maven install过程

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。

作者：zlwen
链接：https://www.jianshu.com/p/2f7080a4858c
来源：简书
简书著作权归作者所有，任何形式的转载都请联系作者获得授权并注明出处。

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