Java FAQ and Cloning

8/6/2019 Java FAQ and Cloning

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Java clone method example

Java clone method is used to present the duplication of object in the Java

programming language. The Java objects are manipulated through referencevariables, we don't have any way to copy an object in java. The clone ( ) is a

constructor that call the clone method of super class in order to get the copy of

object.

Understand with Example

In this Tutorial we want to describe you a code that help you in understanding a

ClonemethodExample.For this we have a primitive class int,The int constructor

return you the object of this class by passing the int value a assigned to i. The

increase method ( ) return you increment value of i. The String toString ( ) return

you the string representation of integer. The ClonemethodExample class include

main method that create array list object 'list'. The for loop run the loop and the list

object call the add method ( ) and add integer variable to it. Finally the println print

the list.

1)Clone ( ) method - This method return you the typecasting that needed to assign

the generic object to a reference of my class object type.

The for loop include the list object that call the iterator method and return the

Iterator object.

The Iterator object e iterate the value from the list and call the next method( ).2)next ( )method - This method return you the next element in the list if it is

present in list

Finally the println print the list of clone value.

ClonemethodExample.java

import java.util.*;

class Int {

private int i;

public Int(int a) { i = a; }

public voidincrease() { i++;
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}

public String toString() { return Integer.toString(i); }}

public class ClonemethodExample {

public static voidmain(String[] args) { ArrayList list = new ArrayList();

for (int i = 0; i < 5; i++) { list.add(new Int(i)); } System.out.println("List is : " + list);

ArrayList list1 = (ArrayList) list.clone();

for (Iterator e = list1.iterator(); e.hasNext();) { ((Int) e.next()).increase(); } System.out.println("List after cloning is: " + list); }}

Output of the program

List is : [0, 1, 2, 3, 4]List after cloning is: [1, 2, 3, 4, 5]

A Cloning Example

public class MainClass {public static voidmain(String[] args) { Employee emp1 = new Employee("M", "A"); emp1.setSalary(40000.0); Employee emp2 = (Employee) emp1.clone(); emp1.setLastName("Smith"); System.out.println(emp1);

System.out.println(emp2); }}

class Employee {private String lastName;

private String firstName;

private Double salary;


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public Employee(String lastName, String firstName) { this.lastName = lastName; this.firstName = firstName; }

public String getLastName() { return this.lastName; }

public voidsetLastName(String lastName) {

this.lastName = lastName; }

public String getFirstName() { return this.firstName; }

public voidsetFirstName(String firstName) {

this.firstName = firstName; }

public Double getSalary() { return this.salary; }

public voidsetSalary(Double salary) { this.salary = salary; }

public Object clone() { Employee emp;

emp = new Employee(this.lastName, this.firstName); emp.setSalary(this.salary); return emp; }

public String toString() { return this.getClass().getName() + "[" + this.firstName + " " + this.lastName + ", " + this.salary + "]"; }}

JDBC and JSP interview questions and answersByadmin| October 9, 2004

1. What is the query used to display all tables names in SQL Server (Query analyzer)?

2. select * from information_schema.tables

3. How many types of JDBC Drivers are present and what are they?- There are 4 types of JDBC

Drivers
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JDBC-ODBC Bridge Driver

Native API Partly Java Driver

Network protocol Driver

JDBC Net pure Java Driver

4. Can we implement an interface in a JSP?- No

5.What is the difference between ServletContext and PageContext?- ServletContext: Gives the

information about the container. PageContext: Gives the information about the Request

6. What is the difference in using request.getRequestDispatcher() and

context.getRequestDispatcher()?- request.getRequestDispatcher(path): In order to create it we need to

give the relative path of the resource, context.getRequestDispatcher(path): In order to create it we need to

give the absolutepath of the resource.

7. How to pass information from JSP to included JSP?- Using tag.8. What is the difference between directive include and jsp include?- : Used to include

static resources during translation time. JSP include: Used to include dynamic content or static content

during runtime.

9. What is the difference between RequestDispatcher and sendRedirect?- RequestDispatcher:

server-side redirect with request and response objects. sendRedirect : Client-side redirect with new request

and response objects.

