pom.xml详解

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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>

groupId : 组织标识，例如：org.codehaus.mojo，在M2_REPO目录下，将是: org/codehaus/mojo目录。
artifactId : 项目名称，例如：my-project，在M2_REPO目录下，将是：org/codehaus/mojo/my-project目录。
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。
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>

groupId , artifactId , version :
scope : compile(default),provided,runtime,test,system
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：

env.X：操作系统环境变量，比如${env.PATH}
project.x：pom文件中的属性，比如：<project><version>1.0</version></project>，引用方式：${project.version}
settings.x：settings.xml文件中的属性，比如：<settings><offline>false</offline></settings>，引用方式：${settings.offline}
Java System Properties：java.lang.System.getProperties()中的属性，比如java.home，引用方式：${java.home}
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>

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