dependencyManagement和dependencies之间的区别是什么? 我在Apache Maven网站上看过文档。 在dependencyManagement下定义的依赖项似乎可以在其子模块中使用,而无需指定版本。

例如:

父项目(Pro-par)在dependencyManagement下定义了一个依赖项:

<dependencyManagement>
  <dependencies>
    <dependency>
      <groupId>junit</groupId>
      <artifactId>junit</artifactId>
      <version>3.8</version>
    </dependency>
 </dependencies>
</dependencyManagement>

然后在Pro-par的子函数中,我可以使用junit:

<dependencies>
  <dependency>
    <groupId>junit</groupId>
    <artifactId>junit</artifactId>
  </dependency>
</dependencies>

但是,我想知道是否有必要在父pom中定义junit ?为什么不在需要的模块中直接定义它呢?


当前回答

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)

其他回答

我不建议使用dependencyManagement。

使用它的唯一好处是您可以在父pom中定义版本,而不需要在子pom中再次定义它。但是如果你有一组项目(特别是微服务项目)。使用dependencyManagement没有任何好处。

不同的项目可能需要不同的依赖关系。为什么要从同一个父pom继承它。尽可能保持简单。如果一个项目需要A依赖项,则将其添加到pom文件中。不要让开发人员感到困惑。

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。

就像你说的;dependencyManagement用于将所有依赖关系信息拉入一个公共POM文件,从而简化子POM文件中的引用。

当您有多个不想在多个子项目下重新输入的属性时,它就变得非常有用。

最后,dependencyManagement可用于定义工件的标准版本,以便在多个项目中使用。

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>

有几个答案概述了maven的< dependencies >和<dependencyManagement>标记之间的区别。

但是,下面简要阐述了几点:

<dependencyManagement> allows to consolidate all dependencies (used at child pom level) used across different modules -- clarity, central dependency version management <dependencyManagement> allows to easily upgrade/downgrade dependencies based on need, in other scenario this needs to be exercised at every child pom level -- consistency dependencies provided in <dependencies> tag is always imported, while dependencies provided at <dependencyManagement> in parent pom will be imported only if child pom has respective entry in its <dependencies> tag.