @PreUpdate
    protected void onUpdate() {
        updatedAt = LocalDateTime.now();
    }

    // Getters
}

Repository Layer

package com.company.repository;

import com.company.domain.Person;
import java.util.List;
import java.util.Optional;
import java.util.UUID;
import org.springframework.data.jpa.repository.JpaRepository;

public interface PersonRepository extends JpaRepository<Person, UUID> {

    // Derived query methods
    List<Person> findByNameContainingIgnoreCase(String name);
    
    Optional<Person> findByEmailIgnoreCase(String email);
    
    List<Person> findByDepartmentId(UUID departmentId);
    
    boolean existsByEmail(String email);
}

Key patterns:

• Extend JpaRepository<Entity, IdType>
• Use derived query methods when simple
• Return Optional for single results that may not exist
• Use IgnoreCase for case-insensitive searches

DTO Layer (Records)

Use Java Records for immutable DTOs with built-in validation.

Request DTO

package com.company.dto;

import com.company.validation.ValidDateRange;
import jakarta.validation.constraints.*;
import java.time.LocalDate;
import java.util.List;

@ValidDateRange  // Custom class-level validator
public record PersonCreateRequest(
    @NotBlank String name,
    @NotBlank @Email String email,
    String designation,
    @NotNull PersonStatus status,
    @NotNull @DecimalMin("0.0") @DecimalMax("10000.0") BigDecimal salary,
    @NotNull LocalDate startDate,
    LocalDate endDate,  // Optional
    List<String> skills
) {}

Response DTO

package com.company.dto;

import java.time.LocalDate;
import java.util.List;
import java.util.UUID;

public record PersonResponse(
    UUID id,
    String name,
    String email,
    String designation,
    PersonStatus status,
    BigDecimal salary,
    LocalDate startDate,
    LocalDate endDate,
    List<String> skills,
    String departmentName  // Flattened from relationship
) {}

Key patterns:

• Use Java Records (immutable, concise)
• Separate Create/Update request DTOs
• Response DTOs flatten relationships
• Validation annotations on request fields

Custom Validation

Annotation

package com.company.validation;

import jakarta.validation.Constraint;
import jakarta.validation.Payload;
import java.lang.annotation.*;

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Constraint(validatedBy = ValidDateRangeValidator.class)
public @interface ValidDateRange {
    String message() default "End date must be after start date";
    Class<?>[] groups() default {};
    Class<? extends Payload>[] payload() default {};
}

Validator

package com.company.validation;

import jakarta.validation.ConstraintValidator;
import jakarta.validation.ConstraintValidatorContext;

public class ValidDateRangeValidator implements ConstraintValidator<ValidDateRange, Object> {

    @Override
    public boolean isValid(Object value, ConstraintValidatorContext context) {
        if (value == null) return true;

        // Use reflection or pattern matching to get dates
        if (value instanceof PersonCreateRequest req) {
            if (req.endDate() == null) return true;
            return req.endDate().isAfter(req.startDate());
        }
        return true;
    }
}

Database Migrations (Flyway)

Store migrations in src/main/resources/db/migration/:

-- V1__create_people_table.sql
CREATE TABLE people (
    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
    name VARCHAR(255) NOT NULL,
    email VARCHAR(255) NOT NULL UNIQUE,
    status VARCHAR(50) NOT NULL,
    salary DECIMAL(10,2) NOT NULL,
    start_date DATE NOT NULL,
    end_date DATE,
    department_id UUID REFERENCES departments(id),
    created_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
    updated_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP
);

CREATE INDEX idx_people_email ON people(email);
CREATE INDEX idx_people_department ON people(department_id);

Naming convention: V{version}__{description}.sql

Application Configuration

# application.yml
spring:
  datasource:
    url: jdbc:postgresql://localhost:5432/myapp
    username: ${DB_USER:postgres}
    password: ${DB_PASSWORD:postgres}
  
  jpa:
    hibernate:
      ddl-auto: validate  # Use Flyway for schema management
    open-in-view: false   # Disable OSIV anti-pattern
    properties:
      hibernate:
        format_sql: true
  
  flyway:
    enabled: true
    locations: classpath:db/migration
  
  # Virtual threads (Java 21+)
  threads:
    virtual:
      enabled: true

server:
  port: ${PORT:8080}

logging:
  level:
    com.company: DEBUG
    org.springframework.web: INFO

Virtual Threads (Java 21+)

Virtual threads dramatically improve throughput for I/O-bound applications by allowing millions of concurrent threads with minimal overhead.

