maven之父子工程&版本控制

<parent> 标签

用于父子工程项目，什么是父子工程？

顾名思义，maven父子项目是一个有一个父项目，父项目下面又有很多子项目的maven工程，当然，子项目下面还可以添加子项目，从而形成一个树形的结构。那么，这么做有什么好处呢？有以下两点：

• 依赖传递
• 打包方便

父工程package 需要指定pom
子工程需要声明parent，否则无法使用父pom的依赖

<dependency> 标签

导入依赖
和dependencyManagement 一起出现，指定的版本优先级更高，通常出现在父pom

<parent> & <dependency>区别

通常parent用于做结构管理，声明子工程继承父工程的
A依赖B C继承B

A = B的依赖 + B自定义class
C = B的依赖

<dependencyManagement> 标签

版本控制，只声明版本，不进行依赖导入

<modules> 标签

执定子工程，如果不指定，不能识别子工程

版本控制优先级

子项目的版本优先更高
子项目 dependency
子项目 dependencyManagement
父项目 dependency
父项目 dependencyManagement

如果出现同一个pom，多个不同版本的相同依赖，最下方优先级最高

父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 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>org.example</groupId>
    <artifactId>mavenparent</artifactId>
    <packaging>pom</packaging>
    <version>1.0-SNAPSHOT</version>
    <modules>
        <module>mavenson1</module>
        <module>mavenson2</module>
    </modules>

    <properties>
        <maven.compiler.source>8</maven.compiler.source>
        <maven.compiler.target>8</maven.compiler.target>
    </properties>
    <dependencies>
        <dependency>
            <groupId>commons-lang</groupId>
            <artifactId>commons-lang</artifactId>
        </dependency>
        <dependency>
            <groupId>log4j</groupId>
            <artifactId>log4j</artifactId>
            <version>1.2.16</version>
        </dependency>
    </dependencies>
    <dependencyManagement>
        <dependencies>
            <dependency>
                <groupId>commons-lang</groupId>
                <artifactId>commons-lang</artifactId>
                <version>2.1</version>
            </dependency>
            <dependency>
                <groupId>log4j</groupId>
                <artifactId>log4j</artifactId>
                <version>1.2.17</version>
            </dependency>
            <dependency>
                <groupId>org.springframework</groupId>
                <artifactId>spring-core</artifactId>
                <version>5.3.24</version>
            </dependency>
        </dependencies>
    </dependencyManagement>

</project>

子pom1

<?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">
    <parent>
        <artifactId>mavenparent</artifactId>
        <groupId>org.example</groupId>
        <version>1.0-SNAPSHOT</version>
    </parent>
    <modelVersion>4.0.0</modelVersion>

    <artifactId>mavenson1</artifactId>

    <properties>
        <maven.compiler.source>8</maven.compiler.source>
        <maven.compiler.target>8</maven.compiler.target>
    </properties>
    <dependencies>
        <dependency>
            <groupId>commons-lang</groupId>
            <artifactId>commons-lang</artifactId>
<!--            <version>2.5</version>-->
        </dependency>
        <dependency>
            <groupId>log4j</groupId>
            <artifactId>log4j</artifactId>
            <version>1.2.16</version>
        </dependency>
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-core</artifactId>
        </dependency>
        <dependency>
            <groupId>org.example</groupId>
            <artifactId>mavenparent</artifactId>
            <version>1.0-SNAPSHOT</version>
        </dependency>
    </dependencies>
<!--    <dependencyManagement>-->
<!--        <dependencies>-->
<!--            <dependency>-->
<!--                <groupId>commons-lang</groupId>-->
<!--                <artifactId>commons-lang</artifactId>-->
<!--                <version>2.6</version>-->
<!--            </dependency>-->
<!--        </dependencies>-->
<!--    </dependencyManagement>-->

</project>

子pom2

<?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">
    <groupId>org.example</groupId>
    <version>1.0-SNAPSHOT</version>
    <modelVersion>4.0.0</modelVersion>

    <artifactId>mavenson2</artifactId>

    <properties>
        <maven.compiler.source>8</maven.compiler.source>
        <maven.compiler.target>8</maven.compiler.target>
    </properties>
    <dependencies>
        <dependency>
            <groupId>org.example</groupId>
            <artifactId>mavenparent</artifactId>
            <version>1.0-SNAPSHOT</version>
        </dependency>
        <dependency>
            <groupId>commons-lang</groupId>
            <artifactId>commons-lang</artifactId>
            <version>2.5</version>
        </dependency>
    </dependencies>
    <dependencyManagement>
        <dependencies>
            <dependency>
                <groupId>commons-lang</groupId>
                <artifactId>commons-lang</artifactId>
                <version>2.4</version>
            </dependency>
        </dependencies>
    </dependencyManagement>
</project>

``