JSP - Java Server Pages

Introduction to JSP

JSP stands for Java Server Pages. JSP is a server side web technology like Servlet. JSP is extension of Servlet not enhancement of Servlet. It does not replace the servlet technology and can be used with Servlets. JSP is used to develop server side web components to generate and render dynamic contents

JSP is Java code embedded in HTML

Such design (Java with in HTML) allows smooth development of presentation layer. A UI designer/developer can develop HTML pages that are rich in UI feature. A Java developer can later easily embed JSP within these HTML pages. In case of servlets all the HTML contents should be embedded in Java. To achieve rich UI feature, servlets become more of HTML than java. Any change in UI, enforces change in Java code. Change in Java code needs compilation of Java file. This becomes more expensive

JSP file extension must be .jsp

Basic example of JSP

The typical request-response flow is depicted for JSP

User sends request to the server from index.jsp
Server receives the request and directs it to LoginServlet.java as per the URL pattern configuration
LoginServlet.java fetches data from request parameters, performs business logic, sets the data in request attributes and forwards the request to welcome.jsp. Not a single line of code to prepare the presentation as a part of response.
welcome.jsp is responsible for rendering all UI stuff, fetches the attributes from request set by LoginServlet.java, displaying the fetched data.

If welcome.jsp was not there then LoginServlet.java has to perform other tasks also like fetching the data, preparing HTML response, setting this data in HTML. All UI stuff servlet has to do means by Java Developer but most likely UI developer are more efficient to do UI stuff. So in case of servlet either Java developer should learn UI technologies or always work closely with any UI developer.

On the other hand in case of JSP this task of Java and HTML can be separated. Let Java developer write LoginServlet.java. Let UI developer write welcome.jsp in HTML only. Later let Java Developer embed the JSP code in welcome.jsp

JSP Life cycle

JSP lifecycle methods

There are mainly three JSP lifecycle method:

public void _jspInit()
public void _jspService(HttpServletRequest req, HttpServletResponse res)
public void _jspDestroy()

JSP lifecycle steps

Container translates the JSP to Servlet (welcome.jsp -> welcome_jsp.java)
Container compile translated servlet (welcome_jsp.java -> welcome_jsp.class)
Container loads servlet class
Container invokes lifecycle method _jspinti() for initialization
Container invokes lifecycle method _jspService()
On further client request container directly invokes _jspService()
On container shut down, container invokes _jspDestroy()

JSP Implicit Objects

Object which are readily available to JSP are implicit object.
Implicit object are created and initialized by web container.
All implicit objects are local variable of _jspService() generated by container from JSP.
These object cannot be defined by the users in JSP page.

JSP Scripting Elements

JSP scripting elements are used to write Java statements in JSP
There are mainly three types of JSP scripting elements
- Scriptlets
- Expressions
- Declarations

Scriptlets

JSP scripting elements are used to write Java statements in JSP
There are mainly three types of JSP scripting elements
- Scriptlets
- Expressions
- Declarations

Syntax

<%

...

%>

Example

<%

int i = 10;

out.println(i);

System.out.println(i);

...

%>

_jspService()

_jspService(){

...

int i = 10;

out.println(i);

System.out.println(i);

...

Expressions

Expression tags are used to print/write the expression values
Any expression will be put in out.print(exp) and then in _jspService()
Semicolons are not allowed inside expression tags

Syntax

<%=

exp

%>

Example

<% String name = “Atul”; %>

<%= “Name” %>

<%= name %>

_jspService()

_jspService(){

...

String name = “Atul”;

out.print(“Name”);

out.print(name);

...

Declarations

Methods definitions, blocks, constructors, class level variable are allowed inside declaration tags
All code inside the declaration will be placed directly inside the translated servlet class and outside the _jspService() method

Syntax

<%!

//global var

//methods

%>

Example

<%!

String name = “Atul”;

void myMethod(){

...

%>

<h1>My Heading</h1>

welcom_jsp.java

public class welcome_jsp extends HttpServlet {

String name = “Atul”;

void myMethod(){

...

public void init(ServletConfig config) throws ServletException {}

public void destroy(){}

protected void service(HttpServletRequest request,

                       HttpServletResponse response) throws ServletException, IOException {

PageContext pageContext = null;

HttpSession session = null

ServletContext application = null;

ServletConfig config = null;

JspWriter out = null;

Object page = null;

...

out.write(“<h1>My Heading</h1>”);

...

int i = 10;

out.print(i);

System.out.println(i);

...

