Maven的聚合和继承（六）

来源：互联网发布：python服务器脚本编辑：程序博客网时间：2024/06/11 16:52

我们使用Maven应用到实际项目的时候，需要将项目分成不同的模块。这个时候，Maven的聚合特性能够把项目的各个模块聚合在一起构件，而Maven的继承特性则能帮助抽取各模块相同的依赖和插件等配置。在简化POM的同时，还能促进各个模块配置的一致性。下面以具体项目来讲解:

1. 项目结构

以上有五个项目分别为user-parent, user-core, user-log, user-dao, user-service. 其中user-parent项目为其他项目的父项目，只是帮助其他模块构建的工具，它本身并无实质的内容，只有一个pom.xml文件。

2. user-parent的pom.xml详情如下（下文简称为父POM）

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<projectxmlns="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.liangbo.user</groupId>
  <artifactId>user-parent</artifactId>
  <version>0.0.1-SNAPSHOT</version>
  <packaging>pom</packaging>
 
  <modules>
      <module>../user-core</module>
      <module>../user-dao</module>
      <module>../user-log</module>
      <module>../user-service</module>
  </modules>
   
  <properties>
    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    <junit.version>4.10</junit.version>
    <mysql.driver>com.mysql.jdbc.Driver</mysql.driver>
    <mysql.url>jdbc:mysql://localhost:3306/mysql</mysql.url>
    <mysql.username>root</mysql.username>
    <mysql.password>password</mysql.password>
  </properties>
   
  <dependencyManagement>
    <dependencies>
        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <version>${junit.version}</version>
            <scope>test</scope>
        </dependency>
         
        <dependency>
            <groupId>com.liangbo.user</groupId>
            <artifactId>user-core</artifactId>
            <version>0.0.1-SNAPSHOT</version>
        </dependency>
         
        <dependency>
            <groupId>com.liangbo.user</groupId>
            <artifactId>user-log</artifactId>
            <version>0.0.1-SNAPSHOT</version>
        </dependency>
         
        <dependency>
            <groupId>com.liangbo.user</groupId>
            <artifactId>user-dao</artifactId>
            <version>0.0.1-SNAPSHOT</version>
        </dependency>
         
        <dependency>
            <groupId>com.liangbo.user</groupId>
            <artifactId>user-log</artifactId>
            <version>0.0.1-SNAPSHOT</version>
            <exclusions>
                <exclusion>
                    <groupId>log4j</groupId>
                    <artifactId>log4j</artifactId>
                </exclusion>
            </exclusions>
        </dependency>
 
        <dependency>
            <groupId>log4j</groupId>
            <artifactId>log4j</artifactId>
            <version>1.2.9</version>
        </dependency>
     
        <dependency>
            <groupId>commons-logging</groupId>
            <artifactId>commons-logging</artifactId>
            <version>1.1.1</version>
        </dependency>
 
        <dependency>
            <groupId>org.hibernate</groupId>
            <artifactId>hibernate-core</artifactId>
            <version>3.6.10.Final</version>
        </dependency>
 
        <dependency>
            <groupId>log4j</groupId>
            <artifactId>log4j</artifactId>
            <version>1.2.16</version>
        </dependency>
     
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <version>5.1.26</version>
        </dependency>
 
        <dependency>
            <groupId>org.slf4j</groupId>
            <artifactId>slf4j-log4j12</artifactId>
            <version>1.7.5</version>
        </dependency>
 
        <dependency>
            <groupId>javassist</groupId>
            <artifactId>javassist</artifactId>
            <version>3.12.1.GA</version>
        </dependency>
         
    </dependencies>
  </dependencyManagement>
   
  <build>
      <pluginManagement>
          <plugins>
              <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-source-plugin</artifactId>
                <version>2.2.1</version>
                <executions>
                    <execution>
                        <phase>package</phase>
                        <goals><goal>jar-no-fork</goal></goals>
                    </execution>
                </executions>
            </plugin>
             
