我是如何理解并使用maven的

前言

一直想写一篇关于Maven的文章，但是不知如何下笔，如果说能使用，会使用Maven的话，一、两个小时足矣，不需要搞懂各种概念。那么给大家来分享下我是如何理解并使用maven的。

什么是Maven？

Maven是一个用于项目构建的工具，通过它便捷的管理项目的生命周期。即项目的jar包依赖，开发，测试，发布打包，主要管理工作是:依赖管理,项目一键构建。

为什么要使用Maven

使用maven构建的项目,整个项目的体积小
maven项目不需要手动导入jar包，通过在pom.xml中添加依赖，引用依赖会自动从maven仓库下载jar包，方便快捷。
项目一键构建:使用maven可以快速地对项目进行编译--测试--运行--打包--安装
maven支持跨平台操作，可在window、linux、mac上使用
maven遵循规范开发有利于提高大型团队的开发效率，降低项目的维护成本，属于主流技术，一般公司都会使用maven来构建项目

maven仓库的配置

ps:下载项目引用需要jar包的时候存放的本地路径

仓库的分类

本地仓库
私服(公司的仓库)
中央仓库

三个仓库之间的关系

三者之间的关系是，当我们在项目中依赖一个jar包时，Maven程序会先去本地仓库中找，如果没找到就回去私服找，如果还是没有，最后就回去中央仓库找。其过程如下图：

本地仓库的配置

找到已安装的maven路径，如：apache-maven-3.3.9\conf 目录下settings.xml 文件并用notepad++打开，ctrl+F找到localRepository标签，将路径设置为D:/repository，如下图：

说明:什么是本地仓库? 就是是由个人将常用到的jar包放入一个仓库中，已备自己在项目中使用，可从别人配置好的jar包仓库拷到自己本地目录，因为仓库一般很大，首次下载需要很长一段时间。

配置私服地址

实际工作中，很多项目都会用到maven私服仓库，一般公司都有统一的maven私服仓库，由于公司都是统一化管理，这时候我们就要配置统一的私服仓库，举例如下：

<mirrors>
    
    mirror>
      <!--This is used to direct the public snapshots repo in the 
          profile below over to a different nexus group -->
      id>nexus-public-snapshots</mirrorOf>public-snapshotsurl>http://192.168.1.118:8888/nexus/content/repositories/apache-snapshots/>
    This sends everything else to /public >nexus>*>http://maven.aliyun.com/nexus/content/groups/public/>
  >

pom文件说明

pom.xml文件，一般描述了maven项目的基本信息，比如groupId，artifactId，version等，一个最简单的pom.xml文件至少需要包含四个元素：modelVersion, groupId, artiffactId和version。

比如一个基本的pom.xml文件如下：

<?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>4.0.0groupId>com.test> //当前项目的信息
    artifactId>maven-demoversion>1.0-SNAPSHOT//SNAPSHOT（快照）表示该项目还在开发中。
project>

其中主要的标签含义如下：

project：pom.xml 文件中的顶层元素；

modelVersion：指明 POM 使用的对象模型的版本。这个值很少改动。

groupId：指明创建项目的组织或者小组的唯一标识。

GroupId 是项目的关键标识，典型的，此标识以组织的完全限定名来定义。比如，org.apache.maven.plugins 是所有 Maven 插件项目指定的 groupId。

artifactId：指明此项目产生的主要产品的基本名称。项目的主要产品通常为一个 JAR 文件。第二，象源代码包通常使用 artifactId 作为最后名称的一部分。典型的产品名称使用这个格式：

version：项目产品的版本号。Maven 帮助你管理版本，可以经常看到 SNAPSHOT 这个版本，表明项目处于开发阶段。

在项目中添加 插件，以及对插件的配置

    build>
        plugins>
            plugin>
                >org.apache.maven.plugins>maven-compiler-plugin//编译插件
                >2.4.3//插件的版本号
                configuration//对插件进行配置
                    source>1.7//源代码编译版本
                    target//目标平台编译版本；
                    encoding>UTF-8//设置插件或资源文件的编码方式。
                >maven-surefire-plugin//执行测试用例的插件
                >2.17suiteXmlFiles>
                        suiteXmlFile>${suiteXmlFile}//测试套件执行路径
                    >
 
                >

如何在pom文件中添加依赖jar包

实际开发中需要引用jar包后，再进行开发，那么在pom中添加依赖呢?

