TheNEXUS

If we take a look at the various plugin configurations, we can see the HSQLDB dependencies duplicated in several places. Unfortunately, dependencyManagement doesn’t apply to plugin dependencies, but we can still use a property to consolidate the versions. Most complex Maven multimodule projects tend to define all versions in the top-level POM. This top-level POM then becomes a focal point for changes that affect the entire project. Think of version numbers as string literals in a Java class; if you are constantly repeating a literal, you’ll likely want to make it a variable so that when it needs to be changed, you have to change it in only one place. Rolling up the version of HSQLDB into a property in the top-level POM yields the following properties element:

<project>
  ...
  <properties>
    <hibernate.annotations.version>3.3.0.ga
      </hibernate.annotations.version>
    <hsqldb.version>1.8.0.7</hsqldb.version>
  </properties>
  ...
</project>

The next thing we notice is that the hibernate3-maven-plugin configuration is duplicated in the simple-webapp and simple-command modules. We can manage the plugin configuration in the top-level POM just as we managed the dependencies in the top-level POM with the dependencyManagement section. To do this, we use the pluginManagement element in the top-level POM’s build element:

<project>
  ...
  <build>
    <pluginManagement>
      <plugins>
        <plugin>
          <groupId>org.apache.maven.plugins</groupId>
          <artifactId>maven-compiler-plugin</artifactId>
          <configuration>
            <source>1.5</source>
            <target>1.5</target>
          </configuration>
        </plugin>
        <plugin>
          <groupId>org.codehaus.mojo</groupId>
          <artifactId>hibernate3-maven-plugin</artifactId>
          <version>2.1</version>
          <configuration>
            <components>
              <component>
                <name>hbm2ddl</name>
                <implementation>annotationconfiguration</implementation>
              </component>
            </components>
          </configuration>
          <dependencies>
            <dependency>
              <groupId>hsqldb</groupId>
              <artifactId>hsqldb</artifactId>
              <version>${hsqldb.version}</version>
            </dependency>
          </dependencies>
        </plugin>
      </plugins>
    </pluginManagement>
  </build>
  ...
</project>