tomcat启动加载类的顺序

今天在写一个用java发email的程序的一点总结：
在tomcat如果要使用mail session,必须保证activation.jar和mail.jar在同一个级别的classpath tomcat5默认是在${CATALINA_HOME}common/lib/下放了mail.jar,但是没有activation.jar，就这一点害的我今天忙了半天。
下面是tomcat启动加载类的顺序，记住在container环境下，类是有层次的，我今天就是把activation.jar 放在了WEB-INF/lib下，mail.jar在${CATALINA_HOME}common/lib/下,结果邮件怎么也发不出去。

Bootstrap: uses JVM classes & looks in $JAVA_HOME/jre/lib/ext         |      System:    uses $CATALINA_HOME/bin/bootstrap.jar         |       and  $JAVA_HOME/lib/tools.jar         |      Common:    uses $CATALINA_HOME/common/lib        /  \     and $CATALINA_HOME/common/classes       /    \    and $CATALINA_HOME/common/endorsed      /      \   >> jsp and servlet API jars are here     /        +---------------------------------+    /                                           |Catalina:  uses $CATALINA_HOME/server/lib       |           and $CATALINA_HOME/server/classes    |           >> catalina.jar is here              |                                             Shared: uses $CATALINA_BASE/shared/lib                                               / \    and $CATALINA_BASE/shared/classes                                              /   \                                         Webapp1  Webapp2 ... <--- **GREEDY** CLASSLOADERS