1、比如我想添加testng.jar包，那么可以通过访问网址 https://mvnrepository.com/ ，然后在搜索框中输入testng,回车

2、点击testng,选择对应版本如6.14.3

3、复制红框中内容，放到dependencies标签内。

在pom中引用完成，并自动下载依赖jar包。

  dependencies https://mvnrepository.com/artifact/org.testng/testng -->
        dependency>org.testng>testng>6.14.3scope>test>

实际效果：

编写一个主类

package com.test.demo;

public class HellowWorld {
    public String sayHello() {
        return "Hello World";
    }

    public static void main( String[] args ) {
        System.out.println(new HellowWorld().sayHello());
    }
}

编写一个测试类

package com.test.demo;
import org.testng.Assert;
 org.testng.annotations.Test;
public class TestHellowWorld {
    @Test
    void testSayHello() {
        HellowWorld hellowWorld = new HellowWorld();
        String result = hellowWorld.sayHello();
        Assert.assertEquals("Hello World",result);
    }
}

maven项目的目录结构

F:\mavendemo>tree
卷新加卷的文件夹 PATH 列表
卷序列号为 5C5B-6DDB
F:.
├─.idea
└─src
├─main
│ ├─java
│ │ └─com
│ │ └─test
│ │ └─demo
│ └─resources
└─test
└─java
└─com
└─test
└─demo

注意上面带红色的目录名，maven项目采用“约定优于配置”的原则，src/main/java约定用于存放源代码，src/main/test用于存放单元测试代码，src/target用于存放编译、打包后的输出文件。这是全世界maven项目的通用约定，请记住这些固定的目录结构。

编译和测试

在项目根目录下运行mvn clean compile命令。执行输出如下所示：

Picked up JAVA_TOOL_OPTIONS: -Dfile.encoding=UTF-8
[INFO] Scanning for projects...
[INFO]
[INFO] ------------------------------------------------------------------------
[INFO] Building maven-demo 1.0-SNAPSHOT
[INFO] ------------------------------------------------------------------------
[INFO]
[INFO] --- maven-clean-plugin:2.5:clean (default-clean) @ maven-demo ---
[INFO] Deleting F:\mavendemo\target
[INFO]
[INFO] --- maven-resources-plugin:2.6:resources (default-resources) @ maven-demo ---
[WARNING] Using platform encoding (UTF-8 actually) to copy filtered resources,i.e. build is platform dependent!
[INFO] Copying 0 resource
[INFO]
[INFO] --- maven-compiler-plugin:2.5.1:compile (default-compile) @ maven-demo ---
[WARNING] File encoding has not been set,using platform encoding UTF-8,i.e. build is platform dependent!
[INFO] Compiling 1 source file to F:\mavendemo\target\classes
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 1.336 s
[INFO] Finished at: 2019-06-23T17:33:29+08:00
[INFO] Final Memory: 13M/309M
[INFO] ------------------------------------------------------------------------

clean：清理输出目录target下生成jar包

compile：编译项目主代码

编译完成后，我们一般都会运行测试代码进行单元测试，虽然很多情况下，我们并没有这么做，但是我还是建议大家通过Maven做一些自动化的单元测试。

测试用例编写完毕之后就可以调用Maven执行测试，运行mvn clean test命令，输出如下：

Picked up JAVA_TOOL_OPTIONS: -Dfile.encoding=UTF-8
[INFO] Scanning for projects...
[INFO]
[INFO] ------------------------------------------------------------------------
[INFO] Building maven-demo 1.0-SNAPSHOT
[INFO] ------------------------------------------------------------------------
[INFO]
[INFO] --- maven-clean-plugin:2.5:clean (default-clean) @ maven-demo ---
[INFO] Deleting F:\mavendemo\target
[INFO]
[INFO] --- maven-resources-plugin:2.6:resources (default-resources) @ maven-demo ---
[WARNING] Using platform encoding (UTF-8 actually) to copy filtered resources,i.e. build is platform dependent!
[INFO] Copying 0 resource
[INFO]
[INFO] --- maven-compiler-plugin:2.5.1:compile (default-compile) @ maven-demo ---
[WARNING] File encoding has not been set,1)"> build is platform dependent!
[INFO] Compiling 1 source file to F:\mavendemo\target\classes
[INFO]
[INFO] --- maven-resources-plugin:2.6:testResources (default-testResources) build is platform dependent!
[INFO] skip non existing resourceDirectory F:\mavendemo\src\test\resources
[INFO]
[INFO] --- maven-compiler-plugin:2.5.1:testCompile (default-testCompile)test-classes
[INFO]
[INFO] --- maven-surefire-plugin:2.17:test (default-test) @ maven-demo ---
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 2.032 s
[INFO] Finished at: 2019-06-23T17:27:28+08:00
[INFO] Final Memory: 15M/309M
[INFO] ------------------------------------------------------------------------

