Expression Lang

Expression Language(EL):

-->Writing java code in jsp is bad practice, so prefer different tags while developing jsp components, performing arithmetic and logical operations through tags may be a complex process or sometimes may not be possible. So we can use "EL" as alternate

Syn:-

1. $ (<expression>)

E.g:-

To enable EL

1. <%@page isELignored="false" %>
2. $ (4+5)-->9

To ignore EL

1. <%@page isELignored="true"%>
2. $ (4+5)-->$ (4+5)

-->EL deals with

a.)EL operators

b.)EL implicit objects(additional objects to existing 9 objects)

c.)EL functions

a.)EL operators:

-->Same as java operators but can be used in different syntaxes

1. ||-->or
2. &&-->and
3. <-->lt
4. <=-->le
5. !=-->ne
6. >-->gt
7. >=-->ge
8. ==-->eq and etc...

E.g(1):-

1. 4<5?$ (4<5) (or) $ (4 lt 5)

E.g(2):-

1. 10>20?$ (10>20) (or)$ (10 gt 20)

Note: EL can be applied on the tags and also on the template text of jsp

EL Implicit objects:

-->EL gives more implicit object supporting java code less jsp development

pageScope, requestScope, sessionScope, applicationScope, cookie, param, paramValues, header, headerValues, initParam and etc...

 xxxScope objects:-

-->Allows to keep data in different scopes and also allows to read data from different scopes

set_values.jsp:-

1. <%request.setAttribute("attr1","val1";
2. session.setAttribute("attr2","val2");
3. application.setAttribute("attr3","val3");%>

get_values.jsp:-

1. attr1(req) data::$ (requestScope.attr1)<br>
2. atrr2(ses) data::$ (sessionScope.attr2)<br>
3. attr3(application) data::$ (applicationScope.attr3)

-->Gives first request to set_values.jsp and other requests to get_values.jsp from same browser and different browser

Param:-

It is useful to read one request param values

Param values:-

-->It is useful to read multiple request param values, it can be also used to get multiple values of one request param

E.g:-

request url:- http://localhost:1025/ELProj1/paramTest.jsp?uname=teja&uname=kumar&pwd=hair

paramTest.jsp

1. uname request param value:$ (param.uname)-->gives teja
2. <br>
3. uname, pwd request param values:$ {paramValues.uname(0)},$ {paramValues.uname(1)},$ {paramValues.pwd(0)}

header:-

Useful to get one request header value

headerValues:-

Useful to get multiple request header values

headerTest.jsp:-

1. user-agent request header values: ${header("user-agent")}
2. <br>
3. Accept request header value: ${headerValues.accept(0)}

Cookie:-

-->Useful to read and display cookie name and cookie values

1. Cookie name holding sessionID: ${cookie.JSESSIONID.name}
2. <br>
3. Cookie value: ${cookie.JSESSIONID.value}

-->HttpSession object allocates memory on server having session id and these session id goes to browser and comes back to web application along with the request, response in the form of cookie

initParam:-

-->Given to read context param values that are configured in web.xml file

E.g:-

web.xml:-

1. <web-app>
2.   <context-param>
3.      <param-name>dbuser</param-name>
4.      <param-value>scott</param-value>
5.   </context-param>
6. </web-app>

initParamTest.jsp

1. DBUser context param values: ${initParam.dbuser}-->gives scott

EL functions:

-->EL function is like developing custom tag i.e we can execute java class methods internally for EL function that is developed

ELProj2

|-->java resources

     |-->src

         |-->org.el

              |-->WishGenerator.java

|-->webcontent

     |-->FxTest.jsp

     |-->WEB-INF

          |-->web.xml

          |-->wish.tld

Steps:-

(1)Developing java class with static method having logic to executes for EL function (Function handler class)

WishGenerator.java

1. package org.el;
2. public class WishGenerator{
3.  public static String sayHello(String user){
4.   return "gud morning"+user;
5.  }
6. }

(2)Configure function name, function handler class in tld file

1. <taglib>
2.    <tlib-version>1.1</tlib-version>
3.    <jsp-version>2.1</jsp-version>
4.    <short-name>myfx</short-name>
5.    <uri>wishfx</uri>
6.    <function>
7.      <name>sayHello</name>
8.      <function-class>org.el.WishGenerator</function-class>
9.     <function-signature>java.lang.String sayHello</function-signature>
10.   </function>
11. </taglib>

(3)Import and use EL function in jsp component

test.jsp

1. <%@ taglib uri="wishfx" prefix="fx"%>
2. Wish Message: ${fx:sayHello("teja")}