How to Resolve Maven Dependency Issues in a Multi-Module Project

What should I do when Maven can't resolve dependencies between modules in a multi-module project?

mvn dependency:build-classpath
mvn exec:java

In a multi-module Maven project, it is common to face issues where one module cannot resolve dependencies from another module. This often results in build errors and can be frustrating, particularly if the project structure seems correct. This explanation will guide you through identifying and resolving these issues step-by-step.

<dependency>
  <groupId>parent_group</groupId>
  <artifactId>B</artifactId>
  <version>0.1-SNAPSHOT</version>
</dependency

Causes

• Incorrect <groupId>, <artifactId>, or <version> specified in the dependency.
• Maven not including the required module in the build lifecycle.
• The child module has not been built or installed before the parent module tries to reference it.

Solutions

• Ensure that the <groupId>, <artifactId>, and <version> specified in the dependency are consistent with the attributes defined in the dependent module's pom.xml.
• Run `mvn clean install` from the parent project directory to install all modules in sequence, ensuring the latest build of all modules is available for dependency resolution.
• Check whether the modules are correctly defined in the parent pom.xml under <modules> section and ensure there are no typos.

Mistake: Using different groupIds or artifactIds in the child module and dependency definitions.

Solution: Ensure all modules have a consistent groupId and artifactId in their respective pom.xml.

Mistake: Forgetting to install the dependency module by running the build lifecycle for the parent project.

Solution: Always run `mvn clean install` at the parent directory level before executing commands requiring dependent modules.

Mistake: Not including a module in the parent pom's <modules> section.

Solution: Double-check that all child modules are properly listed under <modules> in the parent pom.