Maven中settings.xm文件配置经验（持续更新中）

Maven中的settings.xml文件默认是在MAVEN_HOME/conf目录下的，用于配置全局maven行为；但是，一般用户都需要复制一份到用户目录/.m2下，以用其配置本用户的maven行为，这是一条最佳实践。

settings.xm的配置其实在settings.xml文件内部已经有详细的说明，本文主要针对个别细节特别强调下，因为如果不注意这些细节，那么狠容易碰到一些莫名其妙的问题。

1、关于profiles的配置

文件中关于profiles的配置有如下描述：

<!-- profiles
| This is a list of profiles which can be activated in a variety of ways, and which can modify
| the build process. Profiles provided in the settings.xml are intended to provide local machine-
| specific paths and repository locations which allow the build to work in the local environment.
|
| For example, if you have an integration testing plugin - like cactus - that needs to know where
| your Tomcat instance is installed, you can provide a variable here such that the variable is 
| dereferenced during the build process to configure the cactus plugin.
|
| As noted above, profiles can be activated in a variety of ways. One way - the activeProfiles
| section of this document (settings.xml) - will be discussed later. Another way essentially
| relies on the detection of a system property, either matching a particular value for the property,
| or merely testing its existence. Profiles can also be activated by JDK version prefix, where a 
| value of '1.4' might activate a profile when the build is executed on a JDK version of '1.4.2_07'.
| Finally, the list of active profiles can be specified directly from the command line.
|
| NOTE: For profiles defined in the settings.xml, you are restricted to specifying only artifact
|       repositories, plugin repositories, and free-form properties to be used as configuration
|       variables for plugins in the POM.
|
|-->

注意NOTE部分，大体意思是：

注意：对于在settings.xml中定义的profiles，将被限制为只能定义构件仓库（artifact repositories），插件仓库（plugin repositories），和自由形式（free-form）的属性，用作对POM中（使用的）插件的配置变量。

言下之意，只能在settings.xml的profiles里配置仓库和属性，不能配置其他的东西，如配置plugins，配置build，配置project等。

如，试图在settings.xml的profiles中增加如下profile，将会在maven执行mvn clean命令时报错：

<profile>
	<id>cleanC</id>
	
	<activation>
		<activeByDefault>true</activeByDefault>
	</activation>
	
	<build>
		<plugins>
			<plugin>  
				<artifactId>maven-clean-plugin</artifactId>  
				<configuration>  
					<verbose>true</verbose>  
					<filesets>  
						<fileset>  
							<directory>c:/a</directory>  
						</fileset>  
				  </filesets>  
				</configuration>  
			</plugin>
		</plugins>
	</build>
</profile>

出错原因就是不能在settings.xml中使用profiles配置插件。经试验，如果不在profiles中配置plugin，还是会报同样的错，看来还是不要在settings.xml中配置plugin了~