Build order of Maven multimodule project?

The situation is, I have two Maven multimodule projects with the same structure:

Parent
    - Module 1
    - Module 2

When I build project 1, I see that parent is built first (order is parent->module1->module2). However for project 2, parent is built at last (order is module1->module2->parent). Why are the two projects have different build orders? Furthermore, how can I manually control the build order?

Update 1:
Both parent projects are simple POM projects without source code, so I can't explain the build order as per the dependency graph.

Update 2:
The parent POMs are the same except the GAV and child module names:

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <groupId>parent-group-id</groupId>
    <artifactId>parent-artifact-id</artifactId>
    <version>parent-version</version>
    <packaging>pom</packaging>
    <name>parent-name</name>
    <modules>
        <module>module-1</module>
        <module>module-2</module>
    </modules>
</project>

Solution 1:

The build order is determined by the Maven reactor which is a mechanism that automatically ensures correct build order for multimodule builds by sorting the projects.

See the official documentation for how it works.

It says:

The following relationships are honoured when sorting projects:

a project dependency on another module in the build

a plugin declaration where the plugin is another modules in the build

a plugin dependency on another module in the build

a build extension declaration on another module in the build

the order declared in the element (if no other rule applies)

You can't manually control the build order. If you want to change the order you have to make changes to your project that influence the reactor sort order.

Solution 2:

At a high level, the build order is based on a topological sort of the module dependency graph.

EDIT: Question for you. I understand that project 1 contains two modules and so does project 2. But do the modules in project 2 explicitly declare the "parent" pom as a parent? I'm thinking that perhaps your project 1 modules explicitly declare the parent pom, and the project 2 modules don't. Which would mean that the project 2 "parent" isn't really a parent at all and therefore doesn't have to be built before the modules. That's my guess anyway.

Solution 3:

I've been having this issue lately with CentOS 7. I updated Maven to 3.5.3 from 3.0.5 and the problem has been solved. If anyone have this issue as well you can try doing that first.

Solution 4:

Summary

Why?

Maven is using <module> declarations to determine the list of modules to include in the current run. Maven is using <parent> declaration to generate effective POM for each included module, which is then used to execute the build for that module.

In Project 1, each of module1 and module2 specify parent module in <parent> section. In Project 2, neither module1 nor module2 specify parent module in <parent> section.

How can I manually control the build order?

By changing dependencies between modules, including <parent>, <dependencies>, plugin dependencies, etc. as described in the official documentation.

Please check also Introduction to the POM for the discussion on aggregation and inheritance.

Details

There are 4 possible scenarios of module relationships in Maven in terms of aggregation (<module> declaration) and inheritance (<parent> declaration).

Let's assume a module A with packaging pom and a module B (packaging does not matter). The module A could either be a parent or an aggregator. The module B could either reference A as parent or not.

Case 1:

Neither A nor B reference each other - the modules are independent. The Maven can not control build order, each module gets built independently, with manual build order control.

Case 2:

The module A includes the module B in <modules>. The module B does not declare A as parent. The module A is an aggregator for the module B in this case.

Maven invocation mvn -f A/pom.xml will first build B, and then A.
Maven invocation mvn -f B/pom.xml will build B only.

Case 3:

The module A includes the module B in <modules>. The module B declares A as parent. The module A is both the parent and the aggregator (reactor) for the module B in this case.

Maven invocation mvn -f A/pom.xml will first build A, and then B.
Maven invocation mvn -f B/pom.xml will build B only, but will use the POM of A to generate effective POM of B by resolving it either from the local repository or by following /project/parent/relativePath.

Case 4:

The module A does not include the module B in <modules>. The module B declares A as parent. The module A is the parent for the module B in this case.

Maven invocation mvn -f A/pom.xml will build A only.
Maven invocation mvn -f B/pom.xml will build B only, but will use the POM of A to generate effective POM of B by resolving it either from the local repository or by following /project/parent/relativePath.