10.How does JSP handle runtime exceptions?- Using errorPage attribute of page directive and also we

need to specify isErrorPage=true if the current page is intended to URL redirecting of a JSP.

11. How do you delete a Cookie within a JSP?

12. Cookie mycook = new Cookie("name","value");

13. response.addCookie(mycook);

14. Cookie killmycook = new Cookie("mycook","value");

15. killmycook.setMaxAge(0);

16. killmycook.setPath("/");

17. killmycook.addCookie(killmycook);

18.How do I mix JSP and SSI #include?- If youre just including raw HTML, use the #include directive as

usual inside your .jsp file.

19.

But its a little trickier if you want the server to evaluate any JSP code thats inside the included file. If your

data.inc file contains jsp code you will have to use

The is used for including non-JSP files.

20.I made my class Cloneable but I still get Cant access protected method clone. Why?- Some of

the Java books imply that all you have to do in order to have your class support clone() is implement the

Cloneable interface. Not so. Perhaps that was the intent at some point, but thats not the way it works


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currently. As it stands, you have to implement your own public clone() method, even if it doesnt do anything

special and just calls super.clone().

21.Why is XML such an important development?- It removes two constraints which were holding back

Web developments: dependence on a single, inflexible document type (HTML) which was being much

abused for tasks it was never designed for; the complexity of full SGML, whose syntax allows many powerful

but hard-to-program options. XML allows the flexible development of user-defined document types. Itprovides a robust, non-proprietary, persistent, and verifiable file format for the storage and transmission of

text and data both on and off the Web; and it removes the more complex options of SGML, making it easier

to program for.

22.What is the fastest type of JDBC driver?- JDBC driver performance will depend on a number of issues:

the quality of the driver code,

the size of the driver code,

the database server and its load,

network topology,

the number of times your request is translated to a different API.

In general, all things being equal, you can assume that the more your request and response change hands,

the slower it will be. This means that Type 1 and Type 3 drivers will be slower than Type 2 drivers (the

database calls are make at least three translations versus two), and Type 4 drivers are the fastest (only one

translation).

23. How do I find whether a parameter exists in the request object?

24. boolean hasFoo = !(request.getParameter("foo") == null

25. || request.getParameter("foo").equals(""));

or

boolean hasParameter =

request.getParameterMap().contains(theParameter); //(which works in Servlet 2.3+)

26.How can I send user authentication information while makingURLConnection?- Youll want to

use HttpURLConnection.setRequestProperty and set all the appropriate headers to HTTP authorization.

Object Cloning in Java

By pradeep

On 23rd July, 2007

Object Cloning in Java

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Objects in Java are referred using reference types, and there is no direct way to copy the

contents of an object into a new object.

The assignment of one reference to another merely creates another reference to the same

object. Therefore, a special clone() method exists for all reference types in order to provide a

standard mechanism for an object to make a copy of itself. Here are the details you need to

know about cloning Java objects.

Why create a local copy?

The most probable reason for creating a local copy of an object is because you plan to

modify the object, and you don't want to modify the method caller's object. If you decide thatyou need a local copy, you can perform the operation by using the clone() method of the

Object class. The clone() method is defined as protected, but you must redefine it as public

in all subclasses that you might want to clone.

For example, the standard library class ArrayList overrides clone(), so you can call clone() for

ArrayList, like this:

Code: Java

import java.util.*; class MyInt{

privateint i;

public MyInt(int ii){ i = ii; }

publicvoid increment(){ i++; }
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publicString toString(){

returnInteger.toString(i); } }

publicclass Test{

publicstaticvoid main(String[] args){

ArrayList al = newArrayList();for(int i = 0; i < 10; i++ )

al.add(new MyInt(i));

ArrayList al1 = (ArrayList)al.clone(); // Increment all al1's elements: for(Iterator e = al1.iterator(); e.hasNext(); )

((MyInt)e.next()).increment(); } }

The clone() method produces an Object, which must be recast to the proper type. This

example shows how ArrayList's clone() method does not automatically try to clone each of

the objects that the ArrayList contains -- the old ArrayList and the cloned ArrayList are

aliased to the same objects. This is often called a shallow copy, since it's only copying the

"surface" portion of an object. The actual object consists of this "surface," plus all the objects

that the references are pointing to and all the objects those objects are pointing to, etc. This

is often referred to as the "Web of objects." When you copy the entire mess, it is called a

deep copy.