Enable in Spring Boot 3.2+

# application.yml
spring:
  threads:
    virtual:
      enabled: true  # All request handling uses virtual threads

Benefits

• High concurrency: Handle thousands of concurrent requests without thread pool exhaustion
• Simpler code: Write blocking code that scales like async code
• No code changes: Existing synchronous code automatically benefits

When to Use

// ✅ Virtual threads shine for I/O-bound operations
@Service
public class ExternalApiService {
    
    public Data fetchFromMultipleSources(List<String> urls) {
        // Each call blocks, but virtual threads make this efficient
        return urls.stream()
            .map(this::fetchFromUrl)  // Blocking HTTP calls
            .toList();
    }
    
    // Virtual threads handle blocking I/O efficiently
    private Data fetchFromUrl(String url) {
        return restTemplate.getForObject(url, Data.class);  // Blocking is OK!
    }
}

When NOT to Use

// ❌ Avoid for CPU-bound operations
// Virtual threads don't help with pure computation
public BigInteger computeFactorial(int n) {
    // CPU-intensive - use platform threads or parallel streams instead
    return IntStream.rangeClosed(1, n)
        .parallel()  // Uses ForkJoinPool (platform threads)
        .mapToObj(BigInteger::valueOf)
        .reduce(BigInteger.ONE, BigInteger::multiply);
}

Structured Concurrency (Preview in Java 21)

// For parallel operations with proper error handling
try (var scope = new StructuredTaskScope.ShutdownOnFailure()) {
    Future<User> userFuture = scope.fork(() -> userService.getUser(id));
    Future<List<Order>> ordersFuture = scope.fork(() -> orderService.getOrders(id));
    
    scope.join();           // Wait for all tasks
    scope.throwIfFailed();  // Propagate exceptions
    
    return new UserWithOrders(userFuture.resultNow(), ordersFuture.resultNow());
}

Configuration for High Throughput

# For very high concurrency scenarios
spring:
  threads:
    virtual:
      enabled: true

# Increase connection pool to match virtual thread capacity
  datasource:
    hikari:
      maximum-pool-size: 50  # Virtual threads can handle more connections
      minimum-idle: 10

Testing with Testcontainers

package com.company.controller;

import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.autoconfigure.web.servlet.AutoConfigureMockMvc;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.http.MediaType;
import org.springframework.test.context.DynamicPropertyRegistry;
import org.springframework.test.context.DynamicPropertySource;
import org.springframework.test.web.servlet.MockMvc;
import org.testcontainers.containers.PostgreSQLContainer;
import org.testcontainers.junit.jupiter.Container;
import org.testcontainers.junit.jupiter.Testcontainers;

import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.*;
import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.*;

@SpringBootTest
@AutoConfigureMockMvc
@Testcontainers
class PersonControllerTest {

    @Container
    static PostgreSQLContainer<?> postgres = new PostgreSQLContainer<>("postgres:15-alpine");

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

    @Autowired
    private MockMvc mockMvc;

    @Test
    void createPerson_validRequest_returns201() throws Exception {
        String json = """
            {
                "name": "John Doe",
                "email": "john@example.com",
                "status": "ACTIVE",
                "salary": 5000.00,
                "startDate": "2024-01-01"
            }
            """;

        mockMvc.perform(post("/api/v1/people")
                .contentType(MediaType.APPLICATION_JSON)
                .content(json))
            .andExpect(status().isCreated())
            .andExpect(jsonPath("$.id").exists())
            .andExpect(jsonPath("$.name").value("John Doe"));
    }

