用我自己的话来说，你的父项目帮助你提供了两种依赖:

implicit dependencies : all the dependencies defined in the <dependencies> section in your parent-project are inherited by all the child-projects explicit dependencies : allows you to select, the dependencies to apply in your child-projects. Thus, you use the <dependencyManagement> section, to declare all the dependencies you are going to use in your different child-projects. The most important thing is that, in this section, you define a <version> so that you don't have to declare it again in your child-project.

在我看来，<dependencyManagement>(如果我错了请纠正我)只是通过帮助您集中依赖项的版本而有用。它就像一种辅助功能。 作为最佳实践，您的<dependencyManagement>必须在父项目中，其他项目将继承它。一个典型的例子是通过声明Spring父项目来创建Spring项目。

依赖项管理允许合并和集中管理依赖项版本，而不需要添加由所有子继承的依赖项。当你有一组继承共同父项的项目(即多个)时，这尤其有用。

dependencyManagement的另一个极其重要的用例是控制在传递依赖关系中使用的工件的版本。没有例子很难解释。幸运的是，文档中对此进行了说明。

为了便于理解，这里作了解释。 dependencyManagement和dependencies之间的最终区别是声明和添加

The documentation on the Maven site is horrible. What dependencyManagement does is simply move your dependency definitions (version, exclusions, etc) up to the parent pom, then in the child poms you just have to put the groupId and artifactId. That's it (except for parent pom chaining and the like, but that's not really complicated either - dependencyManagement wins out over dependencies at the parent level - but if have a question about that or imports, the Maven documentation is a little better).

After reading all of the 'a', 'b', 'c' garbage on the Maven site and getting confused, I re-wrote their example. So if you had 2 projects (proj1 and proj2) which share a common dependency (betaShared) you could move that dependency up to the parent pom. While you are at it, you can also move up any other dependencies (alpha and charlie) but only if it makes sense for your project. So for the situation outlined in the prior sentences, here is the solution with dependencyManagement in the parent pom:

<!-- ParentProj pom -->
<project>
  <dependencyManagement>
    <dependencies>
      <dependency> <!-- not much benefit defining alpha here, as we only use in 1 child, so optional -->
        <groupId>alpha</groupId>
        <artifactId>alpha</artifactId>
        <version>1.0</version>
        <exclusions>
          <exclusion>
            <groupId>zebra</groupId>
            <artifactId>zebra</artifactId>
          </exclusion>
        </exclusions>
      </dependency>
      <dependency>
        <groupId>charlie</groupId> <!-- not much benefit defining charlie here, so optional -->
        <artifactId>charlie</artifactId>
        <version>1.0</version>
        <type>war</type>
        <scope>runtime</scope>
      </dependency>
      <dependency> <!-- defining betaShared here makes a lot of sense -->
        <groupId>betaShared</groupId>
        <artifactId>betaShared</artifactId>
        <version>1.0</version>
        <type>bar</type>
        <scope>runtime</scope>
      </dependency>
    </dependencies>
  </dependencyManagement>
</project>

<!-- Child Proj1 pom -->
<project>
  <dependencies>
    <dependency>
      <groupId>alpha</groupId>
      <artifactId>alpha</artifactId>  <!-- jar type IS DEFAULT, so no need to specify in child projects -->
    </dependency>
    <dependency>
      <groupId>betaShared</groupId>
      <artifactId>betaShared</artifactId>
      <type>bar</type> <!-- This is not a jar dependency, so we must specify type. -->
    </dependency>
  </dependencies>
</project>

<!-- Child Proj2 -->
<project>
  <dependencies>
    <dependency>
      <groupId>charlie</groupId>
      <artifactId>charlie</artifactId>
      <type>war</type> <!-- This is not a jar dependency, so we must specify type. -->
    </dependency>
    <dependency>
      <groupId>betaShared</groupId> 
      <artifactId>betaShared</artifactId> 
      <type>bar</type> <!-- This is not a jar dependency, so we must specify type. -->
    </dependency>
  </dependencies>
</project>

对不起，我迟到了。

让我尝试使用mvn dependency:tree命令来解释两者的区别

考虑下面的例子

父母POM -我的项目

<modules>
    <module>app</module>
    <module>data</module>
</modules>

<dependencies>
    <dependency>
        <groupId>com.google.guava</groupId>
        <artifactId>guava</artifactId>
        <version>19.0</version>
    </dependency>
</dependencies>

<dependencyManagement>
    <dependencies>
        <dependency>
            <groupId>org.apache.commons</groupId>
            <artifactId>commons-lang3</artifactId>
            <version>3.9</version>
        </dependency>
    </dependencies>
</dependencyManagement>

子POM -数据模块

<dependencies>
    <dependency>
        <groupId>org.apache.commons</groupId>
        <artifactId>commons-lang3</artifactId>
    </dependency>
</dependencies>

子POM -应用程序模块(没有额外的依赖项，因此将依赖项留空)

 <dependencies>
</dependencies>

在运行mvn dependency:tree命令时，我们得到如下结果

Scanning for projects...
------------------------------------------------------------------------
Reactor Build Order:

MyProject
app
data

------------------------------------------------------------------------
Building MyProject 1.0-SNAPSHOT
------------------------------------------------------------------------

--- maven-dependency-plugin:2.8:tree (default-cli) @ MyProject ---
com.iamvickyav:MyProject:pom:1.0-SNAPSHOT
\- com.google.guava:guava:jar:19.0:compile

------------------------------------------------------------------------
Building app 1.0-SNAPSHOT
------------------------------------------------------------------------

--- maven-dependency-plugin:2.8:tree (default-cli) @ app ---
com.iamvickyav:app:jar:1.0-SNAPSHOT
\- com.google.guava:guava:jar:19.0:compile

------------------------------------------------------------------------
Building data 1.0-SNAPSHOT
------------------------------------------------------------------------

--- maven-dependency-plugin:2.8:tree (default-cli) @ data ---
com.iamvickyav:data:jar:1.0-SNAPSHOT
+- org.apache.commons:commons-lang3:jar:3.9:compile
\- com.google.guava:guava:jar:19.0:compile

谷歌guava在每个模块(包括父模块)中都被列为依赖项，而apache commons仅在数据模块中被列为依赖项(甚至不在父模块中)

如果依赖项是在顶级pom的dependencyManagement元素中定义的，子项目就不必显式地列出依赖项的版本。如果子项目确实定义了一个版本，它将覆盖顶层中列出的版本 POM的依赖管理部分。也就是说，dependencyManagement版本仅为 当子进程没有直接声明版本时使用。