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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：
4.0.0
org.maven
echo
1.0.0
pom
api
biz
这里有个比较费解的地方就是
-
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：
4.0.0
org.maven
echo
1.0.0
org.maven
echo-biz
1.0.0
org.maven
echo-api
1.0.0
compile
/Users/allstarw/workspace/own-proj/misc/maven/echo/biz/src/main/java
/Users/allstarw/workspace/own-proj/misc/maven/echo/biz/src/main/scripts
/Users/allstarw/workspace/own-proj/misc/maven/echo/biz/src/test/java
/Users/allstarw/workspace/own-proj/misc/maven/echo/biz/target/classes
/Users/allstarw/workspace/own-proj/misc/maven/echo/biz/target/test-classes
/Users/allstarw/workspace/own-proj/misc/maven/echo/biz/src/main/resources
/Users/allstarw/workspace/own-proj/misc/maven/echo/biz/src/test/resources
/Users/allstarw/workspace/own-proj/misc/maven/echo/biz/target
echo-biz-1.0.0
maven-clean-plugin
2.4.1
default-clean
clean
clean
maven-install-plugin
2.3.1
default-install
install
install
maven-resources-plugin
2.5
default-resources
process-resources
resources
default-testResources
process-test-resources
testResources
maven-surefire-plugin
2.10
default-test
test
test
maven-compiler-plugin
2.3.2
default-testCompile
test-compile
testCompile
default-compile
compile
compile
maven-jar-plugin
2.3.2
default-jar
package
jar
maven-deploy-plugin
2.7
default-deploy
deploy
deploy
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
[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。
作者：zlwen
链接：https://www.jianshu.com/p/2f7080a4858c
來源：简书
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