我們在用maven和eclipse開發WEB應用的時候，需要把servlet-api和jsp-api加入進來，要不然編譯不會通過，加入進來之后在打包的時候maven自動把所有的依賴包都放到lib下面，如果你在tomcat下面運行就會有問題，因為tomcat發現你的web應用的lib中包含了servlet-api，他會報錯。
解決這個問題的方法就是使用<scope>標簽，如下
<dependency>
<groupId>javax.servlet</groupId>
<artifactId>servlet-api</artifactId>
<version>2.4</version>
<scope>provided</scope>
</dependency>
<dependency>
<groupId>javax.servlet</groupId>
<artifactId>jsp-api</artifactId>
<version>2.0</version>
<scope>provided</scope>
</dependency>
這里的意思是說，編譯的時候用到servlet-api和jsp-api，但在打包的時候不用這兩個依賴。
在maven的官方中是這樣描述的
Dependency Scope
Dependency scope is used to limit the transitivity of a depedency, 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.
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 (only available in Maven 2.0.9 or later)
This scope is only used on a dependency of type pom in the <dependencyManagement> section. It indicates that the specified POM should be replaced with the dependencies in that 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.
如果你這樣做了，但使用eclipse+tomcat做測試的時候發現servlet-api還是被打包到lib下面了，你要把maven插件中的WTP也安裝一下，問題應該就解決了。