The Cloneable interface and deep copies

By default, classes in Java do not support cloning; the default implementation of the clone()

method throws a CloneNotSupportedException. You should override implementation of the

clone() method. Remember that you must make it public and, inside the method, your first

action must be super.clone(). Classes that want to allow cloning must implement the marker

interface Cloneable. Since the default implementation of Object.clone only performs a

shallow copy, classes must also override clone to provide a custom implementation when a

deep copy is desired. Basically, if you want to make objects of your class publicly cloneable,

you need code like this:

Code: Java

class Test implementsCloneable {

... publicObject clone() {
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try { return super.clone(); } catch(CloneNotSupportedException e ) { returnnull; } }...

}

If you are happy with a protected clone, which just blindly copied the raw bits of the object,

you don't need to redefine your own version. However, you will usually want a public one.

(Note: You can't create a private or default scope clone; you can only increase the visibility

when you override.)

Possible problems and a solution

Since the clone() method is protected, subclasses have to explicitly agree to be cloneable by

overriding this protected method with a public method. All of the Collections classes do this.

The subclass also has to implement Cloneable for the default cloning mechanism in

Object.clone() to work.

If you have an object that you know has a public clone() method, but you don't know the

type of the object at compile time, you have problems. For instance, say x is declared as an

Object. You can't just call x.clone() because Object.clone() is protected. If Cloneable defined

a public clone() method, you could use ((Cloneable) x).clone(), but it doesn't. You either have

to enumerate all the classes that you think x could be, or you have to resort to reflection.

Another problem arises when you try deep copying of a complex object. You're assuming

that the clone() method of all member object variables also does deep copy; this is too risky

of an assumption. You must control the code in all classes, or you must know that all classes

involved in deep copy operation do such a copy in the right way.

One solution to these problems is to clone using serialization. Serialization is usually used to

send objects off somewhere (such as into a file or over the network) so that somebody else

can reconstruct them later. You can abuse serialization to immediately reconstruct the object

yourself. If the object is serializable at all, the reconstruction should be a faithful copy. In

normal uses of serialisation, the original object is nowhere near a faithful copy; it could be on

the other side of the world at the far end of a network connection. You can be sure that

changing the copy will have no effect on the original.
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Servlet concepts

Q:What's the difference between applets and servlets?

A: There are many fundamentaldifferences

between Applet and Servlet classes, the

Java API documentation for the two types

will show you they have little in common.

Applets are essentially graphical user

interface (GUI) applications that run on the

client side in a network environment,

typically embedded in an HTML page.

Applets are normally based on Abstract

Windowing Toolkit components to maintain

backward-compatibility with the widest

range of browsers' Java implementations.

The application classes are downloaded to

the client and run in a Java Virtual Machine

provided by the browser, in a restrictive

security environment called a "sandbox".

Servlets are used to dynamically

generate HTTP responses and return HTML

content to Web browsers on the server side.

Servlets are often used to validate and

process HTML form submissions and control

a series of user interactions in what is

known as a Web application. Servlets can

be used to control all aspects of the request

and response exchange between a Web

browser and the server, called a servlet

container.


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Actions:Follow-up,clarify orcorrect this

answer. Submit anew question.

Q:Do we open servlet classes directly instead of HTML?

A: Servlets are used to deliver HTML toWeb browsers, but they are not like static

HTML documents. When you set up a

servlet in a Web application it has a URL like

a static HTML document, so you can link to

it, bookmark it or send the URL by email,

just as you would with an standard Web

page. The main difference is that the HTML

sent to the Web browser is composed

dynamically by the servlet and its contents

can be customised based on the details of

the request sent by the Web browser.