打包和运行

打包就是将我们编写的应用打成JAR包或者WAR包，我们执行mvn clean package命令就可以完成打包。mvn clean package命令的输出如下：

Picked up JAVA_TOOL_OPTIONS: -Dfile.encoding=UTF-8for projects...
[INFO]
[INFO] ------------------------------------------------------------------------
[INFO] Building maven-demo 1.0-SNAPSHOT
[INFO] ------------------------------------------------------------------------
[INFO]
[INFO] --- maven-clean-plugin:2.5:clean (default-clean) @ maven-demo ---
[INFO] Deleting F:\mavendemo\target
[INFO]
[INFO] --- maven-resources-plugin:2.6:resources (default-resources) @ maven-demo ---
[WARNING] Using platform encoding (UTF-8 actually) to copy filtered resources,i.e. build is platform dependent!
[INFO] Copying 0 resource
[INFO]
[INFO] --- maven-compiler-plugin:2.5.1:compile (default-compile) @ maven-demo ---
[WARNING] File encoding has not been 
[INFO] Compiling 1 source file to F:\mavendemo\target\classes
[INFO]
[INFO] --- maven-resources-plugin:2.6:testResources (default-testResources) @ maven-demo ---
[INFO] skip non existing resourceDirectory F:\mavendemo\src\test\resources
[INFO]
[INFO] --- maven-compiler-plugin:2.5.1:testCompile (default-testCompile) @ maven-demo ---
[INFO] Compiling 1 source file to F:\mavendemo\target\test-classes
[INFO]
[INFO] --- maven-surefire-plugin:2.17:test (default-test) @ maven-demo ---
[INFO] Surefire report directory: F:\mavendemo\test-output
-------------------------------------------------------
 T E S T S
-------------------------------------------------------
Running com.test.demo.TestHellowWorld
Configuring TestNG with: TestNG652Configurator
Tests run: 1,Failures: 0,Errors: 0,Skipped: 0,Time elapsed: 0.498 sec - in com.test.demo.TestHellowWorld
Picked up JAVA_TOOL_OPTIONS: -Dfile.encoding=UTF-8
Results :
Tests run: 1,Skipped: 0
[INFO]
[INFO] --- maven-jar-plugin:2.4:jar (default-jar) @ maven-demo ---
[INFO] Building jar: F:\mavendemo\target\maven-demo-1.0-SNAPSHOT.jar
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 3.356 s
[INFO] Finished at: 2019-06-23T17:46:00+08:00
[INFO] Final Memory: 17M/311M
[INFO] ------------------------------------------------------------------------

运行完后，会在target目录下生成jar包

如果别的项目要引用这个JAR包时，我们将这个JAR包复制到其它项目的classpath中就OK了。

但是这样拷贝就违背了我们当初想要自动解决依赖的问题，所以如何才能让其它的Maven项目直接引用这个JAR包呢？我们需要执行mvn clean install命令。

Picked up JAVA_TOOL_OPTIONS: -Dfile.encoding=UTF-8SNAPSHOT.jar
[INFO]
[INFO] --- maven-install-plugin:2.4:install (default-install) @ maven-demo ---
[INFO] Installing F:\mavendemo\target\maven-demo-1.0-SNAPSHOT.jar to E:\repository\com\test\maven-demo\1.0-SNAPSHOT\maven-demo-1.0-SNAPSHOT.jar
[INFO] Installing F:\mavendemo\pom.xml to E:\repository\com\test\maven-demo\1.0-SNAPSHOT\maven-demo-1.0-SNAPSHOT.pom
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 4.643 s
[INFO] Finished at: 2019-06-23T20:39:34+08:00
[INFO] Final Memory: 16M/211M
[INFO] ------------------------------------------------------------------------

