The reason why the built-in objects of JSP appear in JavaWEB and the detailed explanation of the nine built-in objects

1: Reasons for the appearance of built-in objects

When a JSP page is requested, JSP will be converted by the Web into _jspServlet (essentially: Servlet) , and then call it according to the Servlet calling method

Because of this conversion process, when the dynamic page is accessed, it will be slower for the first time, but it will be faster when it is opened in the future, because the JSP file has been It has been converted and does not need to be converted again.

During the conversion process, 9 objects are automatically created and generated for use by _jspServlet.

2: Detailed explanation of the nine built-in objects

• out

• request

• response

• session

• ##pageContext

• application

• config

• pageex

##ception
Among them, out request response session is more commonly used,

There are four scopes, which is the life cycle of this object, request, session, application, page

3: Detailed explanation of built-in objects

1:out

Type: Javax.servlet.jsp.JspWriter

Function; Mainly used to output data to the client

Scope: page and each Each page has its own out object

Important methods in out: print()/println()/write() Transmitting data to the client

It can be understood this way, a Jsp file is compiled After that, there is a class, and out corresponds to a page object.

2:request

Type: Javax.servlet.http.HttpServletRequest

Description: After the request from the client is processed by the Servlet container, it is encapsulated by the request object. (A communication between the client and the server is a request (sending a request or getting a response).

Scope: request. After this request is completed, its life cycle is over.

Important methods:

getParameter(key) Gets the data of the submitted form

getParameterValues(key) Gets a set of data of the submitted form

request.getRequestDispatcher("list.jsp" ).forward(request,response) Forward (forward through code)

request.setAttribute(key,object) Set the attributes of the request object

request.gettAttribute(key) Get the request Properties of the object

request.setCharacterEncoding("UTF-8") Re-encode the request data

3:resonpse

Type: Javax.servlet.http. HttpServletResponse

Description: It encloses the JSP response and is then sent to the client in response to the client's request.

Scope: page

Method:

response.sendRedirect("页面")：页面跳转。

response.setCharacterEncoding("gbk")：设置响应编码

4:session

Type: Javax.servlet.http.HttpSession

Description: Represents a session, used to save user information in order to track the status of each user.

Definition: refers to a series of related interaction processes between the client and the server within a period of time.

Scope: session.

The client and server may need to continue to process Data interaction (request and response), this process can be understood as a reply. Tomcat's default session time is 30 minutes. If there is no interaction during this period, the session ends; the next time the client sends a request, the session will be re-created. When A session will be created when the client sends a request for the first time. The life cycle of the session is longer than the request.

Conditions for the end of the session: server shutdown, session expiration, manual termination (one of them is enough)

Method:

session.getid(): Get the id number of the session. The id is automatically assigned by tomcat.

session.isnew(): Determine whether the session is newly created

session.setAttribute(key, object): Set an attribute in the current session

session.getAttribute(key): Get an attribute in the current session

session.removeAttribute( key): Delete the attributes in the current session

session.setMaxInactiveInterval(1000*60*30): Set the current session expiration time (ms). Tomcat's default session time is 30 minutes.

session.invalidate(): Initialize the current session object (generally used when launching, you can delete the data of the current session)

5.pageContext

Type: javax. servlet.jsp.PageContext

Description: The page context of this JSP. (Context can be linked to all information on the current page.)

Scope: page

6:config

Type: javax.servlet.ServletConfig

Description: ServletConfig

of this JSP Scope: page

7:page

Type: java.1ang.Object

Description: Implement processing of this page The instance of the currently requested class (javax.servlet.jsp.HttpJspPage), the converted Servlet class itself

Scope: page

8:exception

Type: java.lang.Exception

Description: The exception object of this JSP page

Scope: page

9:

applicationType: javax.servlet.ServletContext

Description: servlet context obtained from the servlet configuration object

Scope: application

The life cycle of this object is the longest. The application object is created when the server starts. It always exists from the time the server exists until the server terminates, and only one object is retained. All users share one application.

Related articles:

JSP’s internal objects_MySQL

JSP basic summary of JavaWeb

Related videos:

JSP Reference Manual

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