6 expression language

7/27/2019 6 Expression Language

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Expression Language


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Scriptless JSP

A JSP page without any scriptlets is a script-less JSP.

Our attempt is to reduce the clutter of java code in

the JSP so that the page designers do not have the

burden of learning java and also maintenancebecomes easy.

With standard jsp action tags we can achieve some

script-less-ness.

But surely this is not enough.

For instance how will you print request parameters in a

script-less way?


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JSP that uses standard JSP tags to access User object named userthat has been stored in the session object:

First Name :

Last Name :

eMail Address :


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The same JSP that uses EL:

First Name :

${user.firstName}

Last Name :${user.lastName}

eMail Address :

${user.emailAddress}


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Need for EL

Take another example-

Suppose we have a class Car which has an attribute

called engine of class Engine.

Let us attempt to display the attribute of the engine

object given the Car object but in a scriptless way.


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public class Car{

Engine engine;

String model;//other attributes

//getters and setters methods

}

class Engine{String engine_type;

int engine_cc;

//other attributes.

public getEngine_type(){}public getEngine_cc

// other getter and setter methods.

}


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Attempts to display engines engine_type attribute

Using standard actions

Engine Type:

NO THIS WILL RESULT IN AN ERROR


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Using scripting

THIS IS FINE BUT REMEMBER WE

SAID WE WILL ATTEMPT SCRIPTLESS

JSP.

SO WE CONCLUDE

STANDARD ACTIONSARE NOT ENOUGH TOMAKE JSP SCRIPTLESS


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EL to rescue!

Using EL to solve the problem

${car.engine.engine_type}

instead of

But what is EL?


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EL

Expression language

Primary feature of JSP technology version 2.0

EL expressions can be used:

In static text

The value of an el expression in static text is computedand inserted into the current output

In any standard or custom tag attribute that can accept anexpression

If the static text appears in a tag body, the expressionwill not be evaluated if the body is declared to betagdependent

We will see what this is in the custom tag section


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EL Syntax

EL implicit Object OR an

attribute either in page,

request, session or application

scope (searched in that

sequence)

If E1 is implicit object , then

E2 is Attribute

If E1 is Attribute, then E2 is

the property of the object.

${E1.E2}


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ExampleBean

package com.jft;import java.io.Serializable;

public class Emp implements Serializable{private String empno;private String name;public Emp(){empno="402";

name="Rama";}public void setEmpno(String empno){this.empno=empno;}

public void setName(String name){this.name=name; }

public String getEmpno(){return empno;}

public String getName(){return name;

}}


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elexample5.jsp

DisplaysMr. Rama 402

The expressions are evaluated from left to right. Each

expression is evaluated to a String and then concatenated

with any intervening text. The resulting String is then

coerced to the attribute's expected type.


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EL Implicit Objects

pageScope

requestScope

sessionScope

applicationScope

param

paramValues

header

headerValues cookie

initParam

pageContext

Map objects

Note that these objects are

not the same as JSP implicit

object

Like JSP implicit object


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Using . Operator

When the variable is on the left side of the dot, it is

either a Map(Key/Value pair) or a bean(properties)

${E1.E2}

Map(java.util.Map)

Key

Java bean class

property


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Examples

We have seen an example for java bean withproperty

${a.name}

Map Example${pageScope.a.name} Where pageScope is implicit object of map type.


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Using [ ] operator

When the variable is on the left side of the [], it can

be

Map

bean

List

array


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${E1[E2]}

Map(java.util.Map)

Key

Java bean class

property

List(java.util.List)

index

array

index

In case of List and arrays , String index is forced(converted) to an int.


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Ex1: [] Operator with JavaBean property

Servlet Code(jsp in scriptlet)

User user = new User(firstName, lastName, emailAddress);

session.setAttribute(user, user);

JSP Code

Hello ${user[firstName]}

Ex2: [] Operator with an ArrayServlet Code

String[] colours = {red, green, blue};

ServletContext application = this.getServletContext();

application.setAttribute(colours, colours);JSP Code

First colour is ${colours[0]}

Second colour is ${colours[2]}

Second colour is ${colours[2]}

Third colour is ${colours[3]}


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Ex3: [] Operator with a List

Servlet Code

ArrayList users = UserIO.getUsers(path);

session.setAttribute(users, users);

JSP Code

First address in the List:${users[0].emailAddress}

Second adderss in the List:${users[1].emailAddress}

First address in the List:${users[0].emailAddress}

Second adderss in the List:${users[1].emailAddress}


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Example using [] and .

protected void doGet(){java.util.Map carmap=newjava.util.HashMap();

carmap.put("Large","Mercedes");carmap.put("Medium","Getz");

carmap.put("Small","Alto");String[] cartype={"Large","Medium","Small"};

request.setAttribute("carmap",carmap);request.setAttribute("cartype",cartype);request.setAttribute("size","Medium");

request.getRequestDispatcher("i2.jsp").forward(request,response);

}

I JSP


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My car is: ${carmap.Large}
or

My car is: ${carmap["Large"]}

Car type : ${cartype[0]} or Car type :

${cartype["1"]}
My car is: ${carmap[size]}

My car is:${carmap[cartype[0]]}

Both print Mercedes

prints Large

prints Medium

prints Getz. Without double

quotes inside the square

brackets the string value istreated as attribute. The

attribute evaluates and its

value is substituted.

prints Mercedes

In JSP


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Question?

