maven dependencyManagement的作用

从Maven的继承开始说起：

假设有两个子模块sub-1和sub-2，其pom文件分别如下所示：

<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>4.0.0modelVersion>

  <groupId>cn.codecrazy.demogroupId>
  <artifactId>demo-sub1artifactId>
  <version>1.0-SNAPSHOTversion>
  <packaging>jarpackaging>

  <dependencies>
    <dependency>
      <groupId>org.springframeworkgroupId>
      <artifactId>spring-contextartifactId>
      <version>4.3.4.RELEASEversion>
    dependency>
    <dependency>
      <groupId>org.springframeworkgroupId>
      <artifactId>spring-beansartifactId>
      <version>4.3.4.RELEASEversion>
    dependency>
    <dependency>
      <groupId>com.alibabagroupId>
      <artifactId>fastjsonartifactId>
      <version>1.2.31version>
    dependency>
    <dependency>
      <groupId>junitgroupId>
      <artifactId>junitartifactId>
      <version>3.8.1version>
      <scope>testscope>
    dependency>
  dependencies>
project>

<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>4.0.0modelVersion>

  <groupId>cn.codecrazy.demogroupId>
  <artifactId>demo-sub2artifactId>
  <version>1.0-SNAPSHOTversion>
  <packaging>jarpackaging>

  <dependencies>
    <dependency>
      <groupId>org.springframeworkgroupId>
      <artifactId>spring-contextartifactId>
      <version>4.3.4.RELEASEversion>
    dependency>
    <dependency>
      <groupId>org.springframeworkgroupId>
      <artifactId>spring-beansartifactId>
      <version>4.3.4.RELEASEversion>
    dependency>
    <dependency>
      <groupId>junitgroupId>
      <artifactId>junitartifactId>
      <version>3.8.1version>
      <scope>testscope>
    dependency>
  dependencies>
project>

可以看到sub1和sub2中都引入了junit、spring-context和spring-beans依赖。并且sub1中还另外引入了fastjson依赖。我们可以利用maven的继承机制，将这些依赖放到parent父模块中，并在子模块中引入节点。

在sub1和sub2的上层目录中新增一个parent模块，其pom文件如下所示：

<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>4.0.0modelVersion>

  <groupId>cn.codecrazy.demogroupId>
  <artifactId>demo-parentartifactId>
  <version>1.0-SNAPSHOTversion>
  <packaging>pompackaging>

  <dependencies>
    <dependency>
      <groupId>org.springframeworkgroupId>
      <artifactId>spring-contextartifactId>
      <version>4.3.4.RELEASEversion>
    dependency>
    <dependency>
      <groupId>org.springframeworkgroupId>
      <artifactId>spring-beansartifactId>
      <version>4.3.4.RELEASEversion>
    dependency>
    <dependency>
      <groupId>com.alibabagroupId>
      <artifactId>fastjsonartifactId>
      <version>1.2.31version>
    dependency>
    <dependency>
      <groupId>junitgroupId>
      <artifactId>junitartifactId>
      <version>3.8.1version>
      <scope>testscope>
    dependency>
  dependencies>
project>

注意节点为pom方式，之后的sub1和sub2的pom文件可简化如下所示：

<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>4.0.0modelVersion>

  <parent>
    <groupId>cn.codecrazy.demogroupId>
    <artifactId>demo-parentartifactId>
    <version>1.0-SNAPSHOTversion>
    <relativePath>../pom.xmlrelativePath>
  parent>

  <artifactId>demo-sub1artifactId>

project>

 <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>4.0.0modelVersion>

  <parent>
    <groupId>cn.codecrazy.demogroupId>
    <artifactId>demo-parentartifactId>
    <version>1.0-SNAPSHOTversion>
    <relativePath>../pom.xmlrelativePath>
  parent>

  <artifactId>demo-sub2artifactId>

project>

可以看到sub1和sub2模块的pom文件配置大大简化了，由于配置了parent节点，子模块的依赖从父模块得到了继承，即使在子模块中不配置节点，子模块也引入了它的依赖。

但是上述配置存在一个问题，sub2中并不需要fastjson依赖，但由于继承，也同样从父模块中继承了fastjson依赖。换句话说，以后继承自该parent模块的其他子模块也许并不需要parent模块中声明的所有依赖，但是就目前这种配置方式，父模块中的这些依赖都会被无条件的继承，这显然是不合理的，这就需要用到节点。

节点中配置的依赖并不会真正的引入依赖，但是该节点却能够被子模块继承，要在子模块中真正引入依赖，需要将依赖配置在节点下，而这种方式配置的依赖可以省略节点，使用的即是在节点中配置的相应。采用了节点的父模块pom文件如下：

