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El expression

2022-04-21 11:23:00 【Carbon water carbon water】

Tips

${ expression }

Expression Language, Expression language
effect ： Instead of jsp The expression script in the page is in jsp Data output in the page , Relatively speaking, it is more concise
EL The expression is outputting null When it's worth it , Output Empty string

jsp Expression script output null When it's worth it , Output null character string .
jsp All four domain objects in the store have the same name key variable , According to the four fields From small to large Search in the order of , Find it and output it , Except in special situations , Otherwise, only request

EL When an expression gets a value , Not looking for property name , Instead, find the attribute Corresponding get Method , Even if not This attribute , Yes get The method can be

obtain	expression	result / remarks
Object properties	${p.name}
Array	${p.phones}	Address of array
An attribute value of the array	${p.phones[2]}
List aggregate	${p.cities}	[ Elements 1, Elements 2, Elements 3, Elements 4]
List Individual element values in the collection	${p.cities[2]}
Map aggregate	${p.map}	{key1=value1,key2=value2,key3=value3}
Map One of the groups key Value	${p.map.key}

operation

Relationship between operation

Relational operator	say bright	Fan example	result
== or eq	be equal to	${ 5 == 5 } or ${ 5 eq 5 }	true
!= or ne	It's not equal to	${ 5 !=5 } or ${ 5 ne 5 }	false
< or lt	Less than	${ 3 < 5 } or ${ 3 lt 5 }	true
> or gt	Greater than	${ 2 > 10 } or ${ 2 gt 10 }	false
<= or le	Less than	be equal to ${ 5 <= 12 } or ${ 5 le 12 }	true
>= or ge	Greater than	be equal to ${ 3 >= 5 } or ${ 3 ge 5 }	false

Logical operations

Logical operators	say bright	Fan example	result
&& or and	And operation	${ 12 == 12 && 12 < 11 } or ${ 12 == 12 and 12 < 11 }	false
\|\| or or	Or operations	${ 12 == 12 \|\| 12 < 11 } or ${ 12 == 12 or 12 < 11 }	true
! or not	Reverse operation	${ !true } or ${not true }	false

Arithmetic operation

Arithmetic operator	say bright	Fan example	result
+	Add	${ 12 + 18 }	30
-	Subtraction	${ 18 - 8 }	10
*	Multiplication	${ 12 * 12 }	144
/	or div division	${ 144 / 12 } or ${ 144 div 12 }	12
%	or mod modulus	${ 144 % 10 } or ${ 144 mod 10 }	4

empty operation

You can judge whether a data is empty , If it is empty , The output true, Not empty output false

The following situations are empty ：

The value is null When it's worth it , It's empty
When the value is an empty string , It's empty
The value is Object Type array , When the length is zero
list aggregate , The number of elements is zero
map aggregate , The number of elements is zero

The ternary operation

Same as java The ternary operation

. operation and [] Operator

. Point operation ： Can output Bean The value of an attribute in an object
[] In brackets , You can output the value of an element in an ordered collection , Single and double quotation marks are OK

also [] In brackets , You can also output map Collection key Containing special characters key Value .

request.setAttribute("abc.def", "zhangsan");
// use . Can't value 
//${requestScope.abc.def}
${
    requestScope["abc.def"]}

<body>
<%
	Map<String,Object> map = new HashMap<String, Object>();
	map.put("a.a.a", "aaaValue");
	map.put("b+b+b", "bbbValue");
	map.put("c-c-c", "cccValue");
	request.setAttribute("map", map);
%>
	${ map['a.a.a'] } <br>
	${ map["b+b+b"] } <br>
	${ map['c-c-c'] } <br>
</body>

Implied objects

11 A hidden object

Variable	type	effect
pageContext	PageContextImpl	It can get jsp Nine built-in objects in
pageScope	Map<String,Object>	It can get pageContext Data in domain
requestScope	Map<String,Object>	It can get Request Data in domain
sessionScope	Map<String,Object>	It can get Session Data in domain
applicationScope	Map<String,Object>	It can get ServletContext Data in domain
param	Map<String,String>	It can get the value of the request parameter
paramValues	Map<String,String[]>	It can also get the value of the request parameter , Use when getting multiple values .
header	Map<String,String>	It can get the information of the request header
headerValues	Map<String,String[]>	It can get the information of the request header , It can get multiple values
cookie	Map<String,Cookie>	It can get the current request Cookie Information
initParam	Map<String,String>	It can be obtained in web.xml The context parameters configured in the

pageContext

obtain jsp Nine built-in objects in

Can directly pageContext. object . Method , It's fine too pageContext.setAttribute Name the object and use it directly

<% pageContext.setAttribute("req", request); %> 
<%=request.getScheme() %> 
 1. agreement ： ${ req.scheme }
 2. The server  ip：${ pageContext.request.serverName }
 3. Server port ：${ pageContext.request.serverPort }
 4. Get the project path ：${ pageContext.request.contextPath }
 5. Get request method ：${ pageContext.request.method }
 6. Get the client  ip  Address ：${ pageContext.request.remoteHost }
 7. Get the  id  Number ：${ pageContext.session.id }

Get properties in four specific domains

EL expression	Domain
pageScope	pageContext Domain
requestScope	Request Domain
sessionScope	Session Domain
applicationScope	ServletContext Domain

<% 
	pageContext.setAttribute("key", "pageContext"); 		
	request.setAttribute("key", "request"); 
	session.setAttribute("key", "session"); 
	application.setAttribute("key", "application"); 
%> 
	${pageScope.key}
	${requestScope.key}
	${sessionScope.key}
	${applicationScope.key}

param And paramValues

 Output request parameters  username  Value ：${ param.username } 
 Output request parameters  password  Value ：${ param.password } 
 Output request parameters  username  Value ：${ paramValues.username[0] } 
 Output request parameters  hobby  Value ：${ paramValues.hobby[0] } 
 Output request parameters  hobby  Value ：${ paramValues.hobby[1] }

header And headerValues

 Output request header 【User-Agent】 Value ：${ header['User-Agent'] } <br>
 Output request header 【Connection】 Value ：${ header.Connection } <br>
 Output request header 【User-Agent】 Value ：${ headerValues['User-Agent'][0] } <br>

cookie

 obtain  Cookie  The name of ：${ cookie.JSESSIONID.name } 
 obtain  Cookie  Value ：${ cookie.JSESSIONID.value }

initParam

obtain web.xml Configuration in ：

<context-param>
	<param-name>username</param-name>
	<param-value>root</param-value>
</context-param>

<context-param>
	<param-name>url</param-name>
	<param-value>jdbc:mysql:///test</param-value>
</context-param>

 Output  Context-param username  Value ：${ initParam.username } <br>
 Output  Context-param url  Value ：${ initParam.url } <br>

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