Write an EL to print the session id.

${pageContext.session.id}EL implicit object

${session.id} No session in EL implicit object


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Question?

Suppose that there is a size attribute set in the

session scope as well. Write an EL Expression to print

the size attribute in the session scope?

${pageContext.session.size}

${size}

${sessionScope.size}

This returns the request scopes size


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Getting other parameters

${cookie.uname.valu}

Returns the value of uname cookie

${initParam.color}

Can you write EL to get the parameters from theform.


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Form Params

${param.eno}

Gets the request parameter matching eno

${paramValues.hobbies}

Gets the first value from the multiple value

request attribute

${paramValues.hobbies[0]}

Same as the above You could also say${param.eno[0]} for a single

value parameter.


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elexample1.jsp

Enter name:

elexample2.jsp

Welcome, ${param.name}


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elexample3.jsp

Welcome to index page

visit

elexample4.jsp

Value is ${sessionScope.user}


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How to get an HTTP Header

JSP Code

Browser MIME Types: ${header.accept}

Browser compression types: ${header[accept-encoding]}

How to work with Cookies

Servlet Code

Cookie c = new Cookie(emailCookie, emailAddress);

c.setMaxAge(60*60);

c.setPath(/);

response.addCookie(c);

JSP Code

The eMail Cookie : ${cookie.emailcookie.value}

This works because the Cookie class follows all the rules for the

JavaBean, and includes a method named getValue() that can be

used to get the value thats stored within the cookie


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How to get a context initialization parameter

web.xml

myEmail

[email protected]

JSP Code

The context init parameter: ${initParam.myEmail}

How to use pageContext Object

JSP Code

HTTP Request Method: ${pageContext.request.method}

HTTP Response Type:${pageContext.response.contentType}

HTTP Session ID: ${pageContext.session.id}

HTTP contextPath:${pageContext.servletContext.contextPath}


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EL Operators Arithmetic Operators

+

- *

/,div${42/0} gives INFINITY as output and notException

% ,mod

${42%0} gives an Exception

In arithmetic expressions, EL treats the

unknown variable (null value) as Zero(0).


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Relational Operators

==, eq

!=, ne

< , lt

> , gt

=, ge

Logical Operators

&& , and AND

|| , or OR

!, not NOT

In Logical expressions (using logical operatorsand/or relational operator, EL treats the unknown

variable (null value) as false.


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EL reserved words

and eq gt true instanceofor ne le false emptynot lt ge null div mod

Note:${empty obj} returns true if obj is null orempty.


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Examples


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EL Functions

Suppose you need to display the number of times this

jsp page has been visited and you have a static

method countIt() that returns this value. How would

you invoke this function in a scriptless way from jsp? The expression language allows a function to

be invoked in an expression provided the

function is defined as public static method in a

java class.


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Creating a class

public class Counter{static int count;

public static int countIt()

{ return count++;}//other class member definitions

}

TLD file


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TLD file

1.2

CountFunction

counterCounterint

countIt()

WEB-INF/ my.tld


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Use in JSP file

${mycnt:counter()}

Prefixmycntis just a name.

The container identifies the TLD through the uri (name

of TLD) in the taglib directive.

Di bli EL i JSP d DD
http://localhost/var/www/apps/conversion/tmp/scratch_5/EL.ppt


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Disabling EL in JSP and DD In a jsp page we can disable the EL using the is ELIgnored

page directive attribute

In the DD we do it as

. . .

*.jsp

true

. . .


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Ensuring that JSPs are scriptless

*.jsp true


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emailListForm.jsp


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import java.io.IOException;

import java.io.PrintWriter;

import java.util.Date;

import javax.servlet.RequestDispatcher;

import javax.servlet.ServletException;

import javax.servlet.ServletRequest;

import javax.servlet.http.HttpServlet;

import javax.servlet.http.HttpServletRequest;

import javax.servlet.http.HttpServletResponse; import javax.servlet.http.HttpSession;

import com.jft.User1;

public class addEmailListServlet extends HttpServlet {

public void doGet(HttpServletRequest request, HttpServletResponse response)

throws ServletException, IOException {

response.setContentType("text/html");

PrintWriter out = response.getWriter();

Date currentDate = new Date();

request.setAttribute("currentDate", currentDate);

String firstName = request.getParameter("firstName");

String lastName = request.getParameter("lastName");

String emailAddress = request.getParameter("emailAddress");

User1 user = new User1(firstName, lastName, emailAddress);

HttpSession session = request.getSession();

session.setAttribute("user", user);

RequestDispatcher rd = getServletContext().getRequestDispatcher("elexample7.jsp");

rd.forward(request, response); }

}


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Taglib Directive

Defines a tag library and prefix for the custom tagsused in the JSP page

Example:

..


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The directive declares that the JSP file usescustom tags, names the tag library that defines them, andspecifies their tag prefix.

----- taglib Directive

JSP has a set of standard tags for you to use. JSP also

provide a possibility to allow you to create new custom tagslook like HTML and XML tag. In this case, you can use taglibdirective to use your custom tag in your JSP page. For usingcustom tag, you can refer to the custom tag tutorial. Thesyntax of using taglib directive is as follows:

6 expression language

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