<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>4.0.0modelVersion>

  <groupId>cn.codecrazy.demogroupId>
  <artifactId>demo-parentartifactId>
  <version>1.0-SNAPSHOTversion>
  <packaging>pompackaging>
  <dependencyManagement>
    <dependencies>
      <dependency>
        <groupId>org.springframeworkgroupId>
        <artifactId>spring-contextartifactId>
        <version>4.3.4.RELEASEversion>
      dependency>
      <dependency>
        <groupId>org.springframeworkgroupId>
        <artifactId>spring-beansartifactId>
        <version>4.3.4.RELEASEversion>
      dependency>
      <dependency>
        <groupId>com.alibabagroupId>
        <artifactId>fastjsonartifactId>
        <version>1.2.31version>
      dependency>
      <dependency>
        <groupId>junitgroupId>
        <artifactId>junitartifactId>
        <version>3.8.1version>
        <scope>testscope>
      dependency>
    dependencies>
  dependencyManagement>
project>

sub1和sub2模块pom文件相应如下所示：

<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>4.0.0modelVersion>

  <parent>
    <groupId>cn.codecrazy.demogroupId>
    <artifactId>demo-parentartifactId>
    <version>1.0-SNAPSHOTversion>
    <relativePath>../pom.xmlrelativePath>
  parent>

  <artifactId>demo-sub1artifactId>

  <dependencies>
    <dependency>
      <groupId>org.springframeworkgroupId>
      <artifactId>spring-contextartifactId>
    dependency>
    <dependency>
      <groupId>org.springframeworkgroupId>
      <artifactId>spring-beansartifactId>
    dependency>
    <dependency>
      <groupId>com.alibabagroupId>
      <artifactId>fastjsonartifactId>
    dependency>
    <dependency>
      <groupId>junitgroupId>
      <artifactId>junitartifactId>
      <scope>testscope>
    dependency>
  dependencies>
project>

<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>4.0.0modelVersion>

  <parent>
    <groupId>cn.codecrazy.demogroupId>
    <artifactId>demo-parentartifactId>
    <version>1.0-SNAPSHOTversion>
    <relativePath>../pom.xmlrelativePath>
  parent>

  <artifactId>demo-sub2artifactId>

  <dependencies>
    <dependency>
      <groupId>org.springframeworkgroupId>
      <artifactId>spring-contextartifactId>
    dependency>
    <dependency>
      <groupId>org.springframeworkgroupId>
      <artifactId>spring-beansartifactId>
    dependency>
    <dependency>
      <groupId>junitgroupId>
      <artifactId>junitartifactId>
      <scope>testscope>
    dependency>
  dependencies>
project>

可以看到在sub2的pom文件中没有引入fastjson依赖，这样sub2就不会真正的引入fastjson依赖。

节点的另一个好处是，使得多个子模块中引入的相同依赖版本号能够保持一致。

如何引入另外的dependencyManagement？

假设在某个模块demo-other中也有节点管理着一批依赖，现在想在demo-parent模块中将那些依赖管理全部导入，把demo-other模块中的依赖全部复制到demo-parent模块的节点下是可行但是非常不优雅的做法。

比较优雅的做法是用到import依赖范围，即import。demo-parent的pom文件如下所示：

<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>4.0.0modelVersion>

  <groupId>cn.codecrazy.demogroupId>
  <artifactId>demo-parentartifactId>
  <version>1.0-SNAPSHOTversion>
  <packaging>pompackaging>
  <dependencyManagement>
    <dependencies>
      <dependency>
        <groupId>cn.codecrazy.demogroupId>
        <artifactId>demo-otherartifactId>
        <version>1.0-SNAPSHOTversion>
        <type>pomtype>
        <scope>importscope>
      dependency>
      <dependency>
        <groupId>org.springframeworkgroupId>
        <artifactId>spring-contextartifactId>
        <version>4.3.4.RELEASEversion>
      dependency>
      <dependency>
        <groupId>org.springframeworkgroupId>
        <artifactId>spring-beansartifactId>
        <version>4.3.4.RELEASEversion>
      dependency>
      <dependency>
        <groupId>com.alibabagroupId>
        <artifactId>fastjsonartifactId>
        <version>1.2.31version>
      dependency>
      <dependency>
        <groupId>junitgroupId>
        <artifactId>junitartifactId>
        <version>3.8.1version>
        <scope>testscope>
      dependency>
    dependencies>
  dependencyManagement>
project>

注意多出来的部分：

<dependency>
    <groupId>cn.codecrazy.demogroupId>
    <artifactId>demo-otherartifactId>
    <version>1.0-SNAPSHOTversion>
    <type>pomtype>
    <scope>importscope>
dependency>