<!-- 子模块继承之后，提供作用：锁定版本 + 子modlue不用写groupId和version -->
<dependencyManagement>
    <dependencies>
      <!-- spring boot 2.2.2 -->
      <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-dependencies</artifactId>
        <version>2.2.2.RELEASE</version>
        <type>pom</type>
        <scope>import</scope>
      </dependency>
      <!-- spring cloud Hoxton.SR1-->
      <dependency>
        <groupId>org.springframework.cloud</groupId>
        <artifactId>spring-cloud-dependencies</artifactId>
        <version>Hoxton.SR1</version>
        <type>pom</type>
        <scope>import</scope>
      </dependency>
      <!--spring cloud alibaba 2.1.0-->
      <dependency>
        <groupId>com.alibaba.cloud</groupId>
        <artifactId>spring-cloud-alibaba-dependencies</artifactId>
        <version>2.1.0.RELEASE</version>
        <type>pom</type>
        <scope>import</scope>
      </dependency>
      <dependency>
        <groupId>mysql</groupId>
        <artifactId>mysql-connector-java</artifactId>
        <version>${mysql.version}</version>
      </dependency>
      <dependency>
        <groupId>com.alibaba</groupId>
        <artifactId>druid</artifactId>
        <version>${druid.version}</version>
      </dependency>
      <dependency>
        <groupId>org.mybatis.spring.boot</groupId>
        <artifactId>mybatis-spring-boot-starter</artifactId>
        <version>${mybatis.spring.boot.version}</version>
      </dependency>
      <dependency>
        <groupId>org.projectlombok</groupId>
        <artifactId>lombok</artifactId>
        <version>${lombok.version}</version>
      </dependency>
    </dependencies>
  </dependencyManagement>

• dependencyManagement只是声明依赖，不会下载依赖，主要作用是：管理项目依赖
• 如果dependencies里的dependency没有version元素，会寻找dependencyManagement标签里的依赖申明
• 如果dependencies里的dependency有version元素，就使用dependency中的version元素
• 子项目会全部继承夫项目中的dependencies元素
• dependencyManagement只有在子项目中写了该依赖项，并且没有指定具体版本，才会从父项目中继承该项