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tag
The useBean action declares a JavaBean for use in a JSP. Once declared, the bean becomes a scripting variable that can be accessed by both scripting elements and other custom tags used in the JSP.

JSP Syntax

<jsp:useBean
        id="beanInstName"
        scope="page | request | session | application"       
        class="package.class"    
        type="package.class"
</jsp:useBean>

Example

  <jsp:useBean id="stock" scope="request" class="market.Stock" />

Accessing Values

<jsp:useBean id="date" class="java.util.Date" /> 
<p>The date/time is <%= date %>

Using getPropertyName() and setPropertyName()

<jsp:useBean id="students" 
                    class="com.tutorialspoint.StudentsBean"> 
   <jsp:setProperty name="students" property="firstName"
                    value="Zara"/>   
</jsp:useBean>

<p>Student First Name: 
   <jsp:getProperty name="students" property="firstName"/>
</p>

Setting Request Scope

PersonBean myBean = (PersonBean)request.getAttribute("myBean");

if(myBean == null)
{
   myBean = new PersonBean();
   request.setAttribute("myBean", myBean);
}

Accessing in JSP

<jsp:useBean id="myBean" class="PersonBean" scope="request" />

The path with reference to root directory is called absolute. The path with reference to current directory is called relative.

Completely relative:

 <img src="kitten.png"/>

Absolute in all respects:

<img src="http://www.foo.com/images/kitten.png">

TestController.java

@RequestMapping(value="/Control2/{personId}")		
public String TestMe(@PathVariable("personId") Long personIdVal)
{
  System.out.println(personIdVal);
  return "hello";
}

Input

http://localhost:8089/Test2/Control1/Control2/152

Output

Spring MVC uses 2 design Patterns Internally

Front Controller
MVC

How Spring MVC Handles Request

Receive the request from client
Consult Handle Mapper to decide which controller processes the request
Dispatch the request to the controller
Controller processes the request and returns the logical view name and model back to DispatcherServlet
Consult View Resolver for appropriate View for the logical view name from Controller
Pass the model to View implementation for rendering
View renders the model and returns the result to DispatcherServlet
Return the rendered result from view to the client

MappingHandler
DispatcherServlet uses MappingHandler to find out which controller is right one for this request.There are many MappingHandler implementations which uses different strategies to map the request to Controller. By default DispatcherServlet will use BeanNameUrlHandlerMapping and DefaultAnnotationHandlerMapping.

public interface HandlerMapping 
{
      HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception;
}

ViewResolver
With the help of ViewResolver strategy object DispatcherServlet can find out physical view from the logical view name. Similar to MappingHandler there are also many different strategies for resolving the view based on the different view technologies. Most commonly used implementation of ViewResolver is InternalResourceViewResolver.

public interface ViewResolver 
{
      View resolveViewName(String viewName, Locale locale) throws Exception;
}

Creating a Preconfigured MVC Project
web.xml

<?xml version="1.0" encoding="UTF-8"?>
<web-app version="2.5" xmlns="http://java.sun.com/xml/ns/javaee"
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd">

	<!-- The definition of the Root Spring Container shared by all Servlets and Filters -->
	<context-param>
		<param-name>contextConfigLocation</param-name>
		<param-value>/WEB-INF/spring/root-context.xml</param-value>
	</context-param>
	
	<!-- Creates the Spring Container shared by all Servlets and Filters -->
	<listener>
		<listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
	</listener>

	<!-- Processes application requests -->
	<servlet>
		<servlet-name>appServlet</servlet-name>
		<servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
		<init-param>
			<param-name>contextConfigLocation</param-name>
			<param-value>/WEB-INF/spring/appServlet/servlet-context.xml</param-value>
		</init-param>
		<load-on-startup>1</load-on-startup>
	</servlet>
		
	<servlet-mapping>
		<servlet-name>appServlet</servlet-name>
		<url-pattern>/</url-pattern>
	</servlet-mapping>

</web-app>

DispatcherServlet is the first servlet which gets called when you run Preconfigured MVC Project
servlet-context.xml in init param of DispatcherServlet get run for other initialization of internal beans
Apart from servlet-context.xml the other xml file that gets called is root-context.xml in a separate context

Test.java

package com.mugil.spring;

import java.util.Locale;
import org.springframework.stereotype.Controller;
import org.springframework.ui.Model;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;

