我是如何理解並使用maven的

Refain發表於2019-06-23

Maven

前言

一直想寫一篇關於Maven的文章，但是不知如何下筆，如果說能使用，會使用Maven的話，一、兩個小時足矣，不需要搞懂各種概念。那麼給大家來分享下我是如何理解並使用maven的。

什麼是Maven？

Maven是一個用於專案構建的工具，通過它便捷的管理專案的生命週期。即專案的jar包依賴，開發，測試，釋出打包，主要管理工作是:依賴管理,專案一鍵構建。

為什麼要使用Maven

使用maven構建的專案,整個專案的體積小
maven專案不需要手動匯入jar包，通過在pom.xml中新增依賴，引用依賴會自動從maven倉庫下載jar包，方便快捷。
專案一鍵構建:使用maven可以快速地對專案進行編譯--測試--執行--打包--安裝
maven支援跨平臺操作，可在window、linux、mac上使用
maven遵循規範開發有利於提高大型團隊的開發效率，降低專案的維護成本，屬於主流技術，一般公司都會使用maven來構建專案

maven倉庫的配置

ps:下載專案引用需要jar包的時候存放的本地路徑

倉庫的分類

本地倉庫
私服(公司的倉庫)
中央倉庫

三個倉庫之間的關係

三者之間的關係是，當我們在專案中依賴一個jar包時，Maven程式會先去本地倉庫中找，如果沒找到就回去私服找，如果還是沒有，最後就回去中央倉庫找。其過程如下圖：

本地倉庫的配置

找到已安裝的maven路徑，如：apache-maven-3.3.9\conf 目錄下settings.xml 檔案並用notepad++開啟，ctrl+F找到localRepository標籤，將路徑設定為D:/repository，如下圖：

說明:什麼是本地倉庫? 就是是由個人將常用到的jar包放入一個倉庫中，已備自己在專案中使用，可從別人配置好的jar包倉庫拷到自己本地目錄，因為倉庫一般很大，首次下載需要很長一段時間。

配置私服地址

實際工作中，很多專案都會用到maven私服倉庫，一般公司都有統一的maven私服倉庫，由於公司都是統一化管理，這時候我們就要配置統一的私服倉庫，舉例如下：

<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</id>
      <mirrorOf>public-snapshots</mirrorOf>
      <url>http://192.168.1.118:8888/nexus/content/repositories/apache-snapshots/</url>
    </mirror>
    <mirror>
      <!--This sends everything else to /public -->
      <id>nexus</id>
      <mirrorOf>*</mirrorOf>
      <url>http://maven.aliyun.com/nexus/content/groups/public/</url>
    </mirror>
  </mirrors>

pom檔案說明

pom.xml檔案，一般描述了maven專案的基本資訊，比如groupId，artifactId，version等，一個最簡單的pom.xml檔案至少需要包含四個元素：modelVersion, groupId, artiffactId和version。

比如一個基本的pom.xml檔案如下：

<?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>4.0.0</modelVersion>
    <groupId>com.test</groupId> //當前專案的資訊
    <artifactId>maven-demo</artifactId>
    <version>1.0-SNAPSHOT</version>//SNAPSHOT（快照）表示該專案還在開發中。
</project>

其中主要的標籤含義如下：

project：pom.xml 檔案中的頂層元素；

modelVersion：指明 POM 使用的物件模型的版本。這個值很少改動。

groupId：指明建立專案的組織或者小組的唯一標識。

GroupId 是專案的關鍵標識，典型的，此標識以組織的完全限定名來定義。比如，org.apache.maven.plugins 是所有 Maven 外掛專案指定的 groupId。

artifactId：指明此專案產生的主要產品的基本名稱。專案的主要產品通常為一個 JAR 檔案。第二，象原始碼包通常使用 artifactId 作為最後名稱的一部分。典型的產品名稱使用這個格式：

version：專案產品的版本號。Maven 幫助你管理版本，可以經常看到 SNAPSHOT 這個版本，表明專案處於開發階段。

在專案中新增外掛，以及對外掛的配置

    <build>
        <plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-compiler-plugin</artifactId>//編譯外掛
                <version>2.4.3</version>//外掛的版本號
                <configuration>//對外掛進行配置
                    <source>1.7</source>//原始碼編譯版本
                    <target>1.7</target>//目標平臺編譯版本；
                    <encoding>UTF-8</encoding>//設定外掛或資原始檔的編碼方式。
                </configuration>
            </plugin>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-surefire-plugin</artifactId>//執行測試用例的外掛
                <version>2.17</version>//外掛的版本號
                <configuration>//對外掛進行配置
                    <suiteXmlFiles>
                        <suiteXmlFile>${suiteXmlFile}</suiteXmlFile>//測試套件執行路徑
                    </suiteXmlFiles>

                </configuration>
            </plugin>
        </plugins>
    </build>

如何在pom檔案中新增依賴jar包

實際開發中需要引用jar包後，再進行開發，那麼在pom中新增依賴呢?

