El expression

What is? EL expression

EL expression （Expression Language）, Originally defined in JSTL1.0 part , stay JSP2.0 in ,EL from JSTL Split it out , Put in JSP Specification , be called JSP2.0 canonical ⼀ part
stay JSP Chinese envoy ⽤EL expression , Access to objects and variables can be simplified EL expression

EL Expression syntax

Grammar format ：

${ The name of the information to be displayed }

Be careful ：
When the expression does not specify the range of variables or objects , Then the container will start from pageContext—>request—>session—>application Find the variable or object in , We can obtain the specified ⽤ Domain value :

pageScope object ,⽤ Get at present ⻚⾯ The attribute value 
requestScope object ,⽤ To get the attribute value of the request range 
sessionScope object ,⽤ To get the attribute value of the session scope 
applicationScope object ,⽤ To get the property value of the program scope 

grammar ：

 ${requestScope.key}

EL Basic operators in

The term	Definition
Arithmetic type	+ 、-、 * /(div) except 、 %(mod) remainder
The logical model	and、&&、or、!、not、II
Relational type	==、eq、!=、ne、、gt、<=、le、>=、ge. Can be compared with other values ⾏⽐ a , Or Boolean 、 String type 、 Integer or floating point ⽂ Words enter ⾏⽐ a .
Empty	Empty Operators are ⼀ A prefix operator ⽤ It's hard to judge ⼀ Whether the first value is null Or for empty Such as String str =“”; ${empty str} The return value is true;
Conditional type	A ?B :C. according to A The result of the assignment B or C.

Example ：

 Single variable ：${a+10}<br>
 Single variable ：${s}<br>
 Single object ：${key. Property name }

// object type 
Users u=new Users();
 u.setName("CaveWang");
 u.setPass("abc");
 pageContext.setAttribute("u1",u);
 user.name=${u1.name}
 user.pass=${u1.pass}

// A collection of objects 
 List list=new ArrayList();
 list.add("CaveWang");
 list.add(21);
 list.add(u);
 pageContext.setAttribute("list2",list);
 list1=${list2[0]}<br/>
 list2=${list2[1]}<br/>
 list3=${list2[2].pass}<br/>
 
 // Determine whether the variable has a value or exists :${empty key value }
 List list2=new ArrayList();
 list2.add("aa");
 request.setAttribute("list222",list2);

// Judge list Is there any data in :${empty list222}