Maven基础教程之聚合

前言

通过前面几篇文章，我把Maven中的坐标、 仓库 、 依赖 、 生命周期 和插件这五大核心概念进行了总结，掌握了这五大核心概念，基本上也就把住了Maven的脉。从这篇文章开始，我们将从实际的应用场景出发，总结Maven的一些比较常见的用户，通过这些常见的用户来更好的把握Maven，学习Maven。

我们的需求

在前后台分离，微服务大行其道的今天，我们的应用不再是一个超级大的包了，而是分成了多个子项目模块，每个子项目模块都是一个单独的工程项目。这就出现应用多的情况，此时如果你负责一个功能开发时，需要修改多个子项目模块时，就需要去修改不同的工程项目，当你开发完进行联调时，就需要一次构建多个工程项目，当出现问题时，又可能同时修改多个工程项目，此时你是一个一个工程项目的去构建呢？还是希望有一种办法一次性可以构建多个工程项目。

当然了，我们肯定希望存在一种办法，我们通过点击某个按钮就可以开始构建多个工程项目，然后我们去喝杯咖啡的。那我们的这种需求在Maven中是否能实现呢？毫无疑问，Maven是可以搞定这个问题的，这就是Maven中的聚合，通过聚合我们就可以解决这个痛点问题。下面我就通过实际的项目来说说Maven中的聚合到底是个什么鬼。

聚合实战

现在我准备了两个基于Maven的子工程项目，分别是Project-A和Project-B。这两个项目都可以单独编译，单独构建。但是为了能够使用一条命令就可以构建Project-A和Project-B这两个子工程项目，我们需要创建一个额外的名为Project-Aggregator的工程项目，然后通过该模块构建整个项目的所有模块。由于这个Project-Aggregator的工程项目是一个聚合项目，它是不需要src和test目录的，只需要有一个POM就OK了，下面就是这个Project-Aggregator工程项目的POM文件内容：

<?xml version="1.0" encoding="UTF-8"?>

<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>com.jellythink.AggregatorDemo</groupId>
  <artifactId>Project-Aggregator</artifactId>
  <version>1.0-SNAPSHOT</version>
  <packaging>pom</packaging>
  <name>Project-Aggregator</name>

  <modules>
    <module>../Project-A</module>
    <module>../Project-B</module>
  </modules>
</project>

