web.xml配置文件的dtd（http://java.sun.com/dtd/web-app_2_3.dtd）
context-param*, filter*, filter-mapping*, listener*, servlet*,
servlet-mapping*, session-config?, mime-mapping*, welcome-file-list?,
error-page*, taglib*, resource-env-ref*, resource-ref*, security-constraint*,
login-config?, security-role*, env-entry*, ejb-ref*, ejb-local-ref*)>
local-home, local, ejb-link?)>
home, remote, ejb-link?)>
env-entry-type)>
filter-class, init-param*)>
resource-env-ref-type)>
res-sharing-scope?)>
auth-constraint?, user-data-constraint?)>
(servlet-class|jsp-file), init-param*, load-on-startup?, run-as?, security-role-ref*)>
url-pattern*, http-method*)>