Developing robust microservices in Java requires a thoughtful approach to architecture and tooling. As applications grow in complexity, ensuring they remain maintainable, scalable, and efficient becomes a significant challenge.

The right combination of frameworks can make all the difference, providing a solid foundation that simplifies development and promotes long-term stability.

For developers working with Java, combining the Ebean ORM with Avaje Inject offers a powerful solution for building clean, maintainable microservices.

This guide explores a practical approach to structuring your application using these tools.

We will cover how to establish a clear project structure, manage dependencies effectively, implement robust data access layers, and handle database migrations seamlessly.

By following these principles, you can create high-quality Java microservices that are not only performant but also easy to manage and evolve time.

This approach allows you to focus more on business logic and less on boilerplate code, ultimately leading to more successful and sustainable projects.

Establishing a Clear Project Structure

A well-organized project structure is the foundation of any maintainable application. It improves readability, simplifies navigation, and makes it easier for new developers to understand the codebase.

API-First Design

An API-first approach separates the application’s interface from its implementation. By defining the API contracts first, you establish a clear boundary between different parts of the system.

• domain-api Module: This module contains your public API, defined using interfaces. It should only depend on standard libraries like Jakarta annotations, ensuring it remains lightweight and free of implementation details.
• domain Module: This module provides the concrete implementation of the API defined in domain-api. It will contain your application’s core business logic.

This separation ensures that the API definition is stable and independent of the underlying implementation, which is crucial for building scalable Java microservices.

Managing Dependencies with Avaje Inject

Dependency injection is a core principle of modern software design, promoting loose coupling and testability.

Avaje Inject is a compile-time dependency injection framework that offers a lightweight and efficient alternative to runtime-based solutions.

Compile-Time Injection

Unlike traditional DI frameworks that use reflection at runtime, Avaje Inject generates the necessary dependency wiring at compile time.

• Improved Startup Time: By avoiding runtime scanning and reflection, applications start faster, which is particularly beneficial in a microservices environment where services may be frequently restarted.
• Early Error Detection: Dependency issues are caught during compilation rather than at runtime, preventing unexpected failures in production.
• Simplified Tooling: Since all wiring is explicit and generated as standard Java code, it’s easier to understand and debug.

Using avaje-inject simplifies your build configuration and removes the need for complex runtime frameworks, resulting in a cleaner and more efficient application.

Implementing the Data Access Layer

A well-designed data access layer is critical for managing database interactions effectively. Ebean ORM provides a powerful and intuitive way to handle persistence in Java applications.

The Role of Repositories

Repositories act as an abstraction over the data store, centralizing data access logic and separating it from your business logic.

• Define Repository Interfaces: Place your repository interfaces (e.g., UserRepository) in the domain-api module. These interfaces define the contract for data operations.
• Implement Repositories with Ebean: In the domain module, implement these interfaces. Your repository classes can extend io.ebean.BeanRepository, which provides a rich set of default methods for common CRUD operations.

By using the repository pattern with Ebean ORM, you create a clean separation of concerns.

Your service layer interacts with the repository interface, remaining completely unaware of the persistence implementation details.

Handling Database Migrations

Managing database schema changes is a critical aspect of application development. Avaje DB Migration provides a straightforward, developer-friendly approach to handling migrations.

Automating Schema Changes

Avaje DB Migration integrates seamlessly with Ebean to automate the generation and application of database migrations.

• Generate Migrations: When you make changes to your Ebean entity beans, the Ebean agent detects these changes and can generate the corresponding SQL migration script during the build process.
• Version Control: These migration scripts are version-controlled alongside your source code, providing a clear history of schema evolution.
• Automatic Execution: The migration runner can be configured to automatically apply pending migrations when the application starts up, ensuring the database schema is always in sync with the application code.

This automated process simplifies database management, reduces the risk of manual errors, and ensures consistency across different environments.

Wiring Components Together

Once you have your modules, dependencies, and data access layer set up, the final step is to wire everything together in your main application class.

The main Module

The main module serves as the application’s entry point, responsible for bootstrapping the application and starting the server.

• Module Dependencies: This module will depend on your domain module and a web framework like Javalin or Helidon.
• Bootstrapping with Avaje Inject: In your main method, you use io.avaje.inject.BeanScope.builder().build() to create the dependency injection context. Avaje Inject will automatically discover and wire together all the components annotated with @Singleton or @Component.
• Starting the Server: After the BeanScope is initialized, you can retrieve your web server component and start it.

This approach keeps your main module clean and focused on its single responsibility: starting the application.

Testing Your Application

Thorough testing is essential for building reliable software. The modular structure and use of dependency injection make your application highly testable.

Unit and Integration Testing

Your setup should support both unit and integration testing to ensure all parts of the application work correctly.

• Unit Testing: Since business logic is isolated in services that depend on repository interfaces, you can easily mock these dependencies to unit-test your services in isolation.
• Integration Testing: For integration tests, you can use libraries like avaje-test to spin up a test database (e.g., in a Docker container) and run tests against a real database instance. This allows you to verify the entire application stack, from the web layer down to the database.

This testing strategy ensures that you can validate both individual components and their interactions, leading to more robust and reliable Java microservices.

A Path to Better Microservices

Building maintainable applications is about making deliberate choices that promote simplicity, modularity, and clean design.

By leveraging the combination of Avaje Inject and Ebean ORM, you can establish a powerful and efficient development workflow.

This approach simplifies dependency management, streamlines data access, and automates database migrations, allowing you to build high-quality Java microservices that are scalable, testable, and easy to maintain over the long term.