When you open a servlet URL the browser

does not display content of the servlet

class, but a dynamic HTML document

created by the servlet. The servlet class is

written as a standard Java class that

extends the HttpServlet class. In its most

basic form, the HTML output can be created

by a series ofprint() statements on

aPrintWriter. The method that handles

simple Web requests is called doGet(), as

below.

publicfinalvoid doGet(HttpServletRequest

request,HttpServletResponse

response) throws IOException {

PrintWriter output =response.getWriter();
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output.println("");output.println(" ");output.println(" ");

// Other HTML output

output.flush();output.close();

}



Q:What part of the Java platform do servlets and JSP belong to?A: The servlet and JSP APIs are a standard

extension to the core Java API and runtimesystem. Their package names are

prefixedjavax to indicate they are standard

extensions, but that means that they rely

upon a code implementation provided by a

specific vendor.

For example, the Apache Tomcat project

supplies its own implementation of the

servlet and JSP API that is integrated with

the servlet container. Developers must

compile their code using the vendor's

servlet package implementation by

including it in the compiler's classpath. The

servlet container includes the same

package classes in its runtime system and

feeds concrete instances of the servlet

interface types to the servlet's lifecycle

methods.


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Q:How does the JVM execute a servlet compared with aregular Java class?

A: Servlets are standard Java classes andare executed by the Java Virtual Machine in

exactly the same way as any other.

However, the environment or context in

which servlets are executed is different. A

servlet is not invoked directly through

a main()method, the class is loaded and run

by a servlet container.

When you run a servlet container, it reads

its configuration, identifies the servletclasses to make available, and uses the Java

classloader system to load and run the

servlets. When the servlets are first brought

into service, the servlet container calls the

servlet's init() method and passes

the ServletConfig object as an argument.

Whenever a request is made to the servlet,

the container creates a new thread in whichto handle it.



Q:How can I tell when a servlet is instantiated?A: A servlet must be instantiated before itis brought into service by the servlet

container, so one way to check is to make a

request to the servlet and check the

response. If you need to check indirectly,

you can override

theinit(ServletConfig) method and

add log(String) statements to it. This
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method is called after the servlet container

has instantiated the servlet before it is

brought into service.

Actions:Follow-up,clarify orcorrect thisanswer. Submit anew question.

Servlet programming

Q:How can I write a servlet using Javascript?A: Java servlets is a server side technologythat delivers dynamic content to Web

browsers and other clients. Javascript is also

delivered by a Web server, but the code is

only interpreted and executed after it has

been downloaded by the Web browser. This

means that it is not possible to write servlet

code in Javascript.

It is possible to include Javascript in the

output of servlets and Java Server Pages,

just like standard Web pages. It is also

possible to dynamically generate Javascript

using a servlet and use it as the source for a

script tag, though this is only advisable in

rare cases.



Q:Can I use a normal class to handle my requests?A: Servlets are normal Java classes, theycompile and run just like any other class. All

that is required is that servlets implement

thejavax.servlet.Servlet interface. Usually,
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they extend a protocol-specific class such

asjavax.servlet.http.HttpServlet.

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Can I use a normal class to handle my requests?



Q:Can I include normal Java classes in servlets?A: Any Java class can be used in a Web

application, provided you make the classesavailable to the servlet container at

runtime. The Java API classes can be used

directly by adding import statements to

your servlet class. Other supporting classes

can also be imported, but these classes

must be added to

the classes or lib directory of your

application.If you need to configure the supporting

classes, this can be done with standard

servlet configuration features using

the ServletConfig and ServletContext object

s available to

the init(ServletConfig) method.



Q:Can I use a constructor in my servlet?A: A servlet is a normal Java class, sowhen there are no custom constructors,

there is an implicit default constructor with
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no arguments. Servlet containers typically

use the Class.newInstance() method to load

servlets, so you must be careful to add an

explicit default constructor if you add non-

default constructors.




Can I use a constructor in my servlet?



Q:What happens if I add a main method to my servlet?A: It is possible to write a main method fora servlet, but it will not be called by the

servlet container, it is not part of the servlet

lifecycle process. If you invoke your servlet

through the main method using

thejava command it will behave exactly like

a standard Java class, it cannot operate as aWeb application in its own right and cannot

be addressed using HTTP requests. Servlets

must run in a servlet container to deliver

Web applications as intended.



