jsp element types

7/29/2019 JSP Element Types

1/59

JSP element types

Just like any other scripting lang tags,JSP also has got its well defined tags for

performing various tasks like declaringvars,methods,exprs,and calling other jsp

pages.


2/59

JSP element typesJSP tag type description syntax

Directives Specifies translation timeinstructions to the JSPengine.

Declaration Declares and definesmethods and vars

Scriptlet Allows the developer towrite free-form java codein a jsp page

Expression Used as a shortcut to printvalues in the output HTML

of a JSP page

Action Provides request-timeinstructions to the JSPengine

Comment Used for commenting jsp

code


3/59

Directives Directives provide general information about the JSP page to the JSP

engine. There are three types of directives: page, include and taglib.

A page directive informs the JSPengine about the overall properties of a

JSP page. For eg. The foll. Page directive informs the JSP engine thatwe will be using Java as the scripting lang in our JSP page.

An include directive tells the JSP engine to include the contents ofanother file(HTML,JSP,etc.) int the current page. Here is an example ofan include directive:

A taglib directive is used for associating a prefix with a tag library. Thefoll. Is an ex. Of a taglib directive:


4/59

< page a r- s >

The tag names, their attributes and

their values are all case sensitive

The value must be enclosed within apair of single or douuble quotes

A pair of single quotes is equivalent to a

pair of double quotes There must be no space between the

equals sign(=) and the value.


5/59

DeclarationsDeclarations declare and define vars and methods that can be used

in the JSP page.

The var is only intialized only once to 0.

When page is first loaded by JSP engine, and it retains its value insubsequent client requests.


6/59

ScriptletsScriptlets are Java code fragments that embedded in the JSP page.

The scriptlet is executed each time it gets executed.

The scriptlet gets excuted each time the page is accessed, andthe count variable is incremented with each request.


7/59

ExpressionsExpressions act as placeholders for javaExpressions.

Eg.

Never to have a semicolon at the end ofexprn.

welcome! U r visitor number

JSP i i


8/59

JSP i e cyc e trans ationPhase Name Description

Page translation The page is parsed and a Java file

containing the corresponding servletis created

Page compilation The java file is compiled

Load class The compiled class is loaded

Create instance An instance of servlet is created

Call jspInit() This method is called before anyother method to allow initilization

Call _jspService() This method is called for eachrequest

Call jspDestroy() This method is called when servletcontainer decides to take servlet outof service.


9/59

Calling life cycle methods The generated servlet class for a jsp page

implements HttpJspPage interface, which isin javax.servlet.jsp pkg.

The HttpJspPage extends JSP pageinterface of the same pkg, which in turn

extends Servlet interface of thejavax.servlet pkg.

The generated servlet class thusimplements all three interfaces and also

known as pages implementation class.


10/59

The JspPage interface has two methods

jspInit() and jspDestroy()-that must be implemented by all jsp pagesregardless of client server protocol.

Http requests,HttpJspPage interface with one method _jspService().

Public void jspInit();

Public void _jspservice(HttpServletRequest req,HttpServletResponse) throws

javax.servlet.ServletException,java.io.IOException;

Public void jspDestroy();

JspPage interface- two methods jspInit() and

jspDestroy(); _jspservice() is added by jsp engine durning

translation phase making the implementation class asconcrete subclass of three interfaces.


11/59

Jspinit()

The container calls this method to initialize the servlet instance. We normallydefine this method for one time setup such as acquiring resources andinitializing the instance variables that been declared using

declarations

Jspdestroy()

When the container decides to take out instance out of service,it callsjspDestroy(). This is last method called on servlet instance, and it is used toclean up the resources acquired in jspInit().

We are not req to implement jspInit() and jspDestroy(),bcoz implemented bybase class but if wants to override these two methods, then can be done

But cannot define _jspService() bcoz generated by engine automatically.


12/59

Jsp life-cycle example Lets modify our counter ex to add persistence

capabilities to it so that the counter does not

start from 1 each time the server is shutdown and restrated.

Use jspInit() to load the previous value of thecounter from a file when server starts.

Use jspdestroy() to save the final to the filewhen the server shuts down.


