Maven的依赖管理

1.  依赖管理

依赖管理，可以将有关依赖项的所有信息放在共同的POM中，并对子POM中的工件进行更简单的引用。举个例子：

父POM

<project>......<dependencyManagement><dependencies><dependency><groupId>group-a</groupId><artifactId>artifact-a</artifactId><version>1.0</version><exclusions><exclusion><groupId>group-c</groupId><artifactId>excluded-artifact</artifactId></exclusion></exclusions></dependency><dependency><groupId>group-c</groupId><artifactId>artifact-b</artifactId><version>1.0</version><type>war</type><scope>runtime</scope></dependency><dependency><groupId>group-a</groupId><artifactId>artifact-b</artifactId><version>1.0</version><type>bar</type><scope>runtime</scope></dependency></dependencies></dependencyManagement>......
</project>

两个子POM

<project>...<dependencies><dependency><groupId>group-a</groupId><artifactId>artifact-a</artifactId></dependency><dependency><groupId>group-a</groupId><artifactId>artifact-b</artifactId><!-- This is not a jar dependency, so we must specify type. --><type>bar</type></dependency></dependencies>
</project><project>...<dependencies><dependency><groupId>group-c</groupId><artifactId>artifact-b</artifactId><!-- This is not a jar dependency, so we must specify type. --><type>war</type></dependency><dependency><groupId>group-a</groupId><artifactId>artifact-b</artifactId><!-- This is not a jar dependency, so we must specify type. --><type>bar</type></dependency></dependencies>
</project>

在<dependencyManagement>区域，实际上是根据{groupId, artifactId, type, classifier}来进行依赖匹配的。自从<type>字段的默认值是jar以后，最简单的标识只需要设置{groupId, artifactId}即可。

依赖管理的另一个重要的作用是统一管理（控制）依赖的版本。这一点不再赘述。

2.  导入依赖

依赖可以继承，但是目前只支持单继承，可以通过“import”的方式从其它POM中导入依赖。

下面的例子，Z导入了X和Y管理的依赖。

<project><modelVersion>4.0.0</modelVersion><groupId>maven</groupId><artifactId>Z</artifactId><packaging>pom</packaging><version>1.0</version><dependencyManagement><dependencies><dependency><groupId>maven</groupId><artifactId>X</artifactId><version>1.0</version><type>pom</type><scope>import</scope></dependency><dependency><groupId>maven</groupId><artifactId>Y</artifactId><version>1.0</version><type>pom</type><scope>import</scope></dependency></dependencies></dependencyManagement>
</project>

举个例子

<?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 https://maven.apache.org/xsd/maven-4.0.0.xsd"><modelVersion>4.0.0</modelVersion><parent><groupId>com.example</groupId><artifactId>demo-parent</artifactId><version>0.0.1-SNAPSHOT</version></parent><groupId>com.example</groupId><artifactId>demo001</artifactId><version>0.0.1-SNAPSHOT</version><dependencies><dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-web</artifactId></dependency></dependencies><dependencyManagement><dependencies><dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-dependencies</artifactId><version>2.7.11</version><type>pom</type><scope>import</scope></dependency></dependencies></dependencyManagement><build><plugins><plugin><groupId>org.springframework.boot</groupId><artifactId>spring-boot-maven-plugin</artifactId><configuration><excludes><exclude><groupId>org.projectlombok</groupId><artifactId>lombok</artifactId></exclude></excludes></configuration></plugin></plugins></build></project>

3.  版本管理插件

Introduction – Versions Maven Plugin

versions插件用于管理、模块、依赖和插件的版。该插件有很多目标，为了统一管理父子项目版本号，我们这里只用其中三个。

操作很简单，首先，在父工程中引入versions插件

<!-- 父POM中增加插件 -->
<build><plugins><plugin><groupId>org.codehaus.mojo</groupId><artifactId>versions-maven-plugin</artifactId><version>2.16.2</version><configuration><generateBackupPoms>false</generateBackupPoms></configuration></plugin></plugins>
</build>

然后，更新父工程的版本号

mvn versions:set -DnewVersion="2.0.0-SNAPSHOT" //更新整个项目版本

 接着，把子模块的版本更新成父工程的版本号

mvn versions:update-child-modules

 最后提交

mvn versions:commit

4.  BOM

对于一个项目来说，使用“库”中的一个或多个构件是相当常见的。为了使项目中使用的构件的版本与库中分发的版本保持同步。我们可以创建一个“物料清单”（BOM）以供其它项目使用。

简单的来讲，就是多个项目都需要依赖某些包时，为了方便管理版本，使得多个项目中引用的某个包的版本保持一致，这个时候我们就定义一个“库”，在这个库中统一定义包的版本，使用的时候只需要导入这个库即可。

