As Java applications grow in size and complexity, maintaining a monolithic codebase becomes increasingly challenging. Modularity is the key to building scalable, testable, and maintainable systems — and Spring Boot, paired with Spring Data, provides an excellent platform to implement it.

In this blog, we’ll explore how to architect modular Spring Boot applications by separating responsibilities into distinct, reusable modules. We’ll also demonstrate how to structure your Spring Data repositories, entities, and services for clean and decoupled data access.

Why Go Modular?

A modular design divides your application into independent functional units (modules) that encapsulate domain-specific logic. This brings several advantages:

  • Easier to test and maintain
  • Better team collaboration
  • Improved code reuse
  • Clear separation of concerns
  • Simplified migration to microservices

Modular monoliths are often a stepping stone to microservices.

High-Level Module Structure

A common module breakdown looks like this:

├── common                # Shared utilities, base classes, exceptions
├── user-service          # Domain module for user management
│   ├── controller
│   ├── service
│   ├── repository
│   └── model
├── product-service       # Domain module for products
│   ├── controller
│   ├── service
│   ├── repository
│   └── model
└── application           # Main Spring Boot launcher and configuration

Each domain module handles a bounded context — a core concept in Domain-Driven Design (DDD).

Configuring Independent Spring Data Repositories

Each module can define its own @Repository interfaces and enable them selectively.

In UserModuleApplication.java:

@SpringBootApplication
@EnableJpaRepositories(basePackages = "com.example.user.repository")
@EntityScan(basePackages = "com.example.user.model")
public class UserModuleApplication {
public static void main(String[] args) {
SpringApplication.run(UserModuleApplication.class, args);
}
}

This ensures repositories remain encapsulated and scoped within the module.

Applying Domain-Driven Design (DDD)

Structure each module around a domain aggregate. For example, in the user-service module:

User.java

@Entity
public class User {
@Id @GeneratedValue
private Long id;
private String username;
private String email;
}

UserRepository.java

public interface UserRepository extends JpaRepository<User, Long> {
Optional<User> findByUsername(String username);
}

UserService.java

@Service
public class UserService {
private final UserRepository userRepository;

    public UserService(UserRepository userRepository) {
        this.userRepository = userRepository;
    }

    public User create(User user) {
        return userRepository.save(user);
    }
}

Managing Transactions Per Module

Each module can manage its own transactions. You can annotate service classes with @Transactional, or configure dedicated PlatformTransactionManager beans if needed.

@Transactional
public void registerUser(User user) {
userRepository.save(user);
}

If each module connects to its own database, Spring Boot supports multi-datasource configurations, where each module is wired with its own DataSource, EntityManagerFactory, and TransactionManager.

Decoupling with Interfaces and Contracts

Avoid tight coupling between modules by defining interfaces and only exposing them where necessary.

public interface UserLookupService {
Optional<User> getUserById(Long id);
}

This reduces compile-time dependencies and prepares the codebase for service extraction or replacement in the future.

Building with Maven Multi-Module Projects

To physically separate code, use Maven’s multi-module build structure:

Parent POM

<modules>
<module>user-service</module>
<module>product-service</module>
<module>common</module>
<module>application</module>
</modules>

Each module declares its dependencies and scopes appropriately, enforcing boundaries at the build level.

Testing in a Modular Architecture

Each module should have its own:

  • Unit tests for services and models
  • Integration tests for repositories
  • Mocked tests for controller behavior
@DataJpaTest
class UserRepositoryTest {
@Autowired
private UserRepository userRepository;

    @Test
    void testFindByUsername() {
        User user = userRepository.save(new User("alice", "alice@example.com"));
        assertTrue(userRepository.findByUsername("alice").isPresent());
    }
}

Test coverage should reflect the module boundaries.

Best Practices

  • Follow package-by-feature rather than package-by-layer
  • Keep module dependencies acyclic
  • Use the common module for shared constants, DTOs, and exceptions
  • Expose minimal public APIs from each module
  • Keep configuration (like @ComponentScan) explicit and scoped

Conclusion

Modularizing your Spring Boot application improves clarity, maintainability, and testability — especially as teams and codebases grow. With Spring Data, each module can manage its own persistence layer cleanly, setting the stage for microservices or domain isolation.

By combining modular architecture with Spring Boot’s flexibility and Spring Data’s powerful repository abstraction, you can build highly maintainable and scalable enterprise applications.