maven tutorial 2
Project Aggregation
Project Aggregation is similar to Project Inheritance. But instead of specifying the parent POM from the module, it specifies the modules from the parent POM. By doing so, the parent project now knows its modules, and if a Maven command is invoked against the parent project, that Maven command will then be executed to the parent's modules as well. To do Project Aggregation, you must do the following:
Change the parent POMs packaging to the value "pom" .
Specify in the parent POM the directories of its modules (children POMs)
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Example 3
The Scenario
Given the previous original artifact POMs, and directory structure,
com.mycompany.app:my-app:1's POM
<project>
<modelVersion>4.0.0</modelVersion>
<groupId>com.mycompany.app</groupId>
<artifactId>my-app</artifactId>
<version>1</version>
</project>
com.mycompany.app:my-module:1's POM
<project>
<modelVersion>4.0.0</modelVersion>
<groupId>com.mycompany.app</groupId>
<artifactId>my-module</artifactId>
<version>1</version>
</project>
directory structure
.
|-- my-module
| `-- pom.xml
`-- pom.xml
The Solution
If we are to aggregate my-module into my-app, we would only have to modify my-app.
<project>
<modelVersion>4.0.0</modelVersion>
<groupId>com.mycompany.app</groupId>
<artifactId>my-app</artifactId>
<version>1</version>
<packaging>pom</packaging>
<modules>
<module>my-module</module>
</modules>
</project>
In the revised com.mycompany.app:my-app:1, the packaging section and the modules sections were added. For the packaging, it's value was set to "pom", and for the modules section, we have the element <module>my-module</module>. The value of <module> is the relative path from the com.mycompany.app:my-app:1 to com.mycompany.app:my-module:1's POM (by practice, we use the module's artifactId as the module directory's name).
Now, whenever a Maven command processes com.mycompany.app:my-app:1, that same Maven command would be ran against com.mycompany.app:my-module:1 as well. Furthermore, some commands (goals specifically) handle project aggregation differently.
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Example 4
The Scenario
But what if we change the directory structure to the following:
.
|-- my-module
| `-- pom.xml
`-- parent
`-- pom.xml
How would the parent pom specify its modules?
The Solution
The answer? - the same way as Example 3, by specifying the path to the module.
<project>
<modelVersion>4.0.0</modelVersion>
<groupId>com.mycompany.app</groupId>
<artifactId>my-app</artifactId>
<version>1</version>
<packaging>pom</packaging>
<modules>
<module>../my-module</module>
</modules>
</project>
