Build Tools · Lesson 8
Managing Multi-Module Projects
What Multi-Module Builds Provide
A multi-module project groups related applications or libraries under one build. Modules can share dependency versions and build conventions while remaining separately testable and publishable. This structure works well when several services evolve together.
A Shared Java Model
Each module should have a clear responsibility and a small public API. For example, a shared module can define a value object used by an application module.
package academy.shared;
public record LessonReference(String moduleSlug, String lessonSlug) {
public String path() {
return "/" + moduleSlug + "/" + lessonSlug;
}
}
The root build coordinates module order and common settings, while each module declares only its direct dependencies.
Module Relationships
| Module type | Responsibility | Dependency direction |
|---|---|---|
| Shared library | Reusable types | No application dependency |
| Domain module | Business rules | Depends on shared types |
| API module | HTTP boundary | Depends on domain |
| Application | Runtime assembly | Depends on API and infrastructure |
Common Mistake and Practices
Common Mistake: Allowing modules to depend on each other in a cycle. Cycles make builds fragile and usually indicate that a shared abstraction belongs in a lower-level module.
- Keep module boundaries purposeful.
- Declare direct dependencies explicitly.
- Centralize versions without hiding ownership.
- Test modules independently and together.
- Avoid turning every package into a module.
Real-World Use Case
A Java Academy repository has a shared lesson model, a content service, and an HTTP application module. A multi-module build verifies all three together while allowing the shared model to be published separately.
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