Archive for the ‘COMPUTERS’ Category

HTTP Error 500.21 – Internal Server Error, Solution

HTTP Error 500.21 – Internal Server Error
Handler “PageHandlerFactory-Integrated” has a bad module “ManagedPipelineHandler” in its module list

I received above error when I tried to include my ASP.NET application into IIS 7 and browse it. This because of the handlers missing for specific version of Microsoft Dotnet Framework in your IIS 7. I had installed visual studio 10.0 and later I installed IIS 7 So IIS did not pick up all the handlers required for running application built in ASP.NET 4.0 or .NET 3.5 framework. Every time I browse any page of the application from IIS it shows only error with index 500.21, 500.19 ..

Now here is solution you can try out,
Step 1: Check the Control Panel->Programs->Windows features on or off (This is in your WINDOWS 7  OPERATING SYSTEM)
Step 2: Ones you see list of all features in the popup check for Internet Information Services features and should have check-box enabled for ASP.NET.
Step 3: You can now try the following command on your Visual Studio command prompt,

%windir%\Microsoft.NET\Framework\v4.0.21006\aspnet_regiis.exe -i

Note: Make sure you opened the command prompt using ‘Run as Administrator’. While doing above operation you should close all instances of Visual Studio IDE, tools.

This will add all handlers in IIS 7 and try browsing the page you should get the page without error.
You should also check that your application pool is targeted to use appropriate framework version.

Happy troubleshooting..

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Difference Between Const Ptr And Ptr to Const

This question becomes sometimes favorite in C interview. It very easy to confuse and play around in question that goes like this. What is difference between pointer to constant and constant pointer? How do you declare Constant Pointers and Pointers to Constants? If you could able to answer these questions then next question will be what is use of const ptr and ptr to const?

Basically when we use Const keyword in C language. Well, when we have some value in program that is not going to change then we make use of Const keyword. This tell compiler that the value of the variable is not going to change at any point of time during run-time, So this kind of variables are stored at initialized data segment of memory. To be precise Const type of variable values will be stored in initialized read-only area and the address of variable will be stored in initialized read-write area of data segment.

Well as it looks confusing but its easy to remember and play around all sort of questions. Here is the trick to remember and answer these confusing questions.

When someone says, pointer to constant char then start writing in reverse while coding, So in this case I will put first pointer – ie ‘* myptr’ then look at requirement what is const here, it is char. So it will be like – ‘ char * myptr’ and in the end put ‘const’ type. So to declare pointer to const char

const char * myptr;

Now when some says, declare constant pointer to char then, it will be,

char * const myptr;

The trick here is where to put ‘const’ keyword decides the declaration.  In first case ie pointer to a constant character, one cannot use ‘myptr’ pointer to change the value of char, ie value being pointed to.

    char temp = 'x';
    const char * myptr = &temp;
    *myptr = 'J'; // error -  assignment of read-only location *myptr

In case of constant pointer to a character. The location stored in the pointer cannot change. You cannot change where this pointer points.

    char temp = 'x';
    char temp1='y'

    char  *  const myptr = &temp;
    myptr = &temp1; // error -  assignment of read-only location *myptr

Now the most famous question in field of embedded systems. If you are working in embedded system and working on firmware ie programming for microcontrollers then this will be for you, declare const pointer to constant char and what is use of such variable. ie both value pointed by pointer is constant and pointer itself is constant. The answer is very simple its use is to declare program status word. Its special register which contains CPU status and all, simply its status flag of your microcontroller. So this kind of status registers are read only by value as well as by location.

const char * const myptr = &status;

So next time interview question comes on about const pointer remember me. Good Luck.



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Basic Standard Action Tags In JSP Tutorial With Examples JSP Technical Advices

Standard Action Tags In JSP

Standard Action Tags In JSP are meant for performing some action at run time. As the name of the tag itself suggests these tags are used for run time behavior. Unlike scripting tags which are meant for generating related servlet code at the time of translation,Standard Action Tags In JSP generates code for run time behavior.


Examples of Standard Action Tags In JSP are as follows :


  1. <jsp:include file= ”AnotherJsp.jsp”/>
  2. <jsp:forward file=”AnotherJsp.jsp”/>
  3. <jsp:useBean id=”obj” type=”SomeBean” class=”SomeBean” scope=”request”>
  4. <c:set var=”attrName” value=”attrVal” scope=”request”/>


There are so many other tags as well amongst which these are some and important.

