Learning log --2019 / 09/04

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jsp

EL expression

To simplify our jsp code, the specific point is to simplify those in jsp java code written inside.

• Written format

${表达式 }

If the value from the scope, it will first start taking small scope, if not, take down a scope. He has taken the four scopes are not finished, there is no display.

• how to use

1. Remove four values ​​stored in scope.

<%
			pageContext.setAttribute("name", "page");
			request.setAttribute("name", "request");
			session.setAttribute("name", "session");
			application.setAttribute("name", "application");
		%>
		
		按普通手段取值<br>
		
		<%= pageContext.getAttribute("name")%>
		<%= request.getAttribute("name")%>
		<%= session.getAttribute("name")%>
		<%= application.getAttribute("name")%>
		
		<br>使用EL表达式取出作用域中的值<br>
		
		${ pageScope.name }
		${ requestScope.name }
		${ sessionScope.name }
		${ applicationScope.name }

2. If the field is stored in an array

<%
   			String [] a = {"aa","bb","cc","dd"};
   			pageContext.setAttribute("array", a);
   		%>
   		
   		使用EL表达式取出作用域中数组的值<br>
   		
   		${array[0] } , ${array[1] },${array[2] },${array[3] }

3. If the domain is the set of latches

<br>---------集合数据------------<br>
	<%
		List list=new ArrayList();
		list.add("11");
		list.add("12");
		list.add("13");
		list.add("14");
		pageContext.setAttribute("li", list);
	%>
		使用EL表达式取出作用域中的数组的值<br>
		
		${li[0] },	${li[1] },	${li[2] }	,${li[3] }
		

4. Takes a value set Map

	<%
   		Map map = new HashMap();
   		map.put("name", "zhangsna");
   		map.put("age",18);
   		map.put("address","北京..");
   		
   		map.put("address.aa","深圳..");


   		pageContext.setAttribute("map", map);
   	%>
   	使用EL表达式取出作用域中Map的值<br>
   	
   	${map.name } , ${map.age } , ${map.address }  , ${map["address.aa"] }

Value details:

1. The value from the domain. We have pre-existing value.

从域中取值。  得先存值。
   <%

   	//pageContext.setAttribute("name", "zhangsan");
   	session.setAttribute("name", "lisi...");
   %>

   <br>直接指定说了，到这个作用域里面去找这个name<br>
   ${ pageScope.name } 


   <br>//先从page里面找，没有去request找，去session，去application <br>
   ${ name }

   <br>指定从session中取值<br>
   ${ sessionScope.name }  

2. Value ways

If this value is under the target, then the direct use []

<%
		String [] array = {"aa","bb","cc"}
		session.setAttribute("array",array);
	%>


	${ array[1] } --> 这里array说的是attribute的name 

If there is no index, used directly. The way to get

	<%
		User user = new User("zhangsan",18);
		
		session.setAttribute("u", user);
	%>
	
	${ u.name }  , ${ u.age } 

EL generally used expression, the more used, it is removed from an object attribute value, such as a student's name removed.

EL expressions of the 11 built-in objects.

$ {Object name. Members}

• pageContext

Scope related objects

• pageScope
• requestScope
• sessionScope
• applicationScope

Header information related objects

• header
• headerValues

Parameter information related objects

• param

• paramValues

• cookie
  global initialization parameters

• Initfrm