JSP Directives

Directive is a special instructions to container to perform the required task. There are mainly three type of directives

include directive
page directive
taglib directive

1. include directive

include directive is used to include one JSP or HTML in another JSP
During JSP translation include directive line will be replaces with actual content
One or more include directives are allowed inside JSP
include directive can be placed at any position of the page

Syntax

<%@ include file=”filename1.jsp” %>

<%@ include file=”filename2.html” %>

2. page directive

page directive is used to perform page level operations eg. import, thread safe

Syntax

<%@ page

language=””

import=””

session=””

extends=””

isThreadSafe=””

errorPage=””

isErrorPage=””

isELIgnored=””

%>

language:

language attribute is used to specify the language for the scriptlets and declarations
Currently only java is the valid value which is also default value
Value other than java will cause invalid attribute error
Eg: <%@ page language=”java” %>

import:

import attribute is used to specify the packages to be imported for translated servlet
one or more packages can be specified by comma separation
Eg: <%@ page import=”java.io.*,java.util.*””%>

session:

Session object is used to enable and disable session attribute in JSP
By default session object is enabled
Eg: <%@ page session=”false” %>
If disables, session object cannot be used in JSP otherwise compilation error

extends:

HttpJspBase is the default superclass for the translated servlet
extends attributes can be used to replace this default HttpJspBase class by other
If HttpServlet class is used as a superclass using extend attribute, you have to override any of the methods of HttpServlet class
Eg:

<%@ page extends=”javax.servlet.http.HttpServlet” %>

<%@ page import=”java.io.IOException,javax.servlet.ServletException”%>

<%@ page extends=”javax.servlet.http.HttpServlet” %>

<html>

   <body>

      <h1>

<%!

            public void service(HttpServletRequest req, HttpServletresponse) throws IOException, ServletException{

            _jspService(req, res);

%>

      </h1>

   </body>

</html>

isThreadSafe:

isThreaSafe attribute is used to specify the servlet Thread model required
Default Servlet Thread Model is multi-thread model
Default values of isThreadSafe attribute is true
Eg:

<%@ page isThreadSafe=”false” %> //Servlet Single Thread Model

<%@ page isThreadSafe=”true” %> //Servlet Multi Thread Model

errorPage and isErrorPage:

isErrorPage attribute used to specify the JSP page is error page (error.jsp)
error.jsp will be displayed in case of any exception in other JSP files having errorPage=”error.jsp”
In such a way error.jsp acts as centralized JSP for error handling
Both attributes work in coordination with each other

index.jsp

<%@ page errorPage=”error.jsp” %>

<html>

   <body>

      <h1>

         Welcome to Home

      </h1>

      <% int i = 10/0; %>

   </body>

</html>

error.jsp

<%@ page isErrorPage=”true” %>

<html>

   <body>

      <h1>Error Occurred!</h1>

      <%= exception %>

   </body>

</html>

isELIgnored:

isELIgnored attribute is used to enable or disable EL expressions
From Tomcat 6.0, EL Expressions are enabled by defaults

index.jsp

<%@ page isELIgnored=”false” %>

<html>

   <body>

      <% request.setAttribute(“msg”, “My message”); %>

      <h1>Messae: ${msg}</h1>

   </body>

</html>

3. taglib directive

taglib directive is used to use the custom tags in JSP

JSP Standard Actions

<jsp:include>
<jsp:forward>
<jsp:param>

<jsp:useBean>
<jsp:setProperty>
<jsp:getProperty>

<jsp:include>

includes one JSP or HTMl inside another JSP
includes response of included page at runtime
<jsp:include> functionality is same as of RequestDispatcher.include method

Syntax

<jsp:include page=”JSP or HTML file with extension” />

<jsp:forward>

used to forward the control from one JSP to another JSP or HTML
<jsp:forward> functionality is same as of RequestDispatcher.forward method

Syntax

<jsp:forward page=”JSP or HTML file with extension” />

<jsp:param>

used to send parameter data from one JSP to another while including or forwarding
must be used along with <jsp:include> or <jsp:forward>

Syntax

<jsp:param name=”mobile” value=”9876543210” />

<jsp:useBean>

<jsp:useBean> is used for creating Java Bean object

Syntax with example

<jsp:useBean id=”” class=”” scope=”” />

<jsp:useBean id=”user” class=”com.learnjsp.bean.User” scope=”session” />

scope attribute used to tell the scope of the bean eg: request, session etc.

