How to get client machine name and Ip address from Httpservlet?

In HttpServletRequest there are two methods
 a. getRemoteAddr()
 b. getRemoteHost()
 which will provide the necessary information.
 Actually in request header following information is stored.


Ip address of client m/c = request.getRemoteAddr();

Hostname of client =  request.getRemoteHost()

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What is difference between include action and include directive in JSP?

Before starting the difference  discussion,  first, we have to understand the JSP life cycle. when a request first comes to a JSP it will translate into servlet then compile and load into web container for further use. Subsequent requests  will serve by this translated servlet.


Let say we have a JSP page called greet.jsp which includes another JSP call  header.jsp. Now it can be included in two ways
 1. by page directive 
 2. by JSP action tag
1. By page directive : syntax is <%@ include file="header.jsp" %>
now when greet.jsp page translated  into a  servlet all the code written in the header.jsp goes to translated servlet's service()(incase of Httpservlet it's //doget() or dopost()) method. so it acts as one servlet which contains two JSP's code ( header.jsp + greet.jsp  ) and ready for serve subsequent requests. Now in mean time suppose  developer change the content of header.jsp, this change will not reflect to translated servlet as it is already compiled and loaded in the web container  . To make this happen you have to restart web container then it will recompile  but some web container smart enough to pick up this changes. so  include page directive is  not a good option for dynamic pages.


2. By JSP action tag: syntax

<jsp:include page="header.jsp">
 Here inclusion is done at request time so it maintains two servlets unlike page directive so it will show updated value each time.  so action tag is the good choice  for dynamic pages.

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Benifits of Static methods

Static keywords mean,  methods/variables attached to class, not with objects. So this variables/methods is shared with all objects instances of this class.Following advantages, we can get using static.

1. We can invoke a static method without creating objects. So Utility methods can be static

example

public class DateUtils
{
    public static Date getSystemdate()
{

    return  new Date(System.currenMillsInSecond());
}

  public static Date getPrevousDay()
{
}

//etc


}

.2. If you want to declare a Global value which will share among objects. suppose wants to calculate hit count of a servlet

public class Countservlet extends HttpServlet
{

    private static  int count = 0;

public void doGet(HttpServletRequest request,
HttpServletResponse response)
throws IOException
{

     count++;
}

public void doPost(HttpServletRequest request,
HttpServletResponse response)
throws IOException
{

   doGet(request,response);
}



}




3. If you want to make a class level synchronization by synchronizing static you can achieve it.


public class Account
{

private static int min_balance=100;

public synchronize void credit(int amount)
{
  min_balance= amount;
}
}


Now no object can change the state of min_balance  as it is class level synchronization.

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Can we have main method tn abstract class?

Yes, you can use the main method in abstract class. The main method is a static method so it is associated with Class, not with object/ instance. The abstract is applicable to the object so there is no problem if it contains the main method. In main method, you can not create an instance of the abstract class but you can instantiate other concrete class.


Here is a sample program


public abstract class TestAbstract {

public static void main(String[] args){

System.out.println("Hello world");

}

}

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Can we Instantiate Abstract Class?

The main reason is   abstract class may have abstract methods . An abstract method is like template/contract, according to inheritance rule  every concrete subclass must have to provide a concrete definition of the same. So we can say the abstract method is the only declaration.

 Now for a moment we  assume we can create an instance of abstract class


so,  we have an abstract class name Shape and an abstract method getShape()

Abstract class looks like,
public abstract Shape

{

  public abstract getshape(); //Only declaration

}

Now, next step is to create an Object of abstract shape as we assume we can instantiate the abstract class.

 Shape s =new shape();

and call
 s.getshape();


Think what it will return as it is the only declaration!!!! so JVM functionality will break.


Due to that, we can't instantiate the abstract class.

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Can we make constuctor as final

No Constructor can never be Final. Only private,public and protected modifiers can use with the constructor.


We use final in method level because  we don't want subclass inherits that method and overwrite it . But constructor is not a method and it can not be overridden as per java specification .

Let's think in a  different way


For say, constructor supports final now that mean's it can't be inherited but if you think about constructor call hierarchy then you can find the first statement of a constructor is super or this, so if the constructor is  not visible how it  can you call the super class constructor ? so we can't call the super class constructor, call hierarchy is uncompleted  so the object will not be created. For this reason, we can't use final in the constructor.

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Why we called Java as Platform Independent?

Before starting why we called Java as platform independent I want to share a simple incident with you and understand the Java compilation process in terms of that story.


Say Narendra Modi our respected PM giving a speech in Hindi, Next day in every news daily irrespective of language(Hindi, English, Bengali) this speech is published. Now the question is Narendra Modi give that speech in Hindi so how it translates to English or Bengali seamlessly.


No surprise right, there are reporters for every newspaper they analysis that speech and transform it to another language like English or Bengali.


So I can say theoretically Modi's speech is Language independent as next day everyone sees it in their respective languages.


Java, Does it the same way, It takes your code as input like Modi's live speech and there is Java interpreter which is platform dependent, for Linux, Java provides Linux interpreter for Windows a Windows interpreter, Whose task it taking that input and transform it to Machine code the language OS understand. SO it is like the reporters who translate Modi's speech to respective language.

Now coming to Technical part. 

Actually, to be specific Java compilation is a two-step process Compile then Run Compiler makes the byte code and JVM inter-prate this to machine specific language Here is the trick  I am taking about , Might you have seen when you download JDK there is OS specific version  one is for Unix one is for windows version why so, because  of JVM implementation JVM takes bytecode as input and provide m/c language as output so Linux specific JVM makes Linux specific machine code, windows do the same but as both take bytecode as input you can easily export Windows compile class  Linux m/c That's why they called Java as a platform independent .

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