在第一篇里主要介绍了maven的几个核心概念，这一篇里我们就以一个简单的例子来分析整个maven运行的过程。构建所使用的项目结构如下：
项目结构
主要是一个echo项目，其包含了两个module，分别是api和biz。echo项目的pom.xml的内容如下：
<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/xsd/maven-4.0.0.xsd">
<modelVersion>4.0.0</modelVersion>
<groupId>org.maven</groupId>
<artifactId>echo</artifactId>
<version>1.0.0</version>
<packaging>pom</packaging>
<modules>
<module>api</module>
<module>biz</module>
</modules>
</project>
这里有个比较费解的地方就是<packaging>pom</packaging>。若<packaging>元素的内容是jar，那么我们很好理解，也就是说这个项目最终会被打包成一个jar包。那<packaging>元素为pom又是什么意思呢？从字面上的意思来看，这个项目将打包成一个pom。我们不妨去maven仓库里去瞧瞧（前提是已经在项目下运行了mvn install命令）。我们发现在<maven仓库路径>/org/maven/echo/1.0.0目录下有一个echo-1.0.0.pom文件，细心的读者可能已经发现这个文件其实和echo项目中的pom.xml是同一个文件。这样做的目的是什么呢？还记得第一篇中说过的PO对象吧，我们说过PO对象也是有继承关系的，比如说这里echo项目对应的PO对象就是api项目对应的PO对象的父对象（api项目是echo项目的一个module，在api项目的pom.xml中<parent>元素的内容所对应的就是echo项目），而echo项目PO对象的父对象又是哪个呢？答案是Super POM对应的PO对象。这就是maven中project inheritance的概念。当实际执行maven命令的时候，会根据project inheritance关系对项目的pom.xml进行转化，得到真正执行时所用到的pom.xml，即所谓的effective pom。而<packaging>pom</packaging>的作用就在这里。因此可以得到一个结论：所有<packaging>元素为pom的项目其实并不会输出一个可供外部使用，类似于jar包的东西。这类项目的作用有两个：
管理子项目
例如这里的api和biz就是echo项目的两个module。若没有echo这个父项目，我们需要到api和biz两个项目下分别执行mvn install命令才能完成整个构建过程，而有了echo这个父项目之后，我们只需在echo项目中执行mvn install即可，maven会解析pom.xml，发现该项目有api和biz两个module，它会分别到这两个项目下去执行mvn install命令。当module数量比较多的时候，能大大提高构建的效率。管理继承属性
比如api和biz都需要某个依赖，那么在echo项目的pom.xml中声明即可，因为根据PO对象的继承关系，api和biz项目会继承echo项目的依赖，这样就可以减少一些重复的输入。
effective pom包含了当前项目的PO对象，直到Super POM对应的PO对象中的信息。要看一个项目的effective pom，只需在项目中执行
mvn help:effective-pom
命令即可查看。这里顺带说一句，有的同学可能不理解上面这个命令是什么意思。maven命令的语法为
mvn [options] [goal(s)] [phase(s)]
这里出现了第一篇中讲述的两个概念：goal和phase。maven允许你执行一个或者多个goals/phases。很明显这面的命令help:effective-pom并不是一个phase（maven构建过程中的phase请参考上一篇文章），那么也就是说它是一个goal。对这个goal只不过是采用了缩写的形式，其全称是这样的：
org.apache.maven.plugins:maven-help-plugin:2.2:effective-pom
以分号为分隔符，包含了groupId，artifactId，version，goal四部分。若groupId为org.apache.maven.plugins则可以使用上述的简写形式。也就是说
mvn help:effective-pom
mvn org.apache.maven.plugins:maven-help-plugin:2.2:effective-pom
是等价的，都是执行了maven-help-plugin这个plugin中的effective-pom这个goal。
好了，继续回到effective pom。我们说过maven在真正构建的时候用的就是effective pom，那么说明effective pom中包含了构建的所有信息，我们以biz项目中的effective pom为例来看下effective pom长什么样子。在biz项目中执行mvn help:effective-pom命令，你会得到如下输出：
<?xml version="1.0"?>
<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>
<parent>
<groupId>org.maven</groupId>
<artifactId>echo</artifactId>
<version>1.0.0</version>
</parent>
<groupId>org.maven</groupId>
<artifactId>echo-biz</artifactId>
<version>1.0.0</version>
<dependencies>
<dependency>
<groupId>org.maven</groupId>
<artifactId>echo-api</artifactId>
<version>1.0.0</version>
<scope>compile</scope>
</dependency>
</dependencies>
<build>
<sourceDirectory>/Users/allstarw/workspace/own-proj/misc/maven/echo/biz/src/main/java</sourceDirectory>
<scriptSourceDirectory>/Users/allstarw/workspace/own-proj/misc/maven/echo/biz/src/main/scripts</scriptSourceDirectory>
<testSourceDirectory>/Users/allstarw/workspace/own-proj/misc/maven/echo/biz/src/test/java</testSourceDirectory>
<outputDirectory>/Users/allstarw/workspace/own-proj/misc/maven/echo/biz/target/classes</outputDirectory>
<testOutputDirectory>/Users/allstarw/workspace/own-proj/misc/maven/echo/biz/target/test-classes</testOutputDirectory>
<resources>
<resource>
<directory>/Users/allstarw/workspace/own-proj/misc/maven/echo/biz/src/main/resources</directory>
</resource>
</resources>
<testResources>
<testResource>
<directory>/Users/allstarw/workspace/own-proj/misc/maven/echo/biz/src/test/resources</directory>
</testResource>
</testResources>
<directory>/Users/allstarw/workspace/own-proj/misc/maven/echo/biz/target</directory>
