Expression Language in JSP

Expression Language

Cornelius Koo, ST

JavaSchool

2005

Jl. Cemara 2/20, Salatiga

Why EL

public void doPost( HttpServletRequest req,

HttpServletResponse res) throws ServletException, IOException {

Human p = new Human();

p.setName("Evan");

Dog dog = new Dog();

dog.setName("Spike");

p.setDog(dog);

req.setAttribute("person", p);

RequestDispatcher rd =

req.getRequestDispatcher("withscript.jsp");

rd.forward(req,res);

}

Result

• Spike Dog's name is :

jsp.example.bean.Dog@1e0e954

EL

• Dog's name is : ${person.dog.name}

Result

• Dog's name is : Spike

EL Rules

1st n 2nd

• ${ firstThing.secondThing }

• firstThing -> EL Implicit Object or attribute

• secondThing -> a property

EL Implicit Objects

• pageScope map objects

• requestScope map objects

• sessionScope map objects

• applicationScope map objects

• param map objects

• paramValues map objects

• header map objects

• headerValues map objects

• cookie map objects

• initParam map objects

• pageContext ->real reference to pageContext object

Accessing EL

If the expression has a variable followed by a dot, the left-hand

variable MUST be a Map or a bean

The Thing on the right must follow normal Java naming rules for

identifiers

If the expression has a variable followed by a bracket [ ], the left-

hand variable can be a Map, a bean, a List or an array.

If the thing inside the brackets is a String literal (i.e., in quotes), it

can be a Map key or a bean property, or an index into a List or array.

• In Servlet

String[] nameList = { “Aan”, “Sam”, “John”};

request.setAttribute(“name”, nameList);

• In JSP

Name : ${name}

Name : ${name[0]}

Name : ${name[“0”]}

Using [] for bean

• In Servlet

Map musicMap = new HashMap();

musicMap.put(“Ambient”, “Zero”);

request.setAttribute(“musicMap”,musicMap);

• In JSP

• ${musicMap.Ambient}

• ${musicMap[“Ambient”]}

What About This ?

• ${musicMap[Ambient]}

• This time with no “ “

• In Servlet

Map musicMap = new HashMap();

musicMap.put(“Ambient”, “Zero”);

request.setAttribute(“musicMap”,musicMap);

request.setAttribute(“Genre”,”Ambient”);

• In JSP

• ${musicMap[“Genre”]} -> doesn’t work

• ${musicMap[Genre]} -> evaluated

because there’s an attribute object named

Genre and it has a value named “Ambient”

Nesting

• ${musicMap[ musicType[0] ]}

• ${musicMap[“Ambient”]}

• Zero 7

Don’t try…

• Don’t put everything that are not qualified

as an identifiers behind the dot (.)

operator!

• ${musicMap[“Ambient”]} -> ${musicMap.Ambient}

• ${musicList[“1”]} -> X ${musicList.1}

Request Parameter in EL

• In HTML

<form action=“TestBean.jsp”>

Name : <input type=“text” name=“name”>

ID# : <input type=“text” name=“empID”>

<input type=“submit”>

</form>

• In JSP

• ${param.name}

• ${param.empID}

Request Method

• With expression

<%= request.getMethod() %>

• With EL

${pageContext.request.method}

Getting Request Attribute

• In Servlet

request.setAttribute(“person”, p);

• Get it with EL

${requestScope[“person”].name}

Header Information

• With expression

<%= request.getHeader(“host”) %>

• With EL

${header.host}

${header.[“host”]}

Cookie

• With scriptlet

<% Cookie[] cookies = request.getCookies();

for(int i=0; i < cookie.length; i++) {

if ((cookies[i].getName()).

equals(“username”)) {

out.println(cookies[i].getValue());

}

}

%>

• With EL

${cookie.userName.value}

Init Parameter

• DD

<context-param>

<param-name>name</param-name>

<param-value>Rod Johnson</param-value>

</context-param>

• With expression<%= application.getInitParameter(“name”) %>

• With EL

${initParam.name}

Method in EL

package jsp.example.method;

public class DiceRoller {

public static int rollDice() {

return (int) ((Math.random()*6)+1);

}

}

The method must be public and static

el.tld

<?xml version="1.0" encoding="ISO-8859-1"?>

<!DOCTYPE taglib PUBLIC

"-//Sun Microsystems, Inc.//DTD JSP Tag Library 1.1//EN"

"http://java.sun.com/j2ee/dtds/web-jsptaglibrary_1_1.dtd">

<taglib>

<tlibversion>1.2</tlibversion>

<jspversion>2.0</jspversion>

<uri>DiceFunctions</uri>

<function>

<name>rollIt</name>

<function-class>jsp.example.method.DiceRoller</function-class>

<function-signature>int rollDice()</function-signature>

</function>

</taglib>

method.jsp

<%@ taglib prefix="method" uri="DiceFunctions" %>

<html>

<head>

<title>EL Method</title>

</head>

<body>

${method:rollIt()}

</body>

</html>

ELOperator

Arithmetic

• +

• -

• *

• /

• %

Logical

• && or and

• || or or

• ! or not

Relational

• == or eq

• != or ne

• < or lt

• > or gt

• <= or le

• >= or ge

Expression Language in JSP

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