Expression Language in JSP

Expression Language

Cornelius Koo, ST
JavaSchool
2005
Jl. Cemara 2/20, Salatiga

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

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

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”]}

• 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}

• 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}

• With expression
<%= request.getHeader(“host”) %>

• With EL
${header.host}
${header.[“host”]}

• 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}

• 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}

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>

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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