1. instruction
	*Function: used to configure JSP pages and import resource files
	* format:
		<% @ instruction name property name 1 = property value 1 property name 2 = property value 2...% >
	* classification:
		1. page: configure the
			*contentType: equivalent to response.setContentType()
				1. Set mime type and character set of response body
				2. Set the encoding of the current jsp page (only advanced IDE can take effect. If you use low-level tools, you need to set the pageEncoding property to set the character set of the current page)
			*Import: import package
			*errorPage: when an exception occurs to the current page, it will automatically jump to the specified error page
			*isErrorPage: identifies whether or not it is currently an error page.
				*true: Yes, you can use the built-in object exception
				*false: No. Default value. Built in object exception is not allowed


		2. include: included in the page. Import resource file for page
			* <%@include file="top.jsp"%>
		3. taglib: import resources
			* <%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>
				*Prefix: prefix, custom
 2. note:
	1. html comment:
		<! ---- >: only html snippets can be annotated
	2. jsp comment: Recommended
		[% ---%]: all can be annotated


3. Built in objects
	*Objects that do not need to be created and used directly in jsp pages
	*There are nine:
			Variable name real type action
		*PageContext pagecontext the current page shares data, and you can get eight other built-in objects
		*Request HttpServletRequest multiple resources (forwarding) accessed by one request
		*Session HttpSession multiple requests in a session
		*application ServletContext data sharing among all users
		*Response HttpServletResponse response object
		*Page Object the Object of the current page (Servlet)
		*out JspWriter output object, data output to page
		*config ServletConfig Servlet configuration object
		*Exception Throwable exception object

1. Concept: Expression Language
 2. Function: replace and simplify the writing of java code in jsp page
 3. Syntax: ${expression}
4. note:
	*jsp supports el expression by default. If you want to ignore el expressions
		1. Set: isELIgnored="true" in the page instruction of jsp to ignore all el expressions in the current jsp page
		2. \ ${expression}: ignore the current el expression


5. use:
	1. operations:
		*Operator:
			1. Arithmetic operator: + - * / (DIV)% (MOD)
			2. Comparison operator: >=
			3. Logical operator: & & (and) | (or)! (not)
			4. Air transport operator: empty
				*Function: used to judge whether the string, collection, array object is null or the length is 0
				*${empty list}: judge whether the string, collection, array object is null or the length is 0
				*${not empty str}: indicates whether the judgment string, collection, array object is not null and the length is > 0
	2. get value
		1. el expression can only get value from domain object
		2. syntax:
			1. ${domain name. Key name}: get the value of the specified key from the specified domain
				*Domain name:
					1. pageScope		--> pageContext
					2. requestScope 	--> request
					3. sessionScope 	--> session
					4. applicationScope --> application(ServletContext)
				*For example: name = Zhang San is stored in the request domain
				*Get: ${requestScope.name}

			2. ${key name}: indicates whether there is a value corresponding to the key from the smallest field until it is found.

			
			
			3. Get the value of object, List set and Map set
				1. Object: ${domain name. Key name. Property name}
					*In essence, it will call the getter method of the object

				2. List set: ${domain name. Key name [index]}

				3. Map set:
					*${domain name. key name. key name}
					*${domain name. Key name ["key name"]}


	3. Implicit object:
		*11 implicit objects in el expression
		* pageContext: 
			*Get eight other jsp built-in objects
				*${pageContext.request.contextPath}: get virtual directory dynamically

1. Concept: JavaServer pages tag library JSP standard tag library
	*Is an open source free jsp tag provided by Apache organization < tag >

2. Function: used to simplify and replace java code on jsp page		

3. Operation steps:
	1. Import jstl related jar package
	2. Import tag library: taglib instruction: <% @ taglib% >
	3. Use label

4. Common JSTL Tags
	1. if: the if statement equivalent to java code
		1. properties:
            *test must be attribute, accept boolean expression
                *If the expression is true, the if label body content is displayed; if it is false, the label body content is not displayed
                *In general, the test attribute value is used in combination with the el expression
   		 2. note:
       		 *The c:if tag has no else situation. If you want else situation, you can define a c:if tag
	2. choose: switch statement equivalent to java code
		1. Using choose label declaration is equivalent to switch declaration
        2. Using the when tag for judgment is equivalent to case
        3. Using the other way tag to make other statements is equivalent to default

	3. foreach: for statement equivalent to java code

5. exercise:
	*Requirement: there is a list collection of User objects in the request domain. You need to use jstl+el to display the list set data to the table of the jsp page

<%@ page contentType="text/html;charset=UTF-8" language="java" %>

<%@taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>

<html>
<head>
    < title > choose tab < / Title >
</head>
<body>
    <%--
        Complete the case of number number corresponding to the day of the week
            1. Store a number in the domain
            2. Using the choose tag to extract the number is equivalent to the switch declaration
            3. Using when tag to make digital judgment is equivalent to case
            4. Declaration of the other case label is equivalent to default
    --%>
    <%
        request.setAttribute("number",51);
    %>
    <c:choose>
        < C: when test = "${number = = 1}" > Monday < / C: when >
        < C: when test = "${number = = 2}" > Tuesday < / C: when >
        < C: when test = "${number = = 3}" > Wednesday < / C: when >
        < C: when test = "${number = = 4}" > Thursday < / C: when >
        < C: when test = "${number = = 5}" > Friday < / C: when >
        < C: when test = "${number = = 6}" > Saturday < / C: when >
        < C: when test = "${number = = 7}" > Sunday < / C: when >
        < C: otherwise > wrong digital input
    </c:choose>
</body>
</html>

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