13/59


14/59

Welcome !you r visitor number


15/59

Page Directive AttributesA page directive informs the JSp engine about overall properties of aJSp page. This directive applies to the translation unit. There 12attributes for page directive

Attr Name Description Default values

import A comma-sepratedlist of java classesand pkgs that we

want to use in theJSP page

Java.lang.*;

Javax.servlet.*;

Javax.servlet.jsp.*;Javax.servlet.http.*;

session A boolean literal ,whether the jsppage takes part inan Http session.

true

errorPage Specifies relativeURL to another JSPpage that iscapable of handlingerrors on the behalfof current page

null

i E P A b l lit l if i f l


16/59

isErrorPage A bolean literal specifyingwhether the current JSPpage is capable ofhandling errors

false

language Any scripting languagesupported by the JSPengine

java

extends Any valid java class thatimplementsjavax.servlet.jsp.JspPage

Implementationdependent

buffer Specifies the size of

output buffer.If buffersize is specified, its inkilobytes.

Implementation

dependent


17/59

autoflush An boolean literalindiacting whetherthe buffer should be

flushed when it is full

true

Info Any informative textabout jsp page

Impl. dependent

contentType Character encoding

for the output

Text/html;charset=

ISO-8859-1

pageEncoding Specifies thecharacter encodingof the JspPage

ISO-8859-1


18/59


19/59

Hello,


20/59

errorHandler.jsp

Unable to process yourrequest:,


21/59


22/59


23/59

Implicit Variables and implicit objects

Durning translation phase, the Jsp engine

declares and intializes nine commonly usedvariables in the _jspservice() method.

Identifier name Class or interface description

application Interfacejavax.servlet.ServletContext

Refers to the webapplications environment

session Interface

Javax.servlet.http.HttpSession

Refers to the users session

request Interface

Javax.servlet.http.HttpservletRequest

Refers to the currentrequests to the page

response Interface

Javax.servlet.http.HttpservletResponse

Used for sending a responseto the client

out Classjavax.servlet.jsp.Jspwriter

Refers to the output streamfor the page


24/59

page classjava.lang.Object

Refers to thepages servletinstance

pageContext classjavax.servlet.jsp.PageContext

Refers to thepagesenvironment

config Interfacejavax.servlet.ServletConfig

Refers to theservletsconfiguration

exception Classjava.lang.Throwa

ble

Used for errorhandling


25/59

Because the page author does notdeclare these variables explicitly,they

are called implicit variables. The objs are to be created by servlet

container and are called implicit objects.


26/59

Application

The application var. is of the type

javax.servlet.servletContext and it refers tothe env of the web application to which the

jsp belongs.


27/59

Session

Session implicit var, lets clarify that the word session refers to four diff. butrelated things in JSP:

Session, as in a HTTP session,is a concept that logically groups multiple reqsfrom the same client as part of one conversation.

Session, as used in a page directive,refers to the attribute named session.

Its value, which is true or false, determines whether or not the JSP pageparticipates in HTTP Session.

Session, as an implicit obj, refers to the variable session of type

javax.servlet.http.HttpSession.

Session, as a scope of an obj, refers to the lifetime and availability of the object.A session-scoped persists throughout the life of an HTTP session.


28/59

Session ID=


29/59

Request and Response

The request and response implicit vars are of typejavax.servlet.http.HttpServletRequest andjavax.servlet.http.HttpServletResponse, respectively.

HI! Your IP address is


30/59

Login.jsp

File authenticate.jsp

String str=request.getParameter(User);

If(isValid(str))

{

request.removeAttribute(str);

Session.setAttribute(User,str);

pageContext.forward(account.jsp);}

else

{pageContext.forward(loginError.jsp);}

return;%>


31/59

page

The implicit variable page is classjava.lang.Object and it refers to the

instance of the generated servlet. It is declared as Object page=this;

pageConte t


32/59

pageContext

The pageContext var is of type javax.jsp.PageContext. ThepageContext class is an abstarct class, and the JSP engine vendorprovides its concrete subclass.

Stores reference to the implicit objs. The session,application,configand out implicit vars are intialized using the objs retrieved frompageContext.

Provides convenient methods for transferring requests to otherresources.

pageContext.forward(other.jsp);


33/59

Unrecognized attr result infatal translationerrors.

This is to test the page directive.