<jsp:setProperty>

<jsp:setProperty> is used for storing data in Java Bean object

Syntax with example

<jsp:setProperty name=”” property=”” value=”” />

<jsp:setProperty name=”user” property=”userName” value=”Atul” />

<jsp:getProperty>

<jsp:getProperty> is used to fetch data from Java Bean object

Syntax with example

<jsp:getProperty name=”” property=”” />

<jsp:getProperty name=”user” property=”userName” />

Include directive v/s Include action

Scopes

Scope is used to specify the lifespan of an object
Any data stored in object is persisted throughout the scope of object
And this data can be accessed any time within the scope as per requirement
Object that are bound to a particular scope called scoped object
Data can be stored in scope as an attribute, attributes are name-value pair
name of attribute is type of String
value of attribute is type of Object
These are read and write attributes

Methods for using scope object

void setAttribute(String attrName, Object attrValue)

Object getAttribute(String attrName)

void removeAttribute(String attrName)

Enumeration getAttributeNames()

Type of Servlet Scopes

There are mainly three types of servlet scopes

Request scope
Session scope
Context scope

1. Request scope

Data stored in HttpServletRequest will be available until request object destroyed
Scope of such data called request scope
Request data can be accessed by single user within same request and before sending the response

2. Session scope

Data stored in HttpSession object will be available until session persists
Scope of such data called session scope
Session data can be accessed by single user across multiple requests
User should belong to same session

3. Context scope

Data stored in ServletContext will be available throughout the life of web application
Scope of such data is called context scope
Context data can be accessed by multiple users across multiple requests

Type of JSP scopes

There are four types of JSP scopes

Page scope
Request scope
Session scope
Application scope

1. Page scope

Data from page can be accessed within the same JSP only
Data can be stored directly in request, session and application using setAttribute but
To save data in page scope you have to use implicit object pageContext
It is of java.lang.Object type
Methods of PageContext to manage attributes and its values

void setAttribute(String attrName, Object attrValue)

void setAttribute(String attrName, Object attrValue, int scope)

Object getAttribute(String attrName)

Object getAttribute(String attrName, int scope)

Object findAttribute(String attrName)

void removeAttribute(String attrName, int scope)

Enumeration getAttributeNamesInScope(int scope)

2. Request scope

Data stored in HttpServletRequest will be available until request object destroyed
Scope of such data called request scope
Request data can be accessed by single user within same request and before sending the response

3. Session scope

Data stored in HttpSession object will be available until session persists
Scope of such data called session scope
Session data can be accessed by single user across multiple requests
User should belong to same session

4. Application scope

Data stored in ServletContext will be available throughout the life of web application
Scope of such data is called context scope
Context data can be accessed by multiple users across multiple requests

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JSP - Java Server Pages

Introduction to JSP

Basic example of JSP

JSP Life cycle

JSP lifecycle methods

JSP lifecycle steps

JSP Implicit Objects

JSP Scripting Elements

Scriptlets

Syntax

Example

_jspService()

Expressions

Syntax

Example

_jspService()

Declarations

Syntax

Example

welcom_jsp.java

JSP Directives

1. include directive

Syntax

2. page directive

Syntax

language:

import:

session:

extends:

isThreadSafe:

errorPage and isErrorPage:

isELIgnored:

3. taglib directive

JSP Standard Actions

<jsp:include>

Syntax

<jsp:forward>

Syntax

<jsp:param>

Syntax

<jsp:useBean>

Syntax with example

<jsp:setProperty>

Syntax with example

<jsp:getProperty>

Syntax with example

Include directive v/s Include action

Scopes

Methods for using scope object

Type of Servlet Scopes

1. Request scope

2. Session scope

3. Context scope

Type of JSP scopes

1. Page scope

2. Request scope

3. Session scope

4. Application scope

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