            <plugin>
                <groupId>org.codehaus.mojo</groupId>
                <artifactId>sql-maven-plugin</artifactId>
                <version>1.5</version>
                <dependencies>
                    <dependency>
                        <groupId>mysql</groupId>
                        <artifactId>mysql-connector-java</artifactId>
                        <version>5.1.26</version>
                    </dependency>
                </dependencies>
                <configuration>
                    <driver>${mysql.driver}</driver>
                    <url>${mysql.url}</url>
                    <username>${mysql.username}</username>
                    <password>${mysql.password}</password>
                    <sqlCommand>
                        create database IF NOT EXISTS maven_test
                    </sqlCommand>
                </configuration>               
                <executions>
                    <execution>
                        <phase>package</phase>
                        <goals>
                            <goal>execute</goal>
                        </goals>
                    </execution>               
                </executions>
            </plugin>
             
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-rar-plugin</artifactId>
                <version>2.3</version>
                <executions>
                    <execution>
                        <phase>package</phase>
                        <goals><goal>rar</goal></goals>
                    </execution>
                </executions>
            </plugin>
          </plugins>
      </pluginManagement>
  </build>
</project>

上面的pom.xml文件既有Maven的聚合特性又有继承特性。这样做是为了更加方便管理其他模块。其中元素modules，是实现聚合的最核心配置。多个子模块之间的pom有很多的相同的配置，这时候可以全部提取出来放到父项目中。重复往往就意味着更多的劳动和更多的潜在的问题。那么子模块如何来继承他的父项目呢？

3. 下面来看下user-core项目pom.xml的配置

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<projectxmlns="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>
 
  <parent>
      <groupId>com.liangbo.user</groupId>
      <artifactId>user-parent</artifactId>
      <version>0.0.1-SNAPSHOT</version>
      <relativePath>../user-parent/pom.xml</relativePath>
  </parent>
 
  <artifactId>user-core</artifactId>
  <packaging>jar</packaging>
 
  <name>user-core</name>
  <url>http://maven.apache.org</url>
 
  <dependencies>
    <dependency>
      <groupId>junit</groupId>
      <artifactId>junit</artifactId>
    </dependency>
 
    <dependency>
        <groupId>org.hibernate</groupId>
        <artifactId>hibernate-core</artifactId>
    </dependency>
 
    <dependency>
        <groupId>log4j</groupId>
        <artifactId>log4j</artifactId>
    </dependency>
 
    <dependency>
        <groupId>mysql</groupId>
        <artifactId>mysql-connector-java</artifactId>
    </dependency>
 
    <dependency>
        <groupId>org.slf4j</groupId>
        <artifactId>slf4j-log4j12</artifactId>
    </dependency>
 
    <dependency>
        <groupId>javassist</groupId>
        <artifactId>javassist</artifactId>
    </dependency>
  </dependencies>
   
  <build>
      <plugins>
          <plugin>
              <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-source-plugin</artifactId>
          </plugin>
           
          <plugin>
              <groupId>org.codehaus.mojo</groupId>
            <artifactId>sql-maven-plugin</artifactId>
          </plugin>
           
          <plugin>
              <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-rar-plugin</artifactId>
          </plugin>
      </plugins>
  </build>
</project>

其中设置好relativePath非常重要，不然无法找到父POM。这时候子POM中只需要配置groupId和artifactId就可以了。

4. 依赖管理和插件管理

上述子POM配置简单了很多，是因为父POM引用了依赖管理和插件管理的概念。如果父POM没有配置依赖管理和插件管理的话，继承它的子模块会将父所有的依赖和插件都继承下来，显然这是不可行的。接下来讲下如何配置的：

Maven提供的dependencyManagement元素既能让子模块继承到父模块的依赖配置，又能保证子模块依赖使用的灵活性。在dependencyManagement元素下的依赖声明不会引入实际的依赖，不过它可以约束dependencies下的依赖使用。插件管理同上，需要在父POM中加入pluginManagement元素。

最后，说下聚合与继承的关系，多模块Maven中的聚合与继承其实是两个不同的概念，其目的是不同的。前者主要是为了方便快速构建项目，后者主要是为了消除重复配置。

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