Modules

<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">
    <modelVersion>4.0.0</modelVersion>
    <groupId>org.sonatype.mavenbook.ch06</groupId>
    <artifactId>simple-parent</artifactId>
    <packaging>pom</packaging>
    <version>1.0</version>
    <name>Chapter 6 Simple Parent Project</name>

    <modules>
        <module>simple-weather</module>
        <module>simple-webapp</module>
    </modules>

    <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>
            </plugins>
        </pluginManagement>
    </build>

    <dependencies>
        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <version>3.8.1</version>
            <scope>test</scope>
        </dependency>
    </dependencies>

</project>    

simple-parent在pom.xml中列出了项目的子模块，这些模块在modules元素中定义，每个module元素对应了一个simple-parent目录下的子目录。Maven会在这些子目录寻找pom.xml并在构建时，将这些子目录包含到要构建的项目中。build配置和dependencies会被所有子模块继承。使用POM集成允许添加一些全局的依赖。

<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">
    <modelVersion>4.0.0</modelVersion>

    <parent>
        <groupId>org.sonatype.mavenbook.ch06</groupId>
        <artifactId>simple-parent</artifactId>
        <version>1.0</version>
    </parent>

    <artifactId>simple-webapp</artifactId>
    <packaging>war</packaging>
    <name>simple-webapp Maven Webapp</name>

    <dependencies>
        <dependency>
            <groupId>org.apache.geronimo.specs</groupId>
            <artifactId>geronimo-servlet_2.4_spec</artifactId>
            <version>1.1.1</version>
        </dependency>
        <dependency>
            <groupId>org.sonatype.mavenbook.ch06</groupId>
            <artifactId>simple-weather</artifactId>
            <version>1.0</version>
        </dependency>
    </dependencies>

    <build>
        <finalName>simple-webapp</finalName>
        <plugins>
            <plugin>
                <groupId>org.mortbay.jetty</groupId>
                <artifactId>maven-jetty-plugin</artifactId>
            </plugin>
        </plugins>
    </build>

</project>

当Maven执行一个带有子模块的项目的时候，Maven首先载入父POM，然后定位所有的子模块POM。Maven将所有这些项目的POM放到一个成为Maven反应堆中，由它负责分析模块之间的依赖关系。这个反应堆处理组件的排序，以确保相互独立的模块能以适当的顺序被编译和安装。