Servlet start up

Q:Can one JSP or Servlet extend another Servlet or JSPA: It is possible and necessary for servletsto extend other classes, they are standard
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Java classes in most respects. Most servlets

extend the abstract HttpServlet class

included in a servlet container's API

implementation. This abstract class

implements the generic Servlet interface

with HTTP-specific methods and provides a

minimal basis to extend and create your

own servlet classes. If you need to develop

complex behaviour across a set of servlets

you can extend this hierarchy to create

numerous servlet subclasses.

The automatic source code generation and

compilation scheme used to create JSP

servlets means that extension of the JSP

servlet class is more complex and should

generally be avoided.



Q:How do I compile a servlet?A: To compile a servlet, you will need tohave the Java Servlet API classes in your

classpath. Most Java servlet containers

come with a copy, it may be

calledservlet.jar or something similar. The

basic classes are in the

packagesjavax.servlet andjavax.servlet.htt

p.




How do I compile a servlet?
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Actions:Follow-up,clarify or correct this


Q:Why won't my servlet compile?

A: All those "cannot find symbol" errormessages mean that you do not have the

Java servletJAR file on your compiler's

classpath, so it cannot find the servlet class

files. The servletJAR file is normally

distributed with your servlet container. For

Apache Tomcat it is a file

named {CATALINA_HOME}/common/lib/serv

let-api.jar. Add this to your compiler

classpath as follows.




Why won't my servlet compile?



Q:Where is the servlet stored and where does it run?A: When you create a servlet, the codemust be installed in a servlet container

which operates as an HTTP server

application. In the a development

environment the servlet container is often

installed on a developer's own workstation,

or a hardware server in the local network

where it can be accessed privately for

testing. In a production environment the

servlet container is usually installed on a

server that is accessible from the Internet,
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but the Java software arrangement is

basically the same.

Servlet code is compiled to Java class byte

code which is physically located on the

server hardware with the servlet container.

The servlet operates as an extension of the

servlet container and its code is executed

on the server. Web browsers operate as

clients which connect to the servlet

container, issue HTTP requests and receive

HTTP responses from the servlet container,

mostly in the form of HTML pages and other

Web content. The servlet code is not

downloaded or executed by the Web

browser at all, it only receives standard

Web content and renders it accordingly.



Q:What are the basic steps to run a servlet?A: The key steps in creating and running aservlet are outlined below in the simplest

form. There are different techniques that

can be used to complete these stages,

described in other FAQ answers.

1. Write and compile your servlet,

e.g. ExampleServlet.class

2. Create a Web application directory

structure under the webapp directory of your

servlet container (or use an existing one),

e.g.

3. {webapps-
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dir}/example

4. {webapps-dir}/example/WEB-INF

5. {webapps-dir}/example/WEB-INF/classes

6. Place your servlet class file in

the WEB-INF/classes directory for your

application, e.g.

7. {webapps-dir}/example/WEB-

INF/classes/ExampleServlet.class

8. Create a WEB-INF/web.xml file for

your application (or edit an existing one) and

add servlet and servlet-mapping elements

for your servlet, e.g.

9. {webapps-dir}/example/WEB-


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INF/web.xml

10. Start the servlet container.



Q:What is the difference between JAR and WAR files?A: There is no difference between thebinary form ofJAR and WAR files, they both

use zip compression provided by

thejar tool. You can create a WAR file by

navigating to the root directory of your Web

application and typing thejar command, as

below.




What is the difference between JAR and WAR files?



Servlet techniques

Q:Can I access a servlet from a stand alone client?A: It is certainly possible to access aservlet that is hosted in a servlet container.

AnyHTTP client should be able to connect to

a properly configured servlet container and

make requests to a servlet. However,
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servlets do not run in their own right, they

are not server applications.



Q:How can I write a servlet client for testing?A: Marty Hall provides the source code fora basic HTTP client in his book Core Servlets

and Java Server Pages. The source code for

chapter 3 is available online. Look

for WebClient.java and supporting classes.

This application allows you to manually

input the host, request path, HTTP header

values and view the headers returned by a

Web application.



