Tomcat类装载器

context.xml：
<Context>
	<Resource name="mail/session" 
		auth="Container"
		type="javax.mail.Session"
		mail.smtp.host="smtp.sina.com.cn"
		mail.transport.protocol="smtp"
		mail.smtp.auth="true"/>	
</Context>

MailServlet.java：
	Context initCtx = new InitialContext();
	Context envCtx = (Context) initCtx.lookup("java:comp/env");
	Session session = (Session) envCtx.lookup("mail/Session");