1、比如我想新增testng.jar包，那麼可以通過訪問網址 https://mvnrepository.com/ ，然後在搜尋框中輸入testng,回車

2、點選testng,選擇對應版本如6.14.3

3、複製紅框中內容，放到dependencies標籤內。

在pom中引用完成，並自動下載依賴jar包。

  <dependencies>
        <!-- https://mvnrepository.com/artifact/org.testng/testng -->
        <dependency>
            <groupId>org.testng</groupId>
            <artifactId>testng</artifactId>
            <version>6.14.3</version>
            <scope>test</scope>
        </dependency>
    </dependencies>

實際效果：

編寫一個主類

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());
    }
}

編寫一個測試類

package com.test.demo;

import org.testng.Assert;
import org.testng.annotations.Test;

public class TestHellowWorld {

    @Test
    public void testSayHello() {
        HellowWorld hellowWorld = new HellowWorld();
        String result = hellowWorld.sayHello();
        Assert.assertEquals("Hello World", result);
    }
}

maven專案的目錄結構

F:\mavendemo>tree
卷新加捲的資料夾 PATH 列表
卷序列號為 5C5B-6DDB
F:.
├─.idea
└─src
├─main
│ ├─java
│ │ └─com
│ │ └─test
│ │ └─demo
│ └─resources
└─test
└─java
└─com
└─test
└─demo

注意上面帶紅色的目錄名，maven專案採用“約定優於配置”的原則，src/main/java約定用於存放原始碼，src/main/test用於存放單元測試程式碼，src/target用於存放編譯、打包後的輸出檔案。這是全世界maven專案的通用約定，請記住這些固定的目錄結構。

編譯和測試

在專案根目錄下執行mvn clean compile命令。執行輸出如下所示：

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：清理輸出目錄target下生成jar包

compile：編譯專案主程式碼

編譯完成後，我們一般都會執行測試程式碼進行單元測試，雖然很多情況下，我們並沒有這麼做，但是我還是建議大家通過Maven做一些自動化的單元測試。

測試用例編寫完畢之後就可以呼叫Maven執行測試，執行mvn clean test命令，輸出如下：

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, using platform encoding UTF-8, i.e. build is platform dependent!
[INFO] Compiling 1 source file to F:\mavendemo\target\classes
[INFO]
[INFO] --- maven-resources-plugin:2.6:testResources (default-testResources) @ maven-demo ---
[WARNING] Using platform encoding (UTF-8 actually) to copy filtered resources, i.e. 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) @ 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\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] ------------------------------------------------------------------------

打包和執行

打包就是將我們編寫的應用打成JAR包或者WAR包，我們執行mvn clean package命令就可以完成打包。mvn clean package命令的輸出如下：

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, using platform encoding UTF-8, i.e. build is platform dependent!
[INFO] Compiling 1 source file to F:\mavendemo\target\classes
[INFO]
[INFO] --- maven-resources-plugin:2.6:testResources (default-testResources) @ maven-demo ---
[WARNING] Using platform encoding (UTF-8 actually) to copy filtered resources, i.e. 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) @ 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\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, Failures: 0, Errors: 0, 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] ------------------------------------------------------------------------

執行完後，會在target目錄下生成jar包

如果別的專案要引用這個JAR包時，我們將這個JAR包複製到其它專案的classpath中就OK了。

但是這樣拷貝就違背了我們當初想要自動解決依賴的問題，所以如何才能讓其它的Maven專案直接引用這個JAR包呢？我們需要執行mvn clean install命令。

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] --- maven-resources-plugin:2.6:testResources (default-testResources) @ maven-demo ---
[WARNING] Using platform encoding (UTF-8 actually) to copy filtered resources, i.e. 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) @ 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\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: 0.611 sec - in com.test.demo.TestHellowWorld
Picked up JAVA_TOOL_OPTIONS: -Dfile.encoding=UTF-8

Results :

Tests run: 1, Failures: 0, Errors: 0, 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] --- 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] BUILD SUCCESS
[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生成專案骨架

下面通過用命名行建立一個最基本的maven專案

mvn archetype:generate

先建立專案的根目錄，從碟符開始，命令列視窗下輸入

mkdir demotest

cd demotest