使用Archetype生成项目骨架

下面通过用命名行创建一个最基本的maven项目

mvn archetype:generate

先创建项目的根目录，从盘符开始，命令行窗口下输入

mkdir demotest

cd demotest

mvn archetype:generate

首次运行时，mvn会从远程"中央仓库"下载一些必需的文件到"本地仓库" -

如果你有兴趣，可以在等待下载过程中，观察一下"C:\Users\当前用户名\.m2\repository"目录下是不是多了很多文件。

下载完成后，会自动进入交互模式，会让你输入一些基本信息，类似下面这样：

F:\demotest>mvn archetype:generate
Picked up JAVA_TOOL_OPTIONS: -Dfile.encoding=UTF-8
[INFO] Building Maven Stub Project (No POM) 1
[INFO] ------------------------------------------------------------------------
[INFO]
[INFO] >>> maven-archetype-plugin:3.0.0:generate (default-cli) > generate-sources @ standalone-pom >>>
[INFO]
[INFO] <<< maven-archetype-plugin:3.0.0:generate (default-cli) < generate-sources @ standalone-pom <<<
[INFO]
[INFO] --- maven-archetype-plugin:3.0.0:generate (default-cli) @ standalone-pom ---
Downloading: https://repo.maven.apache.org/maven2/org/apache/maven/shared/maven-artifact-transfer/0.9.0/maven-artifact-transfer-0.9.0.pom
Downloaded: https:repo.maven.apache.org/maven2/org/apache/maven/shared/maven-artifact-transfer/0.9.0/maven-artifact-transfer-0.9.0.pom (8 KB at 5.4 KB/sec)
Downloading: https:repo.maven.apache.org/maven2/org/apache/maven/shared/maven-common-artifact-filters/3.0.0/maven-common-artifact-filters-3.0.0.pom
Downloaded: https:repo.maven.apache.org/maven2/org/apache/maven/shared/maven-common-artifact-filters/3.0.0/maven-common-artifact-filters-3.0.0.pom (5 KB at 10.1 KB/sec)
Downloading: https:repo.maven.apache.org/maven2/org/apache/maven/shared/maven-shared-components/22/maven-shared-components-22.pom
Downloaded: https:repo.maven.apache.org/maven2/org/apache/maven/shared/maven-shared-components/22/maven-shared-components-22.pom (5 KB at 11.2 KB/sec)
Downloading: https:repo.maven.apache.org/maven2/org/apache/maven/wagon/wagon-provider-api/2.8/wagon-provider-api-2.8.pom
Downloaded: https:repo.maven.apache.org/maven2/org/apache/maven/wagon/wagon-provider-api/2.8/wagon-provider-api-2.8.pom (2 KB at 3.8 KB/sec)
Downloading: https:repo.maven.apache.org/maven2/org/apache/maven/wagon/wagon/2.8/wagon-2.8.pom
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Downloading: https:repo.maven.apache.org/maven2/org/codehaus/groovy/groovy/1.8.3/groovy-1.8.3.pom
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Downloading: https:repo.maven.apache.org/maven2/antlr/antlr/2.7.7/antlr-2.7.7.pom
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Downloading: https:repo.maven.apache.org/maven2/asm/asm-parent/3.2/asm-parent-3.2.pom
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Downloading: https:repo.maven.apache.org/maven2/asm/asm-commons/3.2/asm-commons-3.2.pom
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Downloading: https:repo.maven.apache.org/maven2/asm/asm-tree/3.2/asm-tree-3.2.pom
Downloaded: https:repo.maven.apache.org/maven2/asm/asm-tree/3.2/asm-tree-3.2.pom (0 B at 0.0 KB/sec)
Downloading: https:repo.maven.apache.org/maven2/asm/asm-util/3.2/asm-util-3.2.pom
Downloaded: https:repo.maven.apache.org/maven2/asm/asm-util/3.2/asm-util-3.2.pom (0 B at 0.0 KB/sec)
Downloading: https:repo.maven.apache.org/maven2/asm/asm-analysis/3.2/asm-analysis-3.2.pom
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Downloading: https:repo.maven.apache.org/maven2/org/codehaus/plexus/plexus-interactivity-api/1.0-alpha-6/plexus-interactivity-api-1.0-alpha-6.pom
Downloaded: https:repo.maven.apache.org/maven2/org/codehaus/plexus/plexus-interactivity-api/1.0-alpha-6/plexus-interactivity-api-1.0-alpha-6.pom (0 B at 0.0 KB/sec)
Downloading: https:repo.maven.apache.org/maven2/org/codehaus/plexus/plexus-interactivity/1.0-alpha-6/plexus-interactivity-1.0-alpha-6.pom