Q:What is URL-rewriting?A: URL-rewriting is a way of maintaining a

session between an HTTP client and a

servlet container which does not use

cookies. Rather than exchange a session ID

in a cookie, the servlet container includes it

in the hyperlink URLs it generates for

servlets and JSP.




What is URL-rewriting?



Q:How can I redirect a request to another URL?
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A: The standard servlet method to redirectone HTTP request to another is

theHttpServletResponsesendRedirect(Strin

g) method. The String argument is the URL

for the address you want to direct people

to.

String otherURL ="http://example.com/otherPage.jsp";

response.sendRedirect(otherURL);

When you call this method it will issue an

HTTP redirect request to the browser,

commit the servlet response and end the

HTTP exchange. The browser should

automatically create a new HTTP request for

the given URL that will be entirely separate

from the original request.

HTTP redirection is different from

the RequestDispatcherforward() method,

which sends the forwarded content in the

original HTTP response stream.



Q:What's the difference between forward, include andredirection?

A: The RequestDispatcherforward() and include() methods are mechanisms that are

internal to a servlet container and do not

affect the public URL of a Web resource.

When you call the forward() method on

a RequestDispatcher with a JSPpath, the

servlet container returns the JSP content on
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the original servlet's URL; this effectively

becomes the response of the servlet itself.

Collection of large number of Servlet Interview Questions. These questions are

frequently asked in the Java Interviews.

Question: What is a Servlet?

Answer:Java Servlets are server side components that provides a powerful

mechanism for developing server side of web application. Earlier CGI was

developed to provide server side capabilities to the web applications. Although CGI

played a major role in the explosion of the Internet, its performance, scalability and

reusability issues make it less than optimal solutions. Java Servlets changes all that.

Built from ground up using Sun'swrite once run anywhere technology java servlets

provide excellent framework for server side processing.

Question: What are the types of Servlet?

Answer: There are two types ofservlets, GenericServlet and HttpServlet. GenericServlet defines the generic or

protocol independent servlet. HttpServlet is subclass of GenericServlet and provides

some http specific functionality linke doGet and doPost methods.

Question: What are the differences between HttpServlet and Generic Servlets?

Answer:HttpServlet Provides an abstract class to be subclassed to create an HTTP

servlet suitable for a Web site. A subclass ofHttpServlet must override at least one

method, usually one of these:

doGet, if the servlet supports HTTP GET requests doPost, for HTTP POST requests

doPut, for HTTP PUT requests

doDelete, for HTTP DELETE requests

init and destroy, to manage resources that are held for the life of the servlet

getServletInfo, which the servlet uses to provide information about itself

There's almost no reason to override the service method. service handles standard

HTTP requests by dispatching them to the handler methods for each HTTP request

type (the doXXXmethods listed above). Likewise, there's almost no reason to

override the doOptions and doTrace methods.GenericServlet defines a generic, protocol-independent servlet. To write an HTTP

servlet for use on the Web, extendHttpServlet instead.

GenericServlet implements

the Servlet and ServletConfig interfaces. GenericServlet may be directly extended
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by a servlet, although it's more common to extend a protocol-specific subclass such

as HttpServlet.

GenericServlet makes writing servlets easier. It provides simple versions of the

lifecycle methods init and destroy and of the methods in

the ServletConfig interface. GenericServlet also implements the log method,declared in the ServletContextinterface.

To write a generic servlet, you need only override the abstract service method.

Question: Differentiate between Servlet and Applet.

Answer: Servlets are server side components that executes on the server whereas

applets are client side components and executes on the web browser. Applets have

GUI interface but there is not GUI interface in case of Servlets.

Question: Differentiate between doGet and doPost method?

Answer: doGet is used when there is are requirement of sending data appended to a

query string in the URL. The doGet models the GET method of Http and it is usedto retrieve the info on the client from some server as a request to it. The doGet

cannot be used to send too much info appended as a query stream. GET puts the

form values into the URL string. GET is limited to about 256 characters (usually a

browser limitation) and creates really ugly URLs.

