maven-聚合与继承

1、聚合-方便快速构建项目

　　多个maven模块要构建需要分别执行一次maven构建命令，怎样只执行一次构建命令就构建多个maven模块呢？maven提供了聚合模块可以满足一次运行，构建多模块的要求

2、继承-消除重复配置，统一管理

　　多个maven模块中的pxm.xml有很多相同的配置，如果简化配置？maven提供了继承机制，抽出重复的配置，统一依赖管理和插件管理

maven工程隐式的继承超级POM，超级POM如下：

<?xml version="1.0" encoding="UTF-8"?>

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distributed with this work for additional information
regarding copyright ownership.  The ASF licenses this file
to you under the Apache License, Version 2.0 (the
"License"); you may not use this file except in compliance
with the License.  You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

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"AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
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specific language governing permissions and limitations
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<!-- START SNIPPET: superpom -->
<project>
  <modelVersion>4.0.0</modelVersion>

  <repositories>
    <repository>   --中央仓库
      <id>central</id>
      <name>Central Repository</name>
      <url>https://repo.maven.apache.org/maven2</url>
      <layout>default</layout>
      <snapshots>
        <enabled>false</enabled>
      </snapshots>
    </repository>
  </repositories>

  <pluginRepositories>
    <pluginRepository>   --中央插件仓库
      <id>central</id>
      <name>Central Repository</name>
      <url>https://repo.maven.apache.org/maven2</url>
      <layout>default</layout>
      <snapshots>
        <enabled>false</enabled>
      </snapshots>
      <releases>
        <updatePolicy>never</updatePolicy>
      </releases>
    </pluginRepository>
  </pluginRepositories>

  <build>  --在子maven工程中设定相同的标签元素的值，则覆盖父POM的标签元素的值
    <directory>${project.basedir}/target</directory> --项目主输出目录
    <outputDirectory>${project.build.directory}/classes</outputDirectory> --主代码输出目录
    <finalName>${project.artifactId}-${project.version}</finalName> --最终构建的名称格式
    <testOutputDirectory>${project.build.directory}/test-classes</testOutputDirectory> --测试代码输出目录
    <sourceDirectory>${project.basedir}/src/main/java</sourceDirectory> --主源码目录
    <scriptSourceDirectory>${project.basedir}/src/main/scripts</scriptSourceDirectory> --脚本源码目录
    <testSourceDirectory>${project.basedir}/src/test/java</testSourceDirectory> --测试源码目录
    <resources>
      <resource> --主资源目录
        <directory>${project.basedir}/src/main/resources</directory>
      </resource>
    </resources>
    <testResources> --测试资源目录
      <testResource>
        <directory>${project.basedir}/src/test/resources</directory>
      </testResource>
    </testResources>
    <pluginManagement>  --申明核心插件，设定了版本
      <!-- NOTE: These plugins will be removed from future versions of the super POM -->
      <!-- They are kept for the moment as they are very unlikely to conflict with lifecycle mappings (MNG-4453) -->
      <plugins>
        <plugin>
          <artifactId>maven-antrun-plugin</artifactId>
          <version>1.3</version>
        </plugin>
        <plugin>
          <artifactId>maven-assembly-plugin</artifactId>
          <version>2.2-beta-5</version>
        </plugin>
        <plugin>
          <artifactId>maven-dependency-plugin</artifactId>
          <version>2.8</version>
        </plugin>
        <plugin>
          <artifactId>maven-release-plugin</artifactId>
          <version>2.3.2</version>
        </plugin>
      </plugins>
    </pluginManagement>
  </build>

  <reporting>
    <outputDirectory>${project.build.directory}/site</outputDirectory>
  </reporting>

  <profiles>
    <!-- NOTE: The release profile will be removed from future versions of the super POM -->
    <profile>
      <id>release-profile</id>

      <activation>
        <property>
          <name>performRelease</name>
          <value>true</value>
        </property>
      </activation>

      <build>
        <plugins>
          <plugin>
            <inherited>true</inherited>
            <artifactId>maven-source-plugin</artifactId>
            <executions>
              <execution>
                <id>attach-sources</id>
                <goals>
                  <goal>jar</goal>
                </goals>
              </execution>
            </executions>
          </plugin>
          <plugin>
            <inherited>true</inherited>
            <artifactId>maven-javadoc-plugin</artifactId>
            <executions>
              <execution>
                <id>attach-javadocs</id>
                <goals>
                  <goal>jar</goal>
                </goals>
              </execution>
            </executions>
          </plugin>
          <plugin>
            <inherited>true</inherited>
            <artifactId>maven-deploy-plugin</artifactId>
            <configuration>
              <updateReleaseInfo>true</updateReleaseInfo>
            </configuration>
          </plugin>
        </plugins>
      </build>
    </profile>
  </profiles>

</project>
<!-- END SNIPPET: superpom -->

以下元素可从超级POM获者其它的POM文件继承：

3、聚合和继承的关系

对于聚合模块，它知道有哪些被聚合的模块，但那些被聚合的模块不知道聚合模块的存在。

对于继承关系的父POM，它不知道有哪些子模块继承它，但那些子模块知道自己继承的父POM

相同点：聚合POM和继承关系中的父POM的packaging必须是pom，

　　　　聚合模块和继承关系中到的父模块除了pom之外都没有实际内容

4、maven聚合工程（+父工程）

在实际项目中，通常聚合模块和继承关系中到的父模块是同一个，以下合并使用。两种常用的多模块的maven工程目录结构：

        