接下来，我们在Project-Aggregator工程项目目录下执行 mvn clean package 命令，就会看到以下输出：

[INFO] Scanning for projects...
[INFO] ------------------------------------------------------------------------
[INFO] Reactor Build Order:
[INFO]
[INFO] Project-A
[INFO] Project-B
[INFO] Project-Aggregator
[INFO]
[INFO] ------------------------------------------------------------------------
[INFO] Building Project-A 1.0-SNAPSHOT
[INFO] ------------------------------------------------------------------------
[INFO]
[INFO] --- maven-clean-plugin:3.1.0:clean (default-clean) @ Project-A ---
[INFO] Deleting E:\Code\Spring\Project-A\target
[INFO]
[INFO] --- maven-resources-plugin:3.0.2:resources (default-resources) @ Project-A ---
[INFO] Using 'UTF-8' encoding to copy filtered resources.
[INFO] skip non existing resourceDirectory E:\Code\Spring\Project-A\src\main\resources
[INFO]
[INFO] --- maven-compiler-plugin:3.8.0:compile (default-compile) @ Project-A ---
[INFO] Changes detected - recompiling the module!
[INFO] Compiling 1 source file to E:\Code\Spring\Project-A\target\classes
[INFO]
[INFO] --- maven-resources-plugin:3.0.2:testResources (default-testResources) @ Project-A ---
[INFO] Using 'UTF-8' encoding to copy filtered resources.
[INFO] skip non existing resourceDirectory E:\Code\Spring\Project-A\src\test\resources
[INFO]
[INFO] --- maven-compiler-plugin:3.8.0:testCompile (default-testCompile) @ Project-A ---
[INFO] Changes detected - recompiling the module!
[INFO] Compiling 1 source file to E:\Code\Spring\Project-A\target\test-classes
[INFO]
[INFO] --- maven-surefire-plugin:2.22.1:test (default-test) @ Project-A ---
[INFO]
[INFO] -------------------------------------------------------
[INFO]  T E S T S
[INFO] -------------------------------------------------------
[INFO] Running com.jellythink.AggregatorDemo.AppTest
[INFO] Tests run: 1, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.032 s - in com.jellythink.AggregatorDemo.AppTest
[INFO]
[INFO] Results:
[INFO]
[INFO] Tests run: 1, Failures: 0, Errors: 0, Skipped: 0
[INFO]
[INFO]
[INFO] --- maven-jar-plugin:3.0.2:jar (default-jar) @ Project-A ---
[INFO] Building jar: E:\Code\Spring\Project-A\target\Project-A-1.0-SNAPSHOT.jar
[INFO]
[INFO] ------------------------------------------------------------------------
[INFO] Building Project-B 1.0-SNAPSHOT
[INFO] ------------------------------------------------------------------------
[INFO]
[INFO] --- maven-clean-plugin:3.1.0:clean (default-clean) @ Project-B ---
[INFO] Deleting E:\Code\Spring\Project-B\target
[INFO]
[INFO] --- maven-resources-plugin:3.0.2:resources (default-resources) @ Project-B ---
[INFO] Using 'UTF-8' encoding to copy filtered resources.
[INFO] skip non existing resourceDirectory E:\Code\Spring\Project-B\src\main\resources
[INFO]
[INFO] --- maven-compiler-plugin:3.8.0:compile (default-compile) @ Project-B ---
[INFO] Changes detected - recompiling the module!
[INFO] Compiling 1 source file to E:\Code\Spring\Project-B\target\classes
[INFO]
[INFO] --- maven-resources-plugin:3.0.2:testResources (default-testResources) @ Project-B ---
[INFO] Using 'UTF-8' encoding to copy filtered resources.
[INFO] skip non existing resourceDirectory E:\Code\Spring\Project-B\src\test\resources
[INFO]
[INFO] --- maven-compiler-plugin:3.8.0:testCompile (default-testCompile) @ Project-B ---
[INFO] Changes detected - recompiling the module!
[INFO] Compiling 1 source file to E:\Code\Spring\Project-B\target\test-classes
[INFO]
[INFO] --- maven-surefire-plugin:2.22.1:test (default-test) @ Project-B ---
[INFO]
[INFO] -------------------------------------------------------
[INFO]  T E S T S
[INFO] -------------------------------------------------------
[INFO] Running com.jellythink.AggregatorDemo.AppTest
[INFO] Tests run: 1, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.046 s - in com.jellythink.AggregatorDemo.AppTest
[INFO]
[INFO] Results:
[INFO]
[INFO] Tests run: 1, Failures: 0, Errors: 0, Skipped: 0
[INFO]
[INFO]
[INFO] --- maven-jar-plugin:3.0.2:jar (default-jar) @ Project-B ---
[INFO] Building jar: E:\Code\Spring\Project-B\target\Project-B-1.0-SNAPSHOT.jar
[INFO]
[INFO] ------------------------------------------------------------------------
[INFO] Building Project-Aggregator 1.0-SNAPSHOT
[INFO] ------------------------------------------------------------------------
[INFO]
[INFO] --- maven-clean-plugin:2.5:clean (default-clean) @ Project-Aggregator ---
[INFO] ------------------------------------------------------------------------
[INFO] Reactor Summary:
[INFO]
[INFO] Project-A .......................................... SUCCESS [  3.513 s]
[INFO] Project-B .......................................... SUCCESS [  1.424 s]
[INFO] Project-Aggregator ................................. SUCCESS [  0.047 s]
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 5.093 s
[INFO] Finished at: 2019-04-22T21:54:08+08:00
[INFO] Final Memory: 19M/274M
[INFO] ------------------------------------------------------------------------

从输出可以看到，我们在构建Project-Aggregator项目时，Project-A和Project-B就会一同被构建，在存在多个项目时，这是非常方便的。

总结

对于聚合，内容不多，但是这种使用手法在实际工作中我们也会碰到，以后你在碰到这种用法时，不要疑惑。希望这篇文章能够帮助到你，更加丰富你的Maven知识，让你对Maven的使用有一个更新的认识。

果冻想，认真玩技术的地方。

2019年4月22日，于内蒙古呼和浩特。