JSP系列：JSP进阶-EL

Posted 2020-07-17

1、简单语法

Expression Language(EL) was added to JSP 2.0 specification. The purpose of EL is to produce scriptless JSP pages. The syntax of EL in a JSP is as follows:

    ${expr}

Here expr specifies the expression itself.

(1)JSP EL中常用的两个操作符就是：“.”和“[]”。

The most common operators in JSP EL are . and []. These two operators allow you to access various attributes of Java Beans and built-in JSP objects.

(2)如果JSP compiler看到${}，就会计算其中表达式的值并进行输出。

When the JSP compiler sees the ${} form in an attribute, it generates code to evaluate the expression and substitues the value of expresson.

(3)如果想控制JSP EL表达式是否有效，需要在page指令中设置。

To deactivate the evaluation of EL expressions, we specify the isELIgnored attribute of the page directive as below: 

<%@ page isELIgnored ="true|false" %>

The valid values of this attribute are true and false. If it is true, EL expressions are ignored when they appear in static text or tag attributes. If it is false, EL expressions are evaluated by the container.

 

2、Basic Operators in EL

JSP Expression Language (EL) supports most of the arithmetic and logical operators supported by Java.Below is the list of most frequently used operators:

Basic Operators in EL

序号	Operator	Description
01	.	Access a bean property or Map entry
02	[ ]	Access an array or List element
03	( )	Group a subexpression to change the evaluation order
04(算术)	+	Addition
05(算术)	-	Subtraction or negation of a value
06(算术)	*	Multiplication
07(算术)	/ or div	Division
08(算术)	% or mod	Modulo (remainder)
09(关系)	== or eq	Test for equality
10(关系)	!= or ne	Test for inequality
11(关系)	< or lt	Test for less than
12(关系)	> or gt	Test for greater than
13(关系)	<= or le	Test for less than or equal
14(关系)	>= or ge	Test for greater than or equal
15(逻辑)	&& or and	Test for logical AND
16(逻辑)	|| or or	Test for logical OR
17(逻辑)	! or not	Unary Boolean complement
18	empty	Test for empty variable values

3、JSP EL Implicit Objects

The JSP expression language supports the following implicit objects:

JSP EL Implicit Objects

序号	Implicit object	Description
01	pageScope	Scoped variables from page scope
02	requestScope	Scoped variables from request scope
03	sessionScope	Scoped variables from session scope
04	applicationScope	Scoped variables from application scope
05	param	Request parameters as strings
06	paramValues	Request parameters as collections of strings
07	header	HTTP request headers as strings
08	headerValues	HTTP request headers as collections of strings
09	initParam	Context-initialization parameters
10	cookie	Cookie values
11	pageContext	The JSP PageContext object for the current page

You can use these objects in an expression as if they were variables. Here are few examples which would clear the concept:

3.1、The pageContext Object

此处的pageContext是EL表达式的implicit Object，它正好对应JSP的九个内置对象中的pageContext。

The pageContext object gives you access to the pageContext JSP object.

Through the pageContext object, you can access the request object. For example, to access the incoming query string for a request, you can use the expression:

    ${pageContext.request.queryString}

3.2、The Scope Objects

The pageScope, requestScope, sessionScope, and applicationScope variables    provide access to variables stored at each scope level.

For example, If you need to explicitly access the box variable in the application scope, you can access it through the applicationScope variable as applicationScope.box.

3.3、The param and paramValues Objects

The param and paramValues objects give you access to the parameter values normally available through the request.getParameter and request.getParameterValues methods.

For example, to access a parameter named "order", use the expression ${param.order} or ${param["order"]}.

3.4、header and headerValues Objects

The header and headerValues objects give you access to the header values normally available through the request.getHeader and request.getHeaders methods.

For example, to access a header named "user-agent", use the expression ${header.user-agent} or ${header["user-agent"]}.

4、JSP EL Important Points

(1)EL expressions are always within curly braces prefixed with $ sign, for example ${expr}

(2)We can disable EL expression in JSP by setting JSP page directive isELIgnored attribute value to TRUE.

(3)JSP EL can be used to get attributes, header, cookies, init params etc, but we can‘t set the values.

(4)JSP EL implicit objects are different from JSP implicit objects except pageContext, don‘t get confused.

(5)JSP EL pageContext implicit object is provided to get additional properties from request, response etc, for example getting HTTP request method.

(6)JSP EL is NULL friendly, if given attribute is not found or expression returns null, it doesn‘t throw any exception. For arithmetic operations, EL treats null as 0 and for logical operations, EL treats null as false.

(7)The [] operator is more powerful than dot operator because we can access list and array data too, it can be nested and argument to [] is evaluated when it’s not string literal.

(8)If you are using Tomcat, the EL expressions are evaluated using

org.apache.jasper.runtime.PageContextImpl.proprietaryEvaluate()

method.

(9)We can use EL functions to call method from a java class.

文章参考：

http://beginnersbook.com/2013/11/jsp-expression-language-el/

http://www.journaldev.com/2064/jsp-expression-language-el-example-tutorial