Downloaded: https:repo.maven.apache.org/maven2/org/codehaus/plexus/plexus-interactivity/1.0-alpha-6/plexus-interactivity-1.0-alpha-6.pom (0 B at 0.0 KB/sec)
Downloading: https:repo.maven.apache.org/maven2/org/codehaus/plexus/plexus-components/1.1.9/plexus-components-1.1.9.pom
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Downloading: https:repo.maven.apache.org/maven2/org/apache/maven/shared/maven-script-interpreter/1.0/maven-script-interpreter-1.0.pom
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Downloading: https:repo.maven.apache.org/maven2/org/apache/ant/ant/1.8.1/ant-1.8.1.pom
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Downloading: https:repo.maven.apache.org/maven2/org/apache/maven/archetype/archetype-catalog/3.0.0/archetype-catalog-3.0.0.jar
Downloading: https:repo.maven.apache.org/maven2/net/sourceforge/jchardet/jchardet/1.0/jchardet-1.0.jar
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Downloaded: https:repo.maven.apache.org/maven2/asm/asm-commons/3.2/asm-commons-3.2.jar (33 KB at 20.2 KB/sec)
Downloading: https:repo.maven.apache.org/maven2/org/codehaus/plexus/plexus-utils/3.0.21/plexus-utils-3.0.21.jar
Downloaded: https:repo.maven.apache.org/maven2/asm/asm-tree/3.2/asm-tree-3.2.jar (22 KB at 10.6 KB/sec)
Downloading: https:repo.maven.apache.org/maven2/org/codehaus/plexus/plexus-interactivity-api/1.0-alpha-6/plexus-interactivity-api-1.0-alpha-6.jar
Downloaded: https:repo.maven.apache.org/maven2/asm/asm/3.2/asm-3.2.jar (43 KB at 19.6 KB/sec)
Downloading: https:repo.maven.apache.org/maven2/org/apache/maven/shared/maven-invoker/2.2/maven-invoker-2.2.jar
Downloaded: https:repo.maven.apache.org/maven2/org/codehaus/plexus/plexus-interactivity-api/1.0-alpha-6/plexus-interactivity-api-1.0-alpha-6.jar (12 KB at 4.7 KB/sec)
Downloading: https:repo.maven.apache.org/maven2/org/apache/maven/shared/maven-artifact-transfer/0.9.0/maven-artifact-transfer-0.9.0.jar
Downloaded: https:repo.maven.apache.org/maven2/org/apache/maven/shared/maven-invoker/2.2/maven-invoker-2.2.jar (30 KB at 8.3 KB/sec)
Downloading: https:repo.maven.apache.org/maven2/org/apache/maven/shared/maven-common-artifact-filters/3.0.0/maven-common-artifact-filters-3.0.0.jar
Downloaded: https:repo.maven.apache.org/maven2/org/codehaus/plexus/plexus-utils/3.0.21/plexus-utils-3.0.21.jar (240 KB at 56.1 KB/sec)
Downloading: https:repo.maven.apache.org/maven2/org/apache/maven/shared/maven-script-interpreter/1.0/maven-script-interpreter-1.0.jar
Downloaded: https:repo.maven.apache.org/maven2/org/apache/maven/shared/maven-artifact-transfer/0.9.0/maven-artifact-transfer-0.9.0.jar (121 KB at 27.4 KB/sec)
Downloading: https:repo.maven.apache.org/maven2/org/apache/ant/ant/1.8.1/ant-1.8.1.jar
Downloaded: https:repo.maven.apache.org/maven2/org/apache/ant/ant/1.8.1/ant-1.8.1.jar (0 B at 0.0 KB/sec)
Downloaded: https:repo.maven.apache.org/maven2/org/apache/maven/shared/maven-script-interpreter/1.0/maven-script-interpreter-1.0.jar (21 KB at 4.0 KB/sec)
Downloaded: https:repo.maven.apache.org/maven2/org/apache/maven/shared/maven-common-artifact-filters/3.0.0/maven-common-artifact-filters-3.0.0.jar (56 KB at 10.6 KB/sec)
Downloaded: https:repo.maven.apache.org/maven2/org/apache/maven/archetype/archetype-common/3.0.0/archetype-common-3.0.0.jar (324 KB at 51.1 KB/sec)
Downloaded: https:repo.maven.apache.org/maven2/org/codehaus/groovy/groovy/1.8.3/groovy-1.8.3.jar (5394 KB at 19.0 KB/sec)
[INFO] Generating project in Interactive mode
.............................................................................

执行这个命令后，后看到很多输出，然后再按照提示一步步操作，一个Maven项目就创建成功了。

总结

到此，关于maven的入门基础知识总结完毕，文章知识点相对繁琐、复杂，还请读者多次阅读和实践，如有错误之处，烦请多指正！

参考文章：

https://www.jellythink.com/archives/502

https://www.cnblogs.com/yjmyzz/p/3495762.html