<finalName>echo-biz-1.0.0</finalName>
<plugins>
<plugin>
<artifactId>maven-clean-plugin</artifactId>
<version>2.4.1</version>
<executions>
<execution>
<id>default-clean</id>
<phase>clean</phase>
<goals>
<goal>clean</goal>
</goals>
</execution>
</executions>
</plugin>
<plugin>
<artifactId>maven-install-plugin</artifactId>
<version>2.3.1</version>
<executions>
<execution>
<id>default-install</id>
<phase>install</phase>
<goals>
<goal>install</goal>
</goals>
</execution>
</executions>
</plugin>
<plugin>
<artifactId>maven-resources-plugin</artifactId>
<version>2.5</version>
<executions>
<execution>
<id>default-resources</id>
<phase>process-resources</phase>
<goals>
<goal>resources</goal>
</goals>
</execution>
<execution>
<id>default-testResources</id>
<phase>process-test-resources</phase>
<goals>
<goal>testResources</goal>
</goals>
</execution>
</executions>
</plugin>
<plugin>
<artifactId>maven-surefire-plugin</artifactId>
<version>2.10</version>
<executions>
<execution>
<id>default-test</id>
<phase>test</phase>
<goals>
<goal>test</goal>
</goals>
</execution>
</executions>
</plugin>
<plugin>
<artifactId>maven-compiler-plugin</artifactId>
<version>2.3.2</version>
<executions>
<execution>
<id>default-testCompile</id>
<phase>test-compile</phase>
<goals>
<goal>testCompile</goal>
</goals>
</execution>
<execution>
<id>default-compile</id>
<phase>compile</phase>
<goals>
<goal>compile</goal>
</goals>
</execution>
</executions>
</plugin>
<plugin>
<artifactId>maven-jar-plugin</artifactId>
<version>2.3.2</version>
<executions>
<execution>
<id>default-jar</id>
<phase>package</phase>
<goals>
<goal>jar</goal>
</goals>
</execution>
</executions>
</plugin>
<plugin>
<artifactId>maven-deploy-plugin</artifactId>
<version>2.7</version>
<executions>
<execution>
<id>default-deploy</id>
<phase>deploy</phase>
<goals>
<goal>deploy</goal>
</goals>
</execution>
</executions>
</plugin>
</plugins>
</build>
</project>
篇幅有点长，省略了部分内容。对比biz项目的pom.xml，我们发现effective pom中增加了Super POM中继承过来的一些配置，比如说<sourceDirectory>定义了biz项目的源码路径，以及Lifecycle中各个phase绑定的goal：
[phase] [goal]
compile maven-compiler-plugin:2.3.2:compile
package maven-jar-plugin:2.3.2:jar
install maven-install-plugin:2.3.1:install
... ...
有了effective pom的概念之后，再来看maven构建的输出日志，是不是有点豁然开朗的感觉？
[INFO] Scanning for projects...
[INFO] ------------------------------------------------------------------------
[INFO] Reactor Build Order:
[INFO]
[INFO] echo
[INFO] echo-api
[INFO] echo-biz
[INFO]
[INFO] ------------------------------------------------------------------------
[INFO] Building echo 1.0.0
[INFO] ------------------------------------------------------------------------
[INFO]
[INFO] --- maven-install-plugin:2.3.1:install (default-install) @ echo ---
[INFO] Installing /Users/allstarw/workspace/own-proj/misc/maven/echo/pom.xml to /Users/allstarw/.m2/repository/org/maven/echo/1.0.0/echo-1.0.0.pom
[INFO]
[INFO] ------------------------------------------------------------------------
[INFO] Building echo-api 1.0.0
[INFO] ------------------------------------------------------------------------
[INFO]
[INFO] --- maven-resources-plugin:2.5:resources (default-resources) @ echo-api ---
[debug] execute contextualize
[WARNING] Using platform encoding (UTF-8 actually) to copy filtered resources, i.e. build is platform dependent!
[INFO] skip non existing resourceDirectory /Users/allstarw/workspace/own-proj/misc/maven/echo/api/src/main/resources
[INFO]
[INFO] --- maven-compiler-plugin:2.3.2:compile (default-compile) @ echo-api ---
[WARNING] File encoding has not been set, using platform encoding UTF-8, i.e. build is platform dependent!
