Every WAR file is considered to be a single module, so all the libs that are in WEB-INF/lib and WEB-INF/classes will be treated the same way. Thus, all classes packaged along with the application will be loaded in the same class loader. For a WAR application, when using Maven, it is very practical to resolve all dependencies; for example, if your application uses the Servlet API, just add the following code on to your project POM:

<dependencies>
  ...
    <dependency>
      <groupId>javax.servlet</groupId>
      <artifactId>servlet-api</artifactId>
      <version>2.5</version>
    </dependency>
  …
</dependencies>

However, if you wish to use the Servlet API provided by the container or by the EAR/lib JARs, you must configure the scope of the dependency...