POM.xml详解

一、什么是POM

Project Object Model，项目对象模型。通过xml格式保存的pom.xml文件。作用类似ant的build.xml文件，功能更强大。该文件用于管理：源代码、配置文件、开发者的信息和角色、问题追踪系统、组织信息、项目授权、项目的url、项目的依赖关系等等。

一个完整的pom.xml文件，放置在项目的根目录下。

 <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>
   <!– The Basics –>
   <groupId>…</groupId>
   <artifactId>…</artifactId>
   <version>…</version>
   <packaging>…</packaging>
   <dependencies>…</dependencies>
   <parent>…</parent>
   <dependencyManagement>…</dependencyManagement>
   <modules>…</modules>
   <properties>…</properties>
   <!– Build Settings –>
   <build>…</build>
   <reporting>…</reporting>
   <!– More Project Information –>
   <name>…</name>
   <description>…</description>
   <url>…</url>
   <inceptionYear>…</inceptionYear>
   <licenses>…</licenses>
   <organization>…</organization>
   <developers>…</developers>
   <contributors>…</contributors>
   <!– Environment Settings –>
   <issueManagement>…</issueManagement>
   <ciManagement>…</ciManagement>
   <mailingLists>…</mailingLists>
   <scm>…</scm>
   <prerequisites>…</prerequisites>
   <repositories>…</repositories>
   <pluginRepositories>…</pluginRepositories>
   <distributionManagement>…</distributionManagement>
   <profiles>…</profiles>
 </project>

二、基本设置

1、maven的协作相关属性

 <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.codehaus.mojo</groupId>
   <artifactId>my-project</artifactId>
   <version>1.0</version>
   <packaging>war</packaging>
 </project>

1. groupId : 组织标识，例如：org.codehaus.mojo，在M2_REPO目录下，将是: org/codehaus/mojo目录。
2. artifactId : 项目名称，例如：my-project，在M2_REPO目录下，将是：org/codehaus/mojo/my-project目录。
3. version : 版本号，例如：1.0，在M2_REPO目录下，将是：org/codehaus/mojo/my-project/1.0目录。
4. packaging : 打包的格式，可以为：pom , jar , maven-plugin , ejb , war , ear , rar , par

2、POM之间的关系

主要用于POM文件的复用。

a）依赖关系：依赖关系列表（dependency list）是POM的重要部分

 <dependencies>
    <dependency>
      <groupId>junit</groupId>
      <artifactId>junit</artifactId>
      <version>4.0</version>
      <scope>test</scope>
    </dependency>
    …
  </dependencies>

1. groupId , artifactId , version :
2. scope : compile(default),provided,runtime,test,system
3. exclusions

b）继承关系：继承其他pom.xml配置的机制。

比如父pom.xml：

 <project>
   [...]
   <dependencies>
     <dependency>
       <groupId>junit</groupId>
       <artifactId>junit</artifactId>
       <version>4.4</version>
       <scope>test</scope>
     </dependency>
   </dependencies>
   [...]
 </project>

在子pom.xml文件继承它的依赖（还可以继承其他的：developers and contributors、plugin lists、reports lists、plugin executions with matching ids、plugin configuration）：

 [...]
 <parent>
 <groupId>com.devzuz.mvnbook.proficio</groupId>
   <artifactId>proficio</artifactId>
   <version>1.0-SNAPSHOT</version>
 </parent>
 [...]

