JSP built-in object scope

• page
• request
• session
• application

Common methods:
setAttribute, getAttribute

page scope: the corresponding built-in object is pageContext
request scope: the corresponding built-in object is request
session scope: the corresponding built-in object is session
application scope: the corresponding built-in object is application.

范围：
page<request<session<application

page is valid only on the current page,
request is valid in one request,
session is valid in one session,
application corresponds to the entire WEB application