Multi Module

Links

Sample

Imagine a sample project with this folder structure:

./pom.xml
./module-a/pom.xml
./module-b/pom.xml

Parent

The parent pom.xml file will look like this:

<project>
    <modelVersion>4.0.0</modelVersion>
    <groupId>com.sample</groupId>
    <version>1.0-SNAPSHOT</version>
    <artifactId>parent-app</artifactId>
    <packaging>pom</packaging>
    <name>Parent App</name>
    <modules>
        <module>module-a</module>
        <module>module-b</module>
    </modules>
</project>

Module

The module pom.xml files, module-a and module-b are virtually the same. They will only differ with the artifactId, packaging and name

<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/maven-v4_0_0.xsd">

  <parent>
    <groupId>com.mycompany</groupId>
    <artifactId>parent-app</artifactId>
    <version>1.0-SNAPSHOT</version>
  </parent>

  <modelVersion>4.0.0</modelVersion>
  <groupId>com.mycompany</groupId>
  <artifactId>module-a</artifactId>
  ...

Note:

It is important to identify the parent from each of the modules.
If you use mvn eclipse:eclipse it looks like you need to import each module into Eclipse separately.
If you don’t specify the version in the module, it appears to inherit the version of the parent (which is probably what you want).

Note: If your module does specify a version, the project files generated by the Eclipse plugin might refer to jar files in the Maven repository rather than the project.
If you use archetype:create it will set-up the pom.xml files correctly.
For the release plugin, mvn-plugin-release, the scm, connection only needs to be specified in the parent pom.

Dependencies

The parent pom.xml file can use the dependencyManagement tag to specify versions of dependencies which can be used in the modules:

<dependencyManagement>
  <dependencies>
    <dependency>
      <groupId>commons-dbcp</groupId>
      <artifactId>commons-dbcp</artifactId>
      <version>1.2.1</version>
    </dependency>
    <dependency>
      <groupId>commons-dbutils</groupId>
      <artifactId>commons-dbutils</artifactId>
      <version>1.0</version>
    </dependency>
  </dependencies>
</dependencyManagement>

The dependencies tag in the module pom.xml file will inherit the version of the artifact from the parent:

<dependencies>
  <dependency>
    <groupId>commons-dbcp</groupId>
    <artifactId>commons-dbcp</artifactId>
    <scope>test</scope>
  </dependency>
  <dependency>
    <groupId>commons-dbutils</groupId>
    <artifactId>commons-dbutils</artifactId>
  </dependency>
</dependencies>

Note: The scope can be changed in the module.

To add a dependency to one of the other modules in the project:

<dependencies>
  <dependency>
    <groupId>com.sample</groupId>
    <artifactId>module-b</artifactId>
    <version>${project.version}</version>
  </dependency>

This is the configuration to use if you want to release all modules with the same version number. Releasing all modules with the same version number will make it much simple to identify the version number of deployed jar files. In any case, the release plugin increments the version number for every module in the project even if no changes have been committed.
If you do want a different version number for a module the release plugin, mvn-plugin-release, will automatically update the version number in the module and the dependencyManagement section of the parent pom e.g.
```
<version>5.3-SNAPSHOT</version>
```
Also see Maven best practices: Use dependency management for multi module projects.

Effective pom

To view the effective pom.xml file for a project:

mvn help:effective-pom

POM Dependency

…declare a pom as a dependency, so that its dependencies would be inherited…

POM Inheritance

JDBC POM:

<project>
  ...
  <artifactId>my-jdbc-project</artifactId>
  <packaging>pom</packaging>
  <dependencies>
    ... add jdbc dependencies here, like mysql jdbc, or sqlserver jdbc ...
  </dependencies>
</project>

Then, in your project that needs to use the jdbc drivers:

  <dependencies>
    <dependency>
      <groupId>...</groupId>
      <artifactId>my-jdbc-project</artifactId>
      <version>...</version>
      <type>pom</type>
    </dependency>
    ....

Site

mvn-plugin-site-multi-module