@Controller
public class Test
{	
	@RequestMapping(value = "/", method = RequestMethod.GET)
	public String testMethod(Locale locale, Model model) 
	{	
		model.addAttribute("testMsg", "Hi There" );		
		return "home";
	}
}

home.jsp

<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>
<%@ page session="false" %>
<html>
<head>
	<title>Home</title>
</head>
<body>
<h1>
	${testMsg}  
</h1>
</body>
</html>

Output(http://localhost:8089/spring/)

Hi There

servlet-context.xml

<?xml version="1.0" encoding="UTF-8"?>
<beans:beans xmlns="http://www.springframework.org/schema/mvc"
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xmlns:beans="http://www.springframework.org/schema/beans"
	xmlns:context="http://www.springframework.org/schema/context"
	xsi:schemaLocation="http://www.springframework.org/schema/mvc http://www.springframework.org/schema/mvc/spring-mvc.xsd
		http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd
		http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsd">
	<annotation-driven />
	<resources mapping="/resources/**" location="/resources/" />
	<beans:bean class="org.springframework.web.servlet.view.InternalResourceViewResolver">
		<beans:property name="prefix" value="/WEB-INF/views/" />
		<beans:property name="suffix" value=".jsp" />
	</beans:bean>

	<context:component-scan base-package="com.mugil.spring" />
</beans:beans>

context:component-scan base-package defines the base location in which the controller to be found
beans:property name=”prefix” defines the folder in which jsp should be found
beans:property name=”suffix” defines the valid prefix
Based on the return type of testMethod method the jsp page name is recognized. In our case it is home.jsp

hello.java

<%@ page language="java" contentType="text/html; charset=ISO-8859-1"
    pageEncoding="ISO-8859-1"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
<title>Insert title here</title>
</head>
<body>
<form name="frmTest" id="frmTest" method="post">		
	<select name="cboArea">
		<option value="Teynampet">Teynampet</option>
		<option value="TNagar">TNagar</option>
		<option value="Adyar">Adyar</option>
	</select>		
	<input type="checkbox" name="cboAgree" value="I Agree"/>I Agree		
	<input type="submit" name="btnSubmit" value="Submit"/>		
</form>
</body>
</html>

TestController.java

package com.mugil.controls;

import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;

@Controller
@RequestMapping("/Control1")
public class TestController 
{	
	@RequestMapping("/Control2")		
	public String TestMe()
	{
		return "hello";
	}

	@RequestMapping(value="/Control2", method=RequestMethod.POST)
	public String TestMe2()
	{
		System.out.println("Hi there");
		return "hello";
	}

	@RequestMapping(value="/Control2", method=RequestMethod.POST, params={"cboArea=TNagar", "cboAgree"})
	public String TestMe3()
	{
		System.out.println("I am in TNagar");
		return "hello";
	}
}

add_person.jsp

<body>
  <form name="frmTest" id="frmTest" method="post" action="/Test2/Control1/displayPerson">
    <input type="text" name="Name" />I Agree
	<select name="Location">
  	  <option value="Teynampet">Teynampet</option>
	  <option value="TNagar">TNagar</option>
	  <option value="Adyar">Adyar</option>
	</select>
    <input type="checkbox" name="cboAgree" value="I Agree"/>I Agree		
    <input type="submit" name="btnSubmit" value="Submit"/>		
  </form>
</body>

Person.java

public class Person 
{
	private String Name;
	private String Location;	
	
	public String getName() {
		return Name;
	}
	public void setName(String name) {
		Name = name;
	}
	public String getLocation() {
		return Location;
	}
	public void setLocation(String location) {
		Location = location;
	}

        @Override
	public String toString() 
        {
	  return "Person [Name=" + Name + ", Location=" + Location + "]";
	}
}

TestController .java

@Controller
@RequestMapping("/Control1")
public class TestController 
{
	@RequestMapping(value="/displayPerson", method=RequestMethod.POST)
	public String TestMe4(@ModelAttribute Person person)
	{
		System.out.println(person); 
		return "add_person";
	}
}

Output

Person [Name=Mac, Location=TNagar]

TestController.java

@RequestMapping(value="/Control2", method=RequestMethod.POST)
public String TestMe2(Model model)
{
  Person objPerson = new Person();
  objPerson.setName("Mugil");
  objPerson.setAddress("Sample Address");
  objPerson.setAge("27");

  model.addAttribute("objPerson", objPerson);
  return "display";
}

display.jsp

Person Name : ${objPerson.name}
Person Location : ${objPerson.location}
Person Age : ${objPerson.age}
Person Address : ${objPerson.address}

public class Person 
{
 private String Name;
 private String Location;
 private String Age;
 private String Address;

 public String getName() {
	return Name;
 }
 public void setName(String name) {
	Name = name;
 }
 public String getLocation() {
	return Location;
 }
 public void setLocation(String location) {
	Location = location;
 }

 .
 .
 .
}

How to Design Spring MVC for View Page

Have a isSuccess attribute in the bean
Use when and choose to decide whether to form element or form value based on the value set in the isSuccess
When isSuccess is set to true the show the form Value or show the form element else display the form element
When adding a new values in through the form the bean would be set to isSuccess true
Next time when the page gets loaded the form values filled before the click of the submit will be displayed a s form values.

