在POM文件中，依赖是常见的，如下：

<dependency>
	<groupId>junit</groupId>
	<artifactId>junit</artifactId>
	<version>4.8.1</version>
	<scope>test</scope> <!--只在测试时有效，编译和运行时无效-->
</dependency>

而 dependency 中的 scope 表示的是依赖的包的作用范围

它的值可以为：

compile：默认值，表示在 编译，运行，测试 阶段均会用到。

provided：编译和测试 时有效，运行时则没有此依赖包，运行时此包由 运行环境（tomcat）提供，比如：servlet-api

runtime：仅运行时有效

test：只在测试阶段有效，其他阶段用不到。

这里的 pom.xml 知识所属为 Maven 的了解。

官方解释地址：http://maven.apache.org/guides/introduction/introduction-to-dependency-mechanism.html#Dependency_Scope 

Dependency Scope

Dependency scope is used to limit the transitivity of a dependency, and also to affect the classpath used for various build tasks.

There are 6 scopes available:

• compile
  This is the default scope, used if none is specified. Compile dependencies are available in all classpaths of a project. Furthermore, those dependencies are propagated to dependent projects.

• provided
  This is much like compile, but indicates you expect the JDK or a container to provide the dependency at runtime. For example, when building a web application for the Java Enterprise Edition, you would set the dependency on the Servlet API and related Java EE APIs to scope provided because the web container provides those classes. This scope is only available on the compilation and test classpath, and is not transitive.

• runtime
  This scope indicates that the dependency is not required for compilation, but is for execution. It is in the runtime and test classpaths, but not the compile classpath.

• test
  This scope indicates that the dependency is not required for normal use of the application, and is only available for the test compilation and execution phases. This scope is not transitive.

• system
  This scope is similar to provided except that you have to provide the JAR which contains it explicitly. The artifact is always available and is not looked up in a repository.

• import
  This scope is only supported on a dependency of type pom in the <dependencyManagement> section. It indicates the dependency to be replaced with the effective list of dependencies in the specified POM's <dependencyManagement> section. Since they are replaced, dependencies with a scope of import do not actually participate in limiting the transitivity of a dependency.