    @Test
    void createPerson_invalidEmail_returns400() throws Exception {
        String json = """
            {
                "name": "John Doe",
                "email": "invalid-email",
                "status": "ACTIVE",
                "startDate": "2024-01-01"
            }
            """;

        mockMvc.perform(post("/api/v1/people")
                .contentType(MediaType.APPLICATION_JSON)
                .content(json))
            .andExpect(status().isBadRequest());
    }
}

Code Quality & Best Practices

Use Optional Correctly

// ✅ Good - use Optional for return types that may be absent
public Optional<User> findByEmail(String email) { }

// ✅ Good - chain Optional operations
return userRepository.findById(id)
    .map(this::toResponse)
    .orElseThrow(() -> new ResponseStatusException(HttpStatus.NOT_FOUND, "User not found"));

// ✅ Good - ifPresent for side effects
userRepository.findByEmail(email).ifPresent(existing -> {
    throw new ResponseStatusException(HttpStatus.CONFLICT, "Email exists");
});

// ❌ Avoid - Optional.get() without check
User user = userRepository.findById(id).get();  // Throws if empty

// ❌ Avoid - Optional for parameters or fields
public void process(Optional<String> name) { }  // Use @Nullable or overloading

Prefer Records for DTOs

// ✅ Good - immutable, concise, auto-generates equals/hashCode/toString
public record UserResponse(
    UUID id,
    String name,
    String email,
    LocalDateTime createdAt
) {}

// ❌ Avoid - verbose POJOs for simple DTOs
public class UserResponse {
    private UUID id;
    private String name;
    // ... getters, setters, equals, hashCode, toString
}

Use Switch Expressions (Java 14+)

// ✅ Good - switch expression with arrow syntax
String message = switch (status) {
    case ACTIVE -> "User is active";
    case INACTIVE -> "User is inactive";
    case PENDING -> "Awaiting verification";
    case SUSPENDED -> "Account suspended";
};

// ✅ Good - exhaustive switch (compiler checks all cases)
int priority = switch (severity) {
    case LOW -> 1;
    case MEDIUM -> 2;
    case HIGH -> 3;
    case CRITICAL -> 4;
};

// ❌ Avoid - old switch with fall-through risks
String message;
switch (status) {
    case ACTIVE:
        message = "User is active";
        break;
    case INACTIVE:
        message = "User is inactive";
        break;
    // Missing cases silently ignored
}

Prefer Stream API for Collections

// ✅ Good - declarative, readable
List<String> activeEmails = users.stream()
    .filter(User::isActive)
    .map(User::getEmail)
    .toList();

boolean hasAdmin = users.stream()
    .anyMatch(u -> u.getRole() == Role.ADMIN);

Map<Role, List<User>> byRole = users.stream()
    .collect(Collectors.groupingBy(User::getRole));

// ❌ Avoid - imperative loops for simple transformations
List<String> activeEmails = new ArrayList<>();
for (User user : users) {
    if (user.isActive()) {
        activeEmails.add(user.getEmail());
    }
}

Avoid Null - Use Empty Collections

// ✅ Good - return empty collection, never null
public List<User> findByDepartment(UUID deptId) {
    return repository.findByDepartmentId(deptId);  // Returns empty list if none
}

// ✅ Good - initialize collections
@ManyToMany
private Set<Skill> skills = new HashSet<>();

// ❌ Avoid - returning null
public List<User> findByDepartment(UUID deptId) {
    var users = repository.findByDepartmentId(deptId);
    return users.isEmpty() ? null : users;  // Caller must null-check
}

Use @Slf4j with Structured Logging

// ✅ Good - structured logging with placeholders
@Slf4j
@Service
public class UserService {
    public void createUser(CreateUserRequest request) {
        // ... create user
        log.info("user.created id={} email={}", user.getId(), user.getEmail());
    }
    
    public void deleteUser(UUID id) {
        log.warn("user.deleted id={} deletedBy={}", id, getCurrentUserId());
    }
}