34/59

Include directive test page


35/59

Included.jsp


36/59

Scopes of JSP page

Scope Name Existence andAccessibility

Application Limited to a single webapplication

Session Limited to a single usersession

Request Limited to a single

request

Page Limited to single pageand a single request


37/59

Action tags

The standard JSP actions

jsp:include

jsp:forward

jsp:useBean

jsp:setPropertyjsp:getProperty


38/59

JavaBeans Components

A Javabeancomponent is Java tech.class with min. following features

Properties defined with accessors andmutators.

No args constructor

No public instance vars The class implements thejava.io.Serializable interface.


39/59

Package p1;import java.io.Serializable;Public class Customerbean implements Serializable

{private String name;Private String email;Private String phone;

Public CustomerBean(){this.name=;

this.email=;this.phone=;}Public void setName(String name){this.name=name;}Public String getName(){Return name;}

Public void setEmail(String email){this.email=email;}


40/59

Public String getEmail()

{

return email;}

Public void setPhone(string phone)

{

this.phone=phone;}

Public String getPhone(){

Return phone;}

}


41/59

If u want to interact with a JavaBeanscomponents using std tags in a JSP

page. U must first declare the bean.The action tag usebean syntax


42/59
mailto:[email protected]:[email protected]


43/59

Property=*|property=propName|property=propName

value=propValue

I l d S D I l d d i


44/59

Include

type

Syntax Donewhen

Includedcontent

parsing

directive

Compilation time

Static Parsed bycontainer

action

Requestprocessing time

Static ordynamic Notparsedbutincludedin place


45/59

When the JSP page is requested, itsends a request to another object, and

the object is included in the requestedJSP page. We use and

The foll. Three constructs areequivalent:


46/59


47/59

In ParamTest1:

First Name is:

Last name:

The file paramTest2.jsp

First name is

Last name is

Looping through all the first names


48/59

Expression Language

EL in incorporated JSP2.0 specifications.The purpose of EL is to aid in producing

scriptless JSP pages. Syntax overview

${expr}

To escape the EL expr from JSP it is/${expr}

Package p1;


49/59

Package p1;

import java.io.*;

import java.sql.*;

public class TrialBean implements Serializable

{

Private String driver;

Private String connection;

Private String statement;

Private String recordset;

Public TrialBean(){}


50/59

Public void setDriver(String driver)

{

this.driver=driver;}

Public void getDriver()

{Return driver;

}

Public void setConnection(String connection)

{

this.connection=connection;}

Public void getConnection (){

Return Connection;

}

Public void setStatement(String statement)

{

this.statement=statement;}

Public void getStatement()

{

Return statement;

}


51/59

Public void setResultSet(String resultset)

{

this. resultset = resultset;}

Public void getResultSet ()

{

return resultSet;

}}


52/59

Custom tags


53/59

Custom tags

Tag libraries are extension of Java Beans in JSP, Although they get andset attributes like JavaBeans,they do much more extra functionality

We can access this extra functionality with XML tags,that is why thy arecalled customs tags.

To use custom tags, need the following

-Tag handler

-Tag handler descriptor

-Deployment descriptor

-taglib directive

-Custom tags in JSP

JavaBeans provide rasy access to get/set methods, but tag libraries provide


54/59

this is static output.

This is static outputagain.


55/59

Package ext;

Import java.io.IOException;

Import java.util.Date;

Import javax.servlet.jsp.*;Import javax.servlet.tagext.*;

Public class HelloTag extends TagSupport{

/*This method will be called when JSP engine encounters the start of a tag

implements by this class*/Public int dostartTag() throws JspTagException{

Return EVAL_BODY_INCLUDE;}

Public int doEndTag() throws JspTagException{

String dateString=new Date().toString();

Try{pageContext.getOut().write(hello world.
);

pageContext.getOut().write(my name is +getClass().getName()+and itis+dateString+
);
}


56/59

Catch(IOException e){

Throw new JspTagException(fatal error:hello tag could not write to JSPout);

Return EVAL_PAGE;

}

}//class

1 0


57/59

1.0

1.1

examples

hello

ext.HelloTag

JSP

simple example>


59/59

/hello

/WEB-INF/tlds/hello.tld

jsp element types

Documents