其中，parent既是聚合工程，也是父工程。

• 新建聚合工程：略
• parent模块的pom.xml文件

<?xml version="1.0" encoding="UTF-8"?>

<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/xsd/maven-4.0.0.xsd">
  <modelVersion>4.0.0</modelVersion>

  <groupId>com.cn</groupId>
  <artifactId>parent</artifactId>
  <packaging>pom</packaging>
  <version>1.0-SNAPSHOT</version>
  <modules>
    <module>../common</module>   --被聚合的common模块
    <module>../mananger</module>
  </modules>

  <name>parent</name>
  <!-- FIXME change it to the project's website -->
  <url>http://www.example.com</url>

  <properties>  --定义属性的值，后续可以被引用
    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    <maven.compiler.source>1.7</maven.compiler.source>
    <maven.compiler.target>1.7</maven.compiler.target>
    <spring-version>3.2.2.RELEASE</spring-version>
  </properties>

  <dependencies>  --需要引入的依赖
    <dependency>
      <groupId>junit</groupId>
      <artifactId>junit</artifactId>
      <version>4.11</version>
      <scope>test</scope>
    </dependency>
  </dependencies>

  <dependencyManagement>  --申明依赖，但实际不会被引入
    <dependencies>
      <dependency>
        <groupId>org.springframework</groupId>
        <artifactId>spring-core</artifactId>
        <version>${spring-version}</version>
      </dependency>
      <dependency>
        <groupId>org.springframework</groupId>
        <artifactId>spring-context</artifactId>
        <version>${spring-version}</version>
      </dependency>
    </dependencies>
  </dependencyManagement>

  <build>
    <pluginManagement><!-- lock down plugins versions to avoid using Maven defaults (may be moved to parent pom) -->  --申明插件，但实际不会被引入
      <plugins>
        <plugin>
          <artifactId>maven-clean-plugin</artifactId>
          <version>3.0.0</version>
        </plugin>
        <!-- see http://maven.apache.org/ref/current/maven-core/default-bindings.html#Plugin_bindings_for_jar_packaging -->
        <plugin>
          <artifactId>maven-resources-plugin</artifactId>
          <version>3.0.2</version>
        </plugin>
        <plugin>
          <artifactId>maven-compiler-plugin</artifactId>
          <version>3.7.0</version>
        </plugin>
        <plugin>
          <artifactId>maven-surefire-plugin</artifactId>
          <version>2.20.1</version>
        </plugin>
        <plugin>
          <artifactId>maven-jar-plugin</artifactId>
          <version>3.0.2</version>
        </plugin>
        <plugin>
          <artifactId>maven-install-plugin</artifactId>
          <version>2.5.2</version>
        </plugin>
        <plugin>
          <artifactId>maven-deploy-plugin</artifactId>
          <version>2.8.2</version>
        </plugin>
      </plugins>
    </pluginManagement>
  </build>
</project>

• 子模块manager的pom.xml

<?xml version="1.0" encoding="UTF-8"?>

<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/xsd/maven-4.0.0.xsd">
    <parent>    --配置继承父工程
        <artifactId>parent</artifactId>
        <groupId>com.cn</groupId>
        <version>1.0-SNAPSHOT</version>
        <relativePath>../parent/pom.xml</relativePath>   --父工程pom.xml的相对位置，未配置则默认为上一层目录下
    </parent>
    <modelVersion>4.0.0</modelVersion>

    <artifactId>mananger</artifactId> --省略了版本和组id，因为从父pom中继承了
    <packaging>war</packaging>

    <name>mananger Maven Webapp</name>
    <!-- FIXME change it to the project's website -->
    <url>http://www.example.com</url>

    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <maven.compiler.source>1.7</maven.compiler.source>
        <maven.compiler.target>1.7</maven.compiler.target>
    </properties>

    <dependencies>
        <dependency>  --实际引入了父pom中申明的jar包，只需指定id
            <groupId>org.springframework</groupId>
            <artifactId>spring-core</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-context</artifactId>
        </dependency>
    </dependencies>

    <build>
       <plugins>
           <plugin>  --实际引入父pom中申明的插件
               <artifactId>maven-compiler-plugin</artifactId>
               <version>3.7.0</version>
               <configuration>
                   <source>1.7</source>
                   <target>1.7</target>
               </configuration>
           </plugin>
       </plugins>
    </build>
</project>

• 执行构建命令，多模块的maven工程构建过程

　　Maven按序读取POM，如果该POM没有继承模块，就构建该模块，否则就先构建其继承的模块，如果该继承的模块还继承其它模块，则进一步先构建继承的模块。在此过程中，构建过的模块不再构建。（反应堆：所有模块组成的构建结构，包含了模块间的继承关系，能够计算出模块的构建顺序）

假设A项目聚合了B、C项目，B、C项目继承D项目，D继承E，则构建顺序为：A、E、D、B、C