POST allows you to have extremely dense forms and pass that to the server without

clutter or limitation in size. e.g. you obviously can't send a file from the client to the

server via GET. POST has no limit on the amount of data you can send and because

the data does not show up on the URL you can send passwords. But this does not

mean that POST is truly secure. For real security you have to look into encryptionwhich is an entirely different topic

Question: What are methods of HttpServlet?Answer: The methods of HttpServlet class are :

* doGet() is used to handle the GET, conditional GET, and HEAD requests

* doPost() is used to handle POST requests

* doPut() is used to handle PUT requests

* doDelete() is used to handle DELETE requests

* doOptions() is used to handle the OPTIONS requests and

* doTrace() is used to handle the TRACE requests

Question: What are the advantages of Servlets over CGI programs?

Answer:Question: What are methods of HttpServlet?

Answer: Java Servlets have a number of advantages over CGI and other API's.

They are:

1. Platform Independence

Java Servlets are 100% pure Java, so it is platform independence. It can run on


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any Servlet enabled web server. For example if you develop an web application

in windows machine running Java web server. You can easily run the same

on apache webserver(if Apache Serve is installed) without modification or

compilation of code. Platform independency of servlets provide a great

advantages over alternatives of servlets.

2. Performance

Due to interpreted nature of java, programs written in java are slow. But the

java servlets runs very fast. These are due to the way servlets run on web

server. For any program initialization takes significant amount of time. But in

case of servlets initialization takes place very first time it receives a request and

remains in memory till times out or server shut downs. After servlet is loaded,

to handle a new request it simply creates a new thread and runs service method

of servlet. In comparison to traditional CGI scripts which creates a new process

to serve the request. This intuitive method of servlets could be use to develop

high speed data driven web sites.

3. Extensibility

Java Servlets are developed in java which is robust, well-designed and object

oriented language which can be extended or polymorphed into new objects. So

the java servlets takes all these advantages and can be extended from existing

class the provide the ideal solutions.

4. Safety

Java provides a very good safety features like memory management, exception

handling etc. Servlets inherits all these features and emerged as a very powerful

web server extension.

5. SecureServlets are server side components, so it inherits the security provided by the

web server. Servlets are also benefited with JavaSecurity Manager.

6. Question: What are the lifecycle methods ofServlet?

Answer: The interfacejavax.servlet.Servlet, defines the three life-cycle

methods. These are:

public void init(ServletConfig config) throws ServletException

public void service( ServletRequest req, ServletResponse res) throws

ServletException, IOException

public void destroy()

The container manages the lifecycle of the Servlet. When a new request cometo a Servlet, the container performs the following steps.

1. If an instance of the servlet does not exist, the web container

* Loads the servlet class.

* Creates an instance of the servlet class.

* Initializes the servlet instance by calling the init method. Initialization is

covered in Initializing a Servlet.
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2. The container invokes the service method, passing request and response

objects.

3. To remove the servlet, container finalizes the servlet by calling the servlet's

destroy method.

7. Question: What are the type of protocols supported by HttpServlet?

Answer: It extends the GenericServlet base classand provides an framework

for handling the HTTP protocol. So, HttpServlet only supports HTTP and

HTTPS protocol.

8. Question: What are the directory Structure of Web Application?Answer:Web component follows the standard directory structure defined in the J2EEspecification.

Directory Structure of Web Component

/

index.htm, JSP, Images etc.. Web-inf

web.xml

classes

servlet classes

lib

jar files

9. Question: What is ServletContext?

Answer:ServletContext is an Interface that defines a set of methods that a servlet usesto communicate with its servlet container, for example, to get the MIME type of a file,dispatch requests, or write to a log file. There is one context per "web application" per

Java Virtual Machine. (A "web application" is a collection of servlets and content

installed under a specific subset of the server's URL namespace such as /catalog and

possibly installed via a .war file.)

10. Question: What is meant by Pre-initialization of Servlet?

Answer:When servlet container is loaded, all the servlets defined in the web.xml filedoes not initialized by default. But the container receives the request it loads the servlet.But in some cases if you want your servlet to be initialized when context is loaded, you

have to use a concept called pre-initialization of Servlet. In case of Pre-initialization, theservlet is loaded when context is loaded. You can specify 1

in between the tag.