How to access the bean values where the bean has sub bean

class Student
{
 String Name;
 Address houseAddress;
 .
 .
}

For accessing values in Student bean the form elements should be

path=Name value="${student.Name}"

For accessing values in Student Address bean the form elements should be

path=Address.houseAddress value="${Student.Address.StreetName}"

ServletConfig

ServletConfig is one of the pre-defined interface.
ServletConfig object is used for developing flexible servlets.
ServletConfig objct exist one per servlet program.
An object of ServletConfig created by the container during its initialization phase.
An object of ServletConfig is available to the servlet during its execution, once the servlet execution is completed, automatically ServletConfig interface object will be removed by the container.
An object of ServletConfig interface contains details at web.xml, of a particular servlet.
The moment when we are using an object of ServletConfig, we need to configure the web.xml by writing tag under tag of web.xml.
When ever compiler executes init() mehod then the ServletConfig will be created in general.
An object of ServletConfig contain the data in the form of key,value pairs, here the keys represents init param names and values are its values, which are represented in the web.xml file

How to Get ServletConfig Object into Servelt
Method1

ServletConfig conf = getServletConfig();

Method2
ServletConfig object will be available in init() method of the servlet.

public void init(ServletConfig config)
{
// …………………
}

How to Retrieve Data from ServletConfig Interface Object

public String getInitParameter(“param name”);
public Enumeration getInitParameterNames();

web.xml

<web-app> 
  <servlet>
     <servlet-name>onServletConfig</servlet-name>
     <servlet-class>java4s.OnServletConfig</servlet-class> 
     <init-param>
        <param-name> n1 </param-name>
        <param-value> 100 </param-value>
     </init-param>
    .
    .
</web-app>

Test.java

ServletConfig conf=getServletConfig(); 
String s1=conf.getInitParameter("n1");

ServletContext
An object of ServletContext is created by the web container at time of deploying the project. This object can be used to get configuration information from web.xml file. There is only one ServletContext object per web application.

Methods of ServletContext interface

public String getInitParameter(String name):Returns the parameter value for the specified parameter name.
public Enumeration getInitParameterNames():Returns the names of the context’s initialization parameters.
public void setAttribute(String name,Object object):sets the given object in the application scope.
public Object getAttribute(String name):Returns the attribute for the specified name.
public Enumeration getInitParameterNames():Returns the names of the context’s initialization parameters as an Enumeration of String objects.

web.xml

    <web-app>  
     ......  
          
      <context-param>  
        <param-name>parametername</param-name>  
        <param-value>parametervalue</param-value>  
      </context-param>  
     ......  
    </web-app>

Test.java

  ServletContext context=getServletContext();    

  //Getting the value of the initialization parameter and printing it  
  String driverName=context.getInitParameter("parametername");

Which HTTP method is non-idempotent?
An idempotent operation can be repeated an arbitrary number of times and the result will be the same as if it had been done only once. In arithmetic, adding zero to a number is idempotent.

A HTTP method is said to be idempotent if it returns the same result every time. HTTP methods GET, PUT, DELETE, HEAD, and OPTIONS are idempotent method and we should implement our application to make sure these methods always return same result. HTTP method POST is non-idempotent method and we should use post method when implementing something that changes with every request.

For example, to access an HTML page or image, we should use GET because it will always return the same object but if we have to save customer information to database, we should use POST method. Idempotent methods are also known as safe methods and we don’t care about the repetitive request from the client for safe methods.

What is ServletContext object?

javax.servlet.ServletContext interface provides access to web application parameters to the servlet.
The ServletContext is unique object and available to all the servlets in the web application. When we want some init parameters to be available to multiple or all of the servlets in the web application, we can use ServletContext object and define parameters in web.xml using element.

We can get the ServletContext object via the getServletContext() method of ServletConfig. Servlet containers may also provide context objects that are unique to a group of servlets and which is tied to a specific portion of the URL path namespace of the host.

@Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException 
   {
            HttpSession session = request.getSession(true);
            ServletContext context = session.getServletContext();
   }

ServletConfig vs ServletContext

Servlet Config	Servlet Context
Servlet config object represent single servlet	It represent whole web application running on particular JVM and common for all the servlet
Its like local parameter associated with particular servlet	Its like global parameter associated with whole application
It’s a name value pair defined inside the servlet section of web.xml file so it has servlet wide scope	ServletContext has application wide scope so define outside of servlet tag in web.xml file.
getServletConfig() method is used to get the config object	getServletContext() method is used to get the context object.
for example shopping cart of a user is a specific to particular user so here we can use servlet config	To get the MIME type of a file or application session related information is stored using servlet context object.

What is difference between ServletResponse sendRedirect() and RequestDispatcher forward() method?

RequestDispatcher forward() is used to forward the same request to another resource whereas ServletResponse sendRedirect() is a two step process. In sendRedirect(), web application returns the response to client with status code 302 (redirect) with URL to send the request. The request sent is a completely new request.
forward() can’t be used to invoke a servlet in another context, we can only use sendRedirect() in this case.
forward() is handled internally by the container whereas sednRedirect() is handled by browser.
We should use forward() when accessing resources in the same application because it’s faster than sendRedirect() method that required an extra network call.
In forward() browser is unaware of the actual processing resource and the URL in address bar remains same whereas in sendRedirect() URL in address bar change to the forwarded resource.

Difference between web server, web container and application server
Web server: serves content to the web using http protocol.
Web container also known as a Servlet container is the component of a web server that interacts with Java servlets. A web container is responsible for managing the lifecycle of servlets, mapping a URL to a particular servlet and ensuring that the URL requester has the correct access rights.

Application server: hosts and exposes business logic and processes.
An application server is a server that hosts the business logic for a system. It often hosts both long running/batch processes and/or a interop services not meant for human consumption (REST/JSON services, SOAP, RPC, etc).
An application server is typically designed and deployed to facilitate longer running processes that will also be more resource intensive.

Real World Scenarios
Web server
You have an online store with only a web server and no application server. The site will provide a display where you can choose a product from. When you submit a query, the site performs a lookup and returns an HTML result back to its client. The web server sends your query directly to the database server (be patient, I will explain this one in our next nugget) and waits for a response. Once received, the web server formulates the response into an HTML file and sends it to your web browser. This back and forth communication between the server and database server happens every time a query is run.

Application server
if the query you want to run has already been done previously and no data has changed since then, the server will generate the results without having to send the request to the database server. This allows a real-time query where a second client can access the same info and receive real time, reliable information without sending another duplicate query to the database server. The server basically acts as an intermediate between the database server and the web server. This allows the information pulled to be reusable while in the first scenario, since this info is embedded in a particular and “customized” HTML page, this is not a reusable process. A second client will have to request the info again and receive another HTML embedded page with the info requested -highly inefficient.

How to make sure a servlet is loaded at the application startup?
Usually servlet container loads a servlet on the first client request but sometimes when the servlet is heavy and takes time to loads, we might want to load it on application startup. We can use load-on-startup element with servlet configuration in web.xml file or use WebServlet annotation loadOnStartup variable to tell container to load the servlet on system startup.

<servlet>
	<servlet-name>foo</servlet-name>
	<servlet-class>com.foo.servlets.Foo</servlet-class>
	<load-on-startup>5</load-on-startup>
</servlet>

If there are multiple servlets with load-on-startup value such as 0,1,2,3 then lower integer value servlet will be loaded first.

How to get the actual path of servlet in server?

getServletContext().getRealPath(request.getServletPath())

Why is _jspService() method starting with an ‘_’ while other life cycle methods do not?

_jspService() cannot be overridden where as other methods jspInit(), jspDestroy() can be overridden.

Difference between Filter and Listener in Servlet
Servlet Filter is used for monitoring request and response from client to the servlet, or to modify the request and response, or to audit and log.

Servlet Listener is used for listening to events in a web containers, such as when you create a session, or place an attribute in an session or if you passivate and activate in another container, to subscribe to these events you can configure listener in web.xml, for example HttpSessionListener.

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CodeThatAint

Using bean Tag in JSP

relative vs absolute path

Passing value in URL

How Spring MVC Works

Spring Project Setup – XML Configuration

Choosing Controllers based on Form Inputs

Binding Form elements with Bean Values

Accessing Bean Values

Servlet Config and Servlet Context

Servlet Interview Questions