// ❌ Avoid - string concatenation in logs
log.info("Created user: " + user.getId() + " with email: " + user.getEmail());

Constructor Injection Over Field Injection

// ✅ Good - constructor injection (immutable, testable)
@Service
public class UserService {
    private final UserRepository userRepository;
    private final EmailService emailService;
    
    public UserService(UserRepository userRepository, EmailService emailService) {
        this.userRepository = userRepository;
        this.emailService = emailService;
    }
}

// ❌ Avoid - field injection (harder to test, mutable)
@Service
public class UserService {
    @Autowired
    private UserRepository userRepository;
    
    @Autowired
    private EmailService emailService;
}

Use Meaningful Validation Messages

// ✅ Good - clear validation messages
public record CreateUserRequest(
    @NotBlank(message = "Name is required")
    String name,
    
    @NotBlank(message = "Email is required")
    @Email(message = "Invalid email format")
    String email,
    
    @NotNull(message = "Start date is required")
    @FutureOrPresent(message = "Start date cannot be in the past")
    LocalDate startDate
) {}

// ❌ Avoid - default messages
public record CreateUserRequest(
    @NotBlank String name,  // Message: "must not be blank" (unclear)
    @Email String email
) {}

Handle Exceptions Properly

// ✅ Good - specific exception handling
@RestControllerAdvice
public class GlobalExceptionHandler {

    @ExceptionHandler(ResponseStatusException.class)
    public ResponseEntity<ErrorResponse> handleResponseStatus(ResponseStatusException ex) {
        return ResponseEntity
            .status(ex.getStatusCode())
            .body(new ErrorResponse(ex.getReason()));
    }

    @ExceptionHandler(MethodArgumentNotValidException.class)
    public ResponseEntity<ErrorResponse> handleValidation(MethodArgumentNotValidException ex) {
        var errors = ex.getBindingResult().getFieldErrors().stream()
            .collect(Collectors.toMap(
                FieldError::getField,
                FieldError::getDefaultMessage
            ));
        return ResponseEntity.badRequest()
            .body(new ErrorResponse("Validation failed", errors));
    }
}

record ErrorResponse(String message, Map<String, String> fieldErrors) {
    ErrorResponse(String message) { this(message, null); }
}

Use Constants for Magic Values

// ✅ Good - named constants
public class Limits {
    public static final int MAX_PAGE_SIZE = 100;
    public static final int DEFAULT_PAGE_SIZE = 20;
    public static final int MAX_NAME_LENGTH = 255;
}

@GetMapping
public List<UserResponse> list(
    @RequestParam(defaultValue = "1") int page,
    @RequestParam(defaultValue = "20") @Max(100) int size
) { }

// ❌ Avoid - magic numbers
if (size > 100) {  // What's special about 100?
    size = 100;
}

Text Blocks for Multi-line Strings (Java 15+)

// ✅ Good - readable multi-line strings
String query = """
    SELECT u.id, u.name, u.email
    FROM users u
    JOIN departments d ON u.department_id = d.id
    WHERE d.name = :deptName
    ORDER BY u.name
    """;

String json = """
    {
        "name": "%s",
        "email": "%s"
    }
    """.formatted(name, email);

// ❌ Avoid - string concatenation
String query = "SELECT u.id, u.name, u.email " +
    "FROM users u " +
    "JOIN departments d ON u.department_id = d.id " +
    "WHERE d.name = :deptName";

Best Practices Summary

1. Layered architecture: Controller → Service → Repository → Entity
2. Constructor injection: Not @Autowired on fields
3. Transactions: @Transactional(readOnly = true) for reads
4. DTOs: Separate request/response, use Java Records
5. Validation: Use Bean Validation + custom validators
6. IDs: Use UUID over Long
7. Enums: Store as STRING not ORDINAL
8. Logging: Structured with context (id={})
9. Errors: ResponseStatusException for HTTP errors
10. Testing: Testcontainers for real database tests
11. Migrations: Flyway for schema versioning
12. Virtual threads: Enable for I/O-bound apps (Java 21+, Spring Boot 3.2+)

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