[INFO] Compiling 1 source file to /Users/allstarw/workspace/own-proj/misc/maven/echo/api/target/classes
[INFO]
[INFO] --- maven-resources-plugin:2.5:testResources (default-testResources) @ echo-api ---
[debug] execute contextualize
[WARNING] Using platform encoding (UTF-8 actually) to copy filtered resources, i.e. build is platform dependent!
[INFO] skip non existing resourceDirectory /Users/allstarw/workspace/own-proj/misc/maven/echo/api/src/test/resources
[INFO]
[INFO] --- maven-compiler-plugin:2.3.2:testCompile (default-testCompile) @ echo-api ---
[INFO] No sources to compile
[INFO]
[INFO] --- maven-surefire-plugin:2.10:test (default-test) @ echo-api ---
[INFO] No tests to run.
[INFO] Surefire report directory: /Users/allstarw/workspace/own-proj/misc/maven/echo/api/target/surefire-reports
-------------------------------------------------------
T E S T S
-------------------------------------------------------
Results :
Tests run: 0, Failures: 0, Errors: 0, Skipped: 0
[INFO]
[INFO] --- maven-jar-plugin:2.3.2:jar (default-jar) @ echo-api ---
[INFO] Building jar: /Users/allstarw/workspace/own-proj/misc/maven/echo/api/target/echo-api-1.0.0.jar
[INFO]
[INFO] --- maven-install-plugin:2.3.1:install (default-install) @ echo-api ---
[INFO] Installing /Users/allstarw/workspace/own-proj/misc/maven/echo/api/target/echo-api-1.0.0.jar to /Users/allstarw/.m2/repository/org/maven/echo-api/1.0.0/echo-api-1.0.0.jar
[INFO] Installing /Users/allstarw/workspace/own-proj/misc/maven/echo/api/pom.xml to /Users/allstarw/.m2/repository/org/maven/echo-api/1.0.0/echo-api-1.0.0.pom
[INFO]
[INFO] ------------------------------------------------------------------------
[INFO] Building echo-biz 1.0.0
[INFO] ------------------------------------------------------------------------
[INFO]
[INFO] --- maven-resources-plugin:2.5:resources (default-resources) @ echo-biz ---
[debug] execute contextualize
[WARNING] Using platform encoding (UTF-8 actually) to copy filtered resources, i.e. build is platform dependent!
[INFO] skip non existing resourceDirectory /Users/allstarw/workspace/own-proj/misc/maven/echo/biz/src/main/resources
[INFO]
[INFO] --- maven-compiler-plugin:2.3.2:compile (default-compile) @ echo-biz ---
[INFO] Nothing to compile - all classes are up to date
[INFO]
[INFO] --- maven-resources-plugin:2.5:testResources (default-testResources) @ echo-biz ---
[debug] execute contextualize
[WARNING] Using platform encoding (UTF-8 actually) to copy filtered resources, i.e. build is platform dependent!
[INFO] skip non existing resourceDirectory /Users/allstarw/workspace/own-proj/misc/maven/echo/biz/src/test/resources
[INFO]
[INFO] --- maven-compiler-plugin:2.3.2:testCompile (default-testCompile) @ echo-biz ---
[INFO] No sources to compile
[INFO]
[INFO] --- maven-surefire-plugin:2.10:test (default-test) @ echo-biz ---
[INFO] No tests to run.
[INFO] Surefire report directory: /Users/allstarw/workspace/own-proj/misc/maven/echo/biz/target/surefire-reports
-------------------------------------------------------
T E S T S
-------------------------------------------------------
Results :
Tests run: 0, Failures: 0, Errors: 0, Skipped: 0
[INFO]
[INFO] --- maven-jar-plugin:2.3.2:jar (default-jar) @ echo-biz ---
[INFO]
[INFO] --- maven-install-plugin:2.3.1:install (default-install) @ echo-biz ---
[INFO] Installing /Users/allstarw/workspace/own-proj/misc/maven/echo/biz/target/echo-biz-1.0.0.jar to /Users/allstarw/.m2/repository/org/maven/echo-biz/1.0.0/echo-biz-1.0.0.jar
[INFO] Installing /Users/allstarw/workspace/own-proj/misc/maven/echo/biz/pom.xml to /Users/allstarw/.m2/repository/org/maven/echo-biz/1.0.0/echo-biz-1.0.0.pom
[INFO] ------------------------------------------------------------------------
[INFO] Reactor Summary:
[INFO]
[INFO] echo .............................................. SUCCESS [0.461s]
[INFO] echo-api .......................................... SUCCESS [2.581s]
[INFO] echo-biz .......................................... SUCCESS [0.382s]
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 3.593s
[INFO] Finished at: Wed Jul 06 00:22:50 CST 2016
[INFO] Final Memory: 10M/156M
[INFO] ------------------------------------------------------------------------
日志的输出十分清楚，分别构建echo，api，biz这三个项目（因为biz项目依赖api项目，因此api项目需要首先构建）。对每个项目构建时，将lifecycle中的phase所对应的goal依次执行。现在你能够看懂maven的输出日志了吧？还有疑问？请留言。