11. Question: What mechanisms are used by a Servlet Containerto

maintain session information?
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Answer:Servlet Container uses Cookies, URL rewriting, and HTTPS protocolinformation to maintain the session.

12. Question: What do you understand by servlet mapping?

Answer:Servlet mapping defines an association between a URL pattern and a servlet.You can use one servlet to process a number of url pattern (request pattern). For

example in case of Struts *.do url patterns are processed by Struts Controller Servlet.

13. Question: What must be implemented by all Servlets?

Answer:The Servlet Interface must be implemented by all servlets.

14. Question: What are the differences between Servlet and Applet?

Answer:Servlets are server side components that runs on the Servlet container.Applets are client side components and runs on the web browsers. Servlets have no GUI

interface.

15. Question: What are the uses of Servlets?

Answer: * Servlets are used to process the client request.* A Servlet can handle multiple request concurrently and be used to develop

high performance system

* A Servlet can be used to load balance among serveral servers, as Servlet caneasily forward request.

16. Question: What are the objects that are received when a servlets

accepts call from client?

Answer: The objects are ServeltRequest and ServletResponse . The

ServeltRequest encapsulates the communication from the client to the

server. While ServletResponse encapsulates the communication from the

Servlet back to the client.17. Question: What is a Session?

Answer: A Session refers to all the request that a single client makes to a server. A

session is specific to the user and for each user a new session is created to track all

the request from that user. Every user has a separate session and separate session

variable is associated with that session. In case of web applications the default time-

out value for session variable is 20 minutes, which can be changed as per the

requirement.

18. Question: What is Session ID?

Answer: A session ID is an unique identification string usually a long, random andalpha-numeric string, that is transmitted between the client and the server. Session

IDs are usually stored in the cookies, URLs (in case url rewriting) and hidden fields

of Web pages.

19. Question: What is Session Tracking?

Answer: HTTP is stateless protocol and it does not maintain the client state. But

there exist a mechanism called "Session Tracking" which helps the servers to


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maintain the state to track the series of requests from the same user across some

period of time.

20. Question: What are different types of Session Tracking?

Answer: Mechanism for Session Tracking are:

a) Cookiesb) URL rewriting

c) Hidden form fields

d) SSL Sessions

21. Question: What is HTTPSession Class?

Answer: HttpSession Class provides a way to identify a user across across multiple

request. The servlet containeruses HttpSession interface to create a session between

an HTTP client and an HTTP server. The session lives only for a specified time

period, across more than one connection or page requestfrom the user.

22. Question: Why do u use Session Tracking in HttpServlet?

Answer: In HttpServlet you can use Session Tracking to track the user state. Sessionis required if you are developing shoppingcart application or in any e-commerce

application.

23. Question: What are the advantage of Cookies over URL rewriting?

Answer: Sessions tracking using Cookies are more secure and fast. Session tracking

using Cookies can also be used with other mechanism of Session Tracking like url

rewriting.

24. Cookies are stored at client side so some clients may disable cookies so we

may not sure that the cookies may work or not.

In url rewriting requites large data transfer from and to the server. So, it leads to

network traffic and access may be become slow.

25. Question: What is session hijacking?

Answer: If you application is not very secure then it is possible to get the access of

system after acquiring or generating the authentication information. Session

hijacking refers to the act of taking control of a user session after successfully

obtaining or generating an authentication session ID. It involves an attacker using

captured, brute forced or reverse-engineered session IDs to get a control of a

legitimate user's Web application session while that session is still in progress.26. Question: What is Session Migration?

Answer: Session Migration is a mechanism of moving the session from one server to

another in case of server failure. Session Migration can be implemented by:

a) Persisting the session into database

b) Storing the session in-memory on multiple servers.
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27. Question: How to track a user session in Servlets?Answer: The interface HttpSession can be used to track the session in the Servlet.

Following code can be used to create session object in the Servlet: HttpSession

session = req.getSession(true);

28. Question: How you can destroy the session in Servlet?Answer: You can call invalidate() method on the session object to destroy the

session. e.g. session.invalidate();

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