pom reference 笔记 二

Inheritance 继承

<packaging>pom</packaging>：对于父项目或聚合（多模块）项目，<packageing>必须为pom

父pom中的以下元素可以被子项目继承：

dependencies developers and contributors plugin lists reports lists plugin executions with matching ids plugin configuration

 

 

 

 

典型的父pom和子pom如下：

 

父pom <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.codehaus.mojo</groupId> <artifactId>my-parent</artifactId> <version>2.0</version> <packaging>pom</packaging> </project> 子pom <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> <parent> <groupId>org.codehaus.mojo</groupId> <artifactId>my-parent</artifactId> <version>2.0</version> <relativePath>../my-parent</relativePath><!-- 相对路劲，非必须，但可以告诉maven，可以在本地查找父pom项目，而不必去仓库--> </parent> <artifactId>my-project</artifactId> </project> 

 

 

和java中所有类的父类都为Object类一样，所有的pom多有最终的super pom，只是我们一直不知道而已，该pom内容可在apache上查阅

 

 

 

除了子类继承父pom的顶级元素，父pom也可以定义顶级元素影响子pom，其中的典型就是依赖管理：

<dependencyManagement><?dependencyManagement>,看看文档中对其作用的解释：

dependencyManagement: is used by POMs to help manage dependency information across all of its children. If the my-parent project uses dependencyManagement to define a dependency on junit:junit:4.0, then POMs inheriting from this one can set their dependency giving the groupId=junit and artifactId=junit only, then Maven will fill in the version set by the parent. The benefits of this method are obvious. Dependency details can be set in one central location, which will propagate to all inheriting POMs. In addition, the version and scope of artifacts which are incorporated from transitive dependencies may also be controlled by specifying them in a dependency management section.

 

 

 

 (or Multi-Module)

 

A project with modules is known as a multimodule, or aggregator project. Modules are projects that this POM lists, and are executed as a group. An pom packaged project may aggregate the build of a set of projects by listing them as modules, which are relative directories to those projects.

<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.codehaus.mojo</groupId>
  <artifactId>my-parent</artifactId>
  <version>2.0</version>
  <packaging>pom</packaging>

  <modules>
    <module>my-project</module>
    <module>another-project</module>
  </modules>
</project>

You do not need to consider the inter-module dependencies yourself when listing the modules, i.e. the ordering of the modules given by the POM is not important. Maven will topologically sort the modules such that dependencies are always build before dependent modules.

 

Properties

Properties are the last required piece in understanding POM basics. Maven properties are value placeholder, like properties in Ant. Their values are accessible anywhere within a POM by using the notation ${X}, where X is the property. They come in five different styles:

1. env.X: Prefixing a variable with "env." will return the shell's environment variable. For example, ${env.PATH} contains the PATH environment variable. Note: While environment variables themselves are case-insensitive on Windows, lookup of properties is case-sensitive. In other words, while the Windows shell returns the same value for %PATH% and %Path%, Maven distinguishes between ${env.PATH} and ${env.Path}. As of Maven 2.1.0, the names of environment variables are normalized to all upper-case for the sake of reliability.
2. project.x: A dot (.) notated path in the POM will contain the corresponding element's value. For example: <project><version>1.0</version></project> is accessible via ${project.version}.
3. settings.x: A dot (.) notated path in the settings.xml will contain the corresponding element's value. For example: <settings><offline>false</offline></settings> is accessible via ${settings.offline}.
4. Java System Properties: All properties accessible via java.lang.System.getProperties() are available as POM properties, such as ${java.home}.
5. x: Set within a <properties /> element. The value may be used as ${someVar}

到这里，pom.xml文件的基础部分就简单看完了，接下来看高级部分