举个例子：

定义一个库（其实就是一个POM）

<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><artifactId>mytest-bom</artifactId><version>1.0.0</version><packaging>pom</packaging><properties><project1Version>1.0.0</project1Version><project2Version>1.0.0</project2Version></properties><dependencyManagement><dependencies><dependency><groupId>com.test</groupId><artifactId>project1</artifactId><version>${project1Version}</version></dependency><dependency><groupId>com.test</groupId><artifactId>project2</artifactId><version>${project2Version}</version></dependency></dependencies></dependencyManagement>
</project>

然后，定义两个项目project1和project2

<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>com.test</groupId><artifactId>mytest-bom</artifactId><version>1.0.0</version></parent><groupId>com.test</groupId><artifactId>parent</artifactId><version>1.0.0</version><packaging>pom</packaging><dependencyManagement><dependencies><dependency><groupId>org.apache.logging.log4j</groupId><artifactId>log4j-core</artifactId><version>2.22.0</version></dependency></dependencies></dependencyManagement></project>

project1

<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>com.test</groupId><artifactId>parent</artifactId><version>1.0.0</version></parent><groupId>com.test</groupId><artifactId>project1</artifactId><version>${project1Version}</version><packaging>jar</packaging><dependencies><dependency><groupId>org.apache.logging.log4j</groupId><artifactId>log4j-core</artifactId></dependency></dependencies>
</project>

project2

<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>com.test</groupId><artifactId>parent</artifactId><version>1.0.0</version></parent><groupId>com.test</groupId><artifactId>project2</artifactId><version>${project2Version}</version><packaging>jar</packaging>
</project>

最后，在其它项目中引用project1和project2

<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><artifactId>use</artifactId><version>1.0.0</version><packaging>jar</packaging><dependencyManagement><dependencies><dependency><groupId>com.test</groupId><artifactId>mytest-bom</artifactId><version>1.0.0</version><type>pom</type><scope>import</scope></dependency></dependencies></dependencyManagement><dependencies><dependency><groupId>com.test</groupId><artifactId>project1</artifactId></dependency><dependency><groupId>com.test</groupId><artifactId>project2</artifactId></dependency></dependencies>
</project>

可以看到，使用的时候，直接导入这个POM，然后按需引用

核心就是在一个公共的pom中定义包的版本和依赖管理，然后在使用的时候导入公共pom

假设现在在做一个项目，其中有两个子项目，在cjs-store-service中需要引用cjs-workflow-provider-api

先定义一个公共的pom

<?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 https://maven.apache.org/xsd/maven-4.0.0.xsd"><modelVersion>4.0.0</modelVersion><groupId>com.example</groupId><artifactId>cjs-dependencies-bom</artifactId><version>1.0.0-SNAPSHOT</version><packaging>pom</packaging><properties><store.version>0.0.4-SNAPSHOT</store.version><workflow.version>0.0.3-SNAPSHOT</workflow.version></properties><dependencyManagement><dependencies><dependency><groupId>com.example</groupId><artifactId>cjs-store-provider-api</artifactId><version>${store.version}</version></dependency><dependency><groupId>com.example</groupId><artifactId>cjs-workflow-provider-api</artifactId><version>${workflow.version}</version></dependency></dependencies></dependencyManagement></project>

项目中引用

<?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 https://maven.apache.org/xsd/maven-4.0.0.xsd"><modelVersion>4.0.0</modelVersion><parent><groupId>com.example</groupId><artifactId>cjs-store</artifactId><version>0.0.4-SNATHOST</version></parent><groupId>com.example</groupId><artifactId>cjs-store-service</artifactId><version>0.0.4-SNATHOST</version><dependencies><dependency><groupId>com.example</groupId><artifactId>cjs-store-dto</artifactId><version>${project.parent.version}</version></dependency><dependency><groupId>com.example</groupId><artifactId>cjs-workflow-provider-api</artifactId></dependency></dependencies><!-- 如果父工程cjs-store的parent不是cjs-dependencies-bom，则这里需要import，否则不需要，因为可以直接通过继承得到 --><dependencyManagement><dependencies><dependency><groupId>com.example</groupId><artifactId>cjs-dependencies-bom</artifactId><version>1.0.0-SNAPSHOT</version><type>pom</type><scope>import</scope></dependency></dependencies></dependencyManagement>
</project>

其实，这种依赖包管理可以学习一下dubbo

5.  项目启动问题

如果你的springboot项目起不来，控制台也不打印任何报错信息，就像这样

那么，你可以在SpringApplication.run()这一行打断点

然后，就可以看到报错了