Maven POM Inheritance and Aggregation

POM inheritance and aggregation look like the same thing from a distance but solve completely different problems. Inheritance is about sharing configuration — a parent POM defines managed dependency versions and plugin defaults that children inherit. Aggregation is about build orchestration — a root POM lists submodules and Maven builds them in the correct order.

Inheritance — configuration sharing

• When a child POM declares , it inherits properties, plugin configuration, and from the parent — inheriting from spring-boot-starter-parent gives you aligned versions for all Spring and Jackson libraries without declaring a single version yourself
• defines which version to use without pulling the dependency in — children still explicitly declare what they need; the parent only controls which version is selected when they do
• A child can inherit from a parent in a completely different repository — that is exactly what every Spring Boot project does with spring-boot-starter-parent; the parent does not need to be part of the same build

Aggregation — build orchestration

• The root POM's block lists subproject directories; running mvn install at the root triggers the Maven Reactor, which computes dependency order automatically
• If service depends on core, Reactor builds core first without any explicit ordering configuration — it infers order from references between modules
• The most useful Reactor command is mvn install -pl :service -am — it builds only the target module and its dependencies, skipping unrelated modules; essential in large multi-module projects with slow full builds

Combining both roles

• Most projects combine both in the root POM: pom with both a block and a section
• The two mechanisms are independent — you can aggregate modules that inherit from different parents, and you can inherit from a parent without being part of its aggregation