Class [oracle.jdbc.OracleDriver] not found

I try to test Vaadin JPAContainer demo ‘jpacontainer-addressbook-demo’ to connect to oracle. But I met exception.

com.vaadin.server.ServiceException: javax.persistence.PersistenceException: Exception [EclipseLink-4003]
 (Eclipse Persistence Services - 2.2.0.v20110202-r8913): org.eclipse.persistence.exceptions.DatabaseException Exception Description: Configuration error. Class [oracle.jdbc.OracleDriver]
 not found.

My persistence.xml

<persistence-unit name="oracle">
 <provider>org.eclipse.persistence.jpa.PersistenceProvider</provider>
 <exclude-unlisted-classes>false</exclude-unlisted-classes>
 <properties>
  <property name="javax.persistence.jdbc.url" value="jdbc:oracle:thin:@localhost:1521:orcl"/>
  <property name="javax.persistence.jdbc.user" value="xxx"/>
  <property name="javax.persistence.jdbc.password" value="xxx"/>
  <property name="javax.persistence.jdbc.driver" value="oracle.jdbc.OracleDriver"/>
 </properties>
</persistence-unit>

I have put ojdbc6.jar to WEB-INF/lib. And I cound found ojdbc6.jar in WEB-INF\lib\ of jpacontainer-addressbook-demo-3.2.0.war. I dont understand why it have this exception. Any hints would be appreciated.

I found the solution by following this page.
http://www.mkyong.com/maven/how-to-add-oracle-jdbc-driver-in-your-maven-local-repository/

But I dont know why cant just directly put jar file to lib folder. Could any one help to explain? Thanks.