？表示可选，最多一个；*表示0个或多个。
web-app
(icon?,
display-name?,
description?,
distributable?,
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*)