"Go" JSP built-in object details (top)

First question: How many types of built-in objects do JSPs have?
To answer this question, first of all, the basic knowledge and technology of JSP have a deep understanding and mastery. There are nine main
Built-in Objects: Application object, config object, exception object, out object, Page object, PageContext object, Request object, Reponse object, Session object.
The second question: What types of JSP built-in objects are they? What are the roles of these objects? What are their common methods?
(1) Application object: an example of Javax.servlet.ServletContext. This instance represents the Web application itself that the JSP belongs to and can be used to exchange information between JSPs or Servlets. The common methods are:

Method	return value type	Method description
Getmajorversion ()	Int	Get the main servlet API version
Getminorversion ()	Int	Get the minor servlet API version
Getserverinfo ()	String	Get Server version
GetMimeType ()	String	Gets the MIME type of the specified file
GetContext ()	ServerContext	Gets the application context for the specified local
Getrealpath	String	Gets the absolute path of the specified path
Getinitparameter (name)	String	Get servlet initialization parameters

Getinitparameter (String paramname) is used to obtain the configuration parameters for the Web application, which should be configured with the Context-param element in the. xml file, each of the <context-param: The/> element configures a parameter.
Note: Each Web App has only one ServletContext instance, and the JSP page can access the instance through application built-in objects, which in the servlet must be obtained by code: ServletContext sc= Getservletconfig (). Get ServletContext ();
(2) Config object: javax.servlet.ServletConfig instance, which represents the configuration information of JSP, in fact, JSP page usually does not need to be configured, there is no configuration information. More for use in Servlets.

Method	return value type	Method description
Getinitparameter (name)	String	Get servlet initialization parameters
Getinitparameternames ()	Enumeration	Get all initialization parameter names of the servlet
Getservletcontext ()	ServletContext	Get Current application context
Getservletname ()	String	Get the servlet name

(3) Exception object: java.lang.Throwable instance, this property can only be used if the page is an error handling page, that is, the Iserrorpage property of the compile instruction page is true.
(4) Out object: javax. An instance of Servlet.jsp.JspWriter that represents the output stream of a JSP page, which is used to output content and form an HTML page. The essence of <%=%> expression is the Out.writer method.

Method	return value type	Method description
Clear ()	void	Clear the output on a webpage
Clearbuffer ()	void	Clear Buffer Contents
Close ()	void	Close buffer, clear all contents
GetBufferSize ()	Int	Get buffer size
Getremaining ()	Int	Get buffer remaining size
Isautofulsh ()	Boolean	Get information about whether the buffer is automatically purged
Print (String str)	void	For page output
println (String str)	void	Page output and line wrapping

(5) Page object: Represents the page itself, usually does not have much effect. This is the this in the servlet.
(6) PageContext object: an instance of PageContext that represents the JSP page context, which can be used to access shared data for a page. Common methods are: Getservletcontext and Getservletconfig methods. See below for a detailed description.
(7) Request object: an instance of javax.servlet.http.HttpServletRequest that encapsulates a request and the client's request parameters are encapsulated inside the object. The methods for obtaining parameters are:

Method	return value type	Method description
GetParameter (String name)	String	Gets the parameter value named name for the parameter
Getparameternames ()	Enumeration	Gets the name of all parameters, which can be used in combination with the previous method to get the values of all parameters
Getparametervalues (String name)	String[]	Gets all parameters named name of the parameter, such as the parameter is multiple checkboxes
Getparametermap ()	Map	Gets the map instance encapsulated by all parameters, and returns an array of values with the corresponding parameter named ID through the map instance's string[] get ("id") method

The other methods are:

Method	return value type	Method description
GetHeader (String name)	String	Gets the header named name for the specified header
Getheadername ()	Enumeration	Get all the header names
Getintheader (String name)	Int	Gets the header named name, with the content returned as an integer type
Getdateheader (String name)	Long	Gets the header named name, with the content returned as a date type
GetCookies ()	Cookies	Get the relevant cookie
Getcontextpath ()	String	Get the path to the context
GetMethod ()	String	Get how clients are submitted
Getprotocol ()	String	Get the HTTP protocol used
GetQueryString ()	String	Gets the requested string
Getrequestsessionid ()	String	Gets the session ID of the client
Getrequesturi ()	String	Gets the requested URI
GETREMOTEADDR ()	String	Get Client IP Address

(8) Reponse object: an instance of Javax.servlet.http.HttpServletReponse that represents the response of the server to the client, usually using the object directly, rather than the out object, unless a non-character response needs to be generated. The response object is often used for redirection, and the commonly used method is the Getoutstream,sendredirect method.

Method	return value	Method description
Addcookie (Cookie cookie)	void
Adddateheader (String name,long date)	void
AddHeader (String name,string value)	void
Addintheader (String name,int value)	void
Setdateheader (String name,long date)	void
SetHeader (String name,string value)	void
Setintheader (String name,int value)	void
Senderror (int SC)	void	Transfer Status Code
Senderror (int sc,string msg)	void	Transfer status codes and error messages
SetStatus (int SC)	void	Set the status code
Sendredirect (String URL)	void	Page redirection for page jumps

Note: Here the Response object's Sendredirect (String URL) method Setting page redirection will change the browser address bar information, so also called the client jump.
The instance 1:response object implements the automatic refresh of the page: just the early JSP page plus
<%– Use the Setintheader of the response object to set the value of the refresh (in seconds) to automate the page refresh –%> <% response.addintheader ("Refresh", 10); %>
Example 2: Implementation of automatic page jump: You can use the Response object's SetHeader () method to add a header titled Refresh, and set the page jump time and jump page, so that the page automatically jump. <% Response.setheader ("Refresh", "10; Url=http://www.baidu.com "); %>
Here, use the SetHeader method to add a header titled "Refresh" with a value of "10,url=http://www.baidu.com".
(9) Session object: Javax.servlet.http.HttpServletReponse instance, represents a reply. When the client browser establishes a connection to the site, the session begins, and when the client closes the browser, the reply ends.

Method	return value type	Method description
GetId ()	String	Gets the ID of the session
GetCreationTime ()	Long	Gets the session's build time
Getlashaccessedtime ()	Long	Get user last Send request time via session
Getmaxinactiveinterval ()	Long	Gets the session life cycle, which expires if this time is exceeded
Invalidate ()	void	Clear Session Contents
IsNew ()	Boolean	Determine if the session is "new"
Setmaxinactiveinterval ()	void	Sets the session life cycle, which expires if this time is exceeded

The third question: Under what circumstances are JSP built-in objects generated?
A JSP page corresponds to a servlet class with three methods per servlet class:
Init method: Initializes the Jsp/servlet method.
Destory method: Destroys the Jsp/servlet method.
Service method: The method that responds to a user request.
The request object and the Reponse object are the parameters of the service method, the Application object, the Page object, the Out object, the PageContext object, and the session object are the local variables generated within the service's method.

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