在这种机制下，maven还提供了一个类似java.lang.Object的顶级父pom.xml文件：

 <project>
   <modelVersion>4.0.0</modelVersion>
   <name>Maven Default Project</name>
   <repositories>
     <repository>
       <id>central</id>
       <name>Maven Repository Switchboard</name>
       <layout>default</layout>
       <url>http://repo1.maven.org/maven2</url>
       <snapshots>
         <enabled>false</enabled>
       </snapshots>
     </repository>
   </repositories>
   <pluginRepositories>
     <pluginRepository>
       <id>central</id>
       <name>Maven Plugin Repository</name>
       <url>http://repo1.maven.org/maven2</url>
       <layout>default</layout>
       <snapshots>
         <enabled>false</enabled>
       </snapshots>
       <releases>
         <updatePolicy>never</updatePolicy>
       </releases>
     </pluginRepository>
   </pluginRepositories>
   <build>
     <directory>target</directory>
     <outputDirectory>target/classes</outputDirectory>
     <finalName>${project.artifactId}-${project.version}</finalName>
     <testOutputDirectory>target/test-classes</testOutputDirectory>
     <sourceDirectory>src/main/java</sourceDirectory>
     <scriptSourceDirectory>src/main/scripts</scriptSourceDirectory>
     <testSourceDirectory>src/test/java</testSourceDirectory>
     <resources>
       <resource>
         <directory>src/main/resources</directory>
       </resource>
     </resources>
     <testResources>
       <testResource>
         <directory>src/test/resources</directory>
       </testResource>
     </testResources>
     <pluginManagement>
        <plugins>
          <plugin>
            <artifactId>maven-antrun-plugin</artifactId>
            <version>1.1</version>
          </plugin>
          <plugin>
            <artifactId>maven-assembly-plugin</artifactId>
            <version>2.2-beta-2</version>
          </plugin>
          <plugin>
            <artifactId>maven-clean-plugin</artifactId>
            <version>2.2</version>
          </plugin>
          <plugin>
            <artifactId>maven-compiler-plugin</artifactId>
            <version>2.0.2</version>
          </plugin>
          <plugin>
            <artifactId>maven-dependency-plugin</artifactId>
            <version>2.0</version>
          </plugin>
          <plugin>
            <artifactId>maven-deploy-plugin</artifactId>
            <version>2.3</version>
          </plugin>
          <plugin>
            <artifactId>maven-ear-plugin</artifactId>
            <version>2.3.1</version>
          </plugin>
          <plugin>
            <artifactId>maven-ejb-plugin</artifactId>
            <version>2.1</version>
          </plugin>
          <plugin>
            <artifactId>maven-install-plugin</artifactId>
            <version>2.2</version>
          </plugin>
          <plugin>
            <artifactId>maven-jar-plugin</artifactId>
            <version>2.2</version>
          </plugin>
          <plugin>
            <artifactId>maven-javadoc-plugin</artifactId>
            <version>2.4</version>
          </plugin>
          <plugin>
            <artifactId>maven-plugin-plugin</artifactId>
            <version>2.4.1</version>
          </plugin>
          <plugin>
            <artifactId>maven-rar-plugin</artifactId>
            <version>2.2</version>
          </plugin>
          <plugin>
            <artifactId>maven-release-plugin</artifactId>
            <version>2.0-beta-7</version>
          </plugin>
          <plugin>
            <artifactId>maven-resources-plugin</artifactId>
            <version>2.2</version>
          </plugin>
          <plugin>
            <artifactId>maven-site-plugin</artifactId>
            <version>2.0-beta-6</version>
          </plugin>
          <plugin>
            <artifactId>maven-source-plugin</artifactId>
            <version>2.0.4</version>
          </plugin>
          <plugin>
             <artifactId>maven-surefire-plugin</artifactId>
             <version>2.4.2</version>
          </plugin>
          <plugin>
            <artifactId>maven-war-plugin</artifactId>
            <version>2.1-alpha-1</version>
          </plugin>
        </plugins>
      </pluginManagement>
   </build>
   <reporting>
     <outputDirectory>target/site</outputDirectory>
   </reporting>
   <profiles>
     <profile>
       <id>release-profile</id>
       <activation>
         <property>
           <name>performRelease</name>
           <value>true</value>
         </property>
       </activation>
       <build>
         <plugins>
           <plugin>
             <inherited>true</inherited>
             <groupId>org.apache.maven.plugins</groupId>
             <artifactId>maven-source-plugin</artifactId>
             <executions>
               <execution>
                 <id>attach-sources</id>
                 <goals>
                   <goal>jar</goal>
                 </goals>
               </execution>
             </executions>
           </plugin>
           <plugin>
             <inherited>true</inherited>
             <groupId>org.apache.maven.plugins</groupId>
             <artifactId>maven-javadoc-plugin</artifactId>
             <executions>
               <execution>
                 <id>attach-javadocs</id>
                 <goals>
                   <goal>jar</goal>
                 </goals>
               </execution>
             </executions>
           </plugin>
           <plugin>
             <inherited>true</inherited>
             <groupId>org.apache.maven.plugins</groupId>
             <artifactId>maven-deploy-plugin</artifactId>
             <configuration>
               <updateReleaseInfo>true</updateReleaseInfo>
             </configuration>
           </plugin>
         </plugins>
       </build>
     </profile>
   </profiles>
 </project>

可以通过下面命令查看当前pom.xml受到超pom.xml文件的影响：
mvn help:effective-pom

c）聚合关系：用于将多个maven项目聚合为一个大的项目。

 <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.codehaus.mojo</groupId>
   <artifactId>my-parent</artifactId>
   <version>2.0</version>
   <modules>
     <module>my-project<module>
   </modules>
 </project>

3、属性

maven的属性，是值的占位符，类似EL，类似ant的属性，比如${X}，可用于pom文件任何赋值的位置。有以下分类：

1. env.X：操作系统环境变量，比如${env.PATH}
2. project.x：pom文件中的属性，比如：<project><version>1.0</version></project>，引用方式：${project.version}
3. settings.x：settings.xml文件中的属性，比如：<settings><offline>false</offline></settings>，引用方式：${settings.offline}
4. Java System Properties：java.lang.System.getProperties()中的属性，比如java.home，引用方式：${java.home}
5. 自定义：在pom文件中可以：<properties><installDir>c:/apps/cargo-installs</installDir></properties>，引用方式：${installDir}

4、构建设置

构建有两种build标签：

 <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">
   …
   <!– "Project Build" contains more elements than just the BaseBuild set –>
   <build>…</build>
   <profiles>
     <profile>
       <!– "Profile Build" contains a subset of "Project Build"s elements –>
       <build>…</build>
     </profile>
   </profiles>
 </project>

build中的主要标签：Resources和Plugins。

Resources：用于排除或包含某些资源文件

 <resources>
   <resource>
     <targetPath>META-INF/plexus</targetPath>
     <filtering>false</filtering>
     <directory>${basedir}/src/main/plexus</directory>
     <includes>
       <include>configuration.xml</include>
     </includes>
     <excludes>
       <exclude>**/*.properties</exclude>
     </excludes>
   </resource>
 </resources>

Plugins：设置构建的插件

 <build>
    …
    <plugins>
      <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-jar-plugin</artifactId>
        <version>2.0</version>
        <extensions>false</extensions>
        <inherited>true</inherited>
        <configuration>
          <classifier>test</classifier>
        </configuration>
        <dependencies>…</dependencies>
        <executions>…</executions>
      </plugin>