Let us take these tags individually as see how do they contribute for Standard Action Tags In JSP


  1. <jsp:include file= ”AnotherJsp.jsp”/> : This tag is used for run time inclusion of another JSP page in an existing one. This inclusion is not similar to inclusion with include directive. Include directives are meant for static inclusion. This means that the code from the included JSP will be included int the including JSP at the translation time. But in the case of <jsp:include file= ”AnotherJsp.jsp”/> tag a run time call to the included JSP is done and the response from the included JSP is added to the response of the including JSP.
  2. <jsp:forward file=”AnotherJsp.jsp”/> : This tag is used for forwarding the request to another JSP at run time. The difference here from run time or dynamic inclusion is that here the response from the forwarded JSP only goes to the user and the response from the first JSP from where the forward happens gets flushed out.
  3. <jsp:useBean id=”obj” type=”SomeBean” class=”SomeBean” scope=”request”> : This tag is used to create or fetch a java bean object ,set or get its property value/s and retrieve or set this java bean object in certain scope.
  4. <c:set var=”attrName” value=”attrVal” scope=”request”/> : This tag is used to set an attribute in a particular scope.This is an important tag as our MVC applications rely heavily on attributes
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Explain Different Types Of Tags In JSP Interview Asked Questions And Answers

Different Types Of Tags In JSP


Java Sever Page (JSP) is an abstraction for servlet in which we have less java code and more tags(HTML and JSP). A JSP page is basically used for views with dynamic data(unlike HTML) and evolved basically for people who do not know java and still work at j2ee server side programming.

Depending upon the type of the usage the tags in JSP page are categorized as :


  1. Scripting Tags
  2. Standard Action Tags
  3. Custom Tags

Scripting Tags : These tags are meant for compile time and translation.This means that these tags are meant for translation of code to the servlet. Different types of scripting tags are:

a. Directive Tag

b. Declarative Tag

c. Scriptlet Tag

d. Expression Tag


a. Directive Tag : There are three types of directive tags :

1.Page Directive : It is used for imports,encoding etc.

Example: <%@ page import=”…” %>

2.Include Directive : Its is used for static inclusion of JSP and HTML pages

Example: <%@ include file=”anotherJsp.jsp” %>

3.Tag lib Directive: It is used for including tag libraries

Example: <%@ taglib uri=”…” prefix = “..”%>

b. Declarative Tag : These tags are used to declare variables or methods for the servlet translated servlet class.

Example: <%! int i =0;%>


c. Scriptlet Tag : These tags are used to write java code inside the JSP page.What goes inside the scriptlet tag is merged into the service method of the translated servlet code.

Example: <% System.out.println(“Hello”);%>


d. Expression Tag : These tags are used to print expressions.What is written inside this tag goes as an argument for out.print() inside the service method of the translated servlet.

Example : <%= “Hello” %>


It should be noted that we do not use semicolon inside expression tag because it will go as an argument in ou.print() function,hence will result in syntax error.While in case of declarative tag and scirptlet tag we use semicolon.

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How To Get Servlet Init Parameters In JSP

How To Get Servlet Init Parameters In JSP

We know that we can declare init parameters for JSP file in web.xml file as we do in the case of servlet.The entry in the web.xml file will be as follows:


Now comes the question that how can be access or retrieve those initialization parameters in the JSP page.Among the life cycle methods of a JSP page jspInit() method is one of the methods which can be overridden.The jspInit() method is called by the init() method of the servlet which is the result of the translation of the JSP page.To access the initialization parameters is one of the cases where we override the jspInit() method.When we need to declare a method we use the declarative tag as follows :

 public void jspInit()
   ServletConfig sConfig = getServletCOnfig();
   String str= sConfig.getInitParameter(“paramName”);

Here we have overridden the jspInit() method in the declarative tag.Since this method is called by the init method of the translated servlet code we can access the servlet config object with the help of getServletConfig() method as it is passed by the server to the servlet through init() method. This method (jspInit()) will appear as an overridden method in the translated servlet.The point should be noted here that init() method in the servlet calls this jpInit() life cycle method of the  jsp page so it can access the servlet config object.The life cycle method _jspService() is not permitted to be overridden .Initialization parameters for the the JSP page can be understood as the configuration parameters to be accessed by the translated servlet.After all a JSP page is a servlet at the end of the day and it is the translated servlet only which processes the requests from the clients.




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Declare Servlet Init Parameters For JSP Page In web.xml File JSP Initialization FAQ

How To Declare Initialization Parameters For JSP Page In web.xml file

Servlet initialization parameters are declared  inside web.xml page and are used to pass the value to the servlet as a part of configuration.Now the question comes that since we do not make an entry for JSP inside web.xml page,how to pass the init parameters to the JSP page.We should remember that a JSP page is a servlet at the end of the day.A JSP is ultimately converted into a servlet then only it processes requests.Like the entry for init parameters in web.xml in case of servlet we make an entry for the JSP page as well,but with a slight difference.The web.xml file looks like this :


Here we can find that the same way as we declare  init paramters for servlet within the servlet tags,the same way we make an entry for the JSP page so that the init parameters can be declared for that particular JSP page.The only difference is that instead of he <servlet-class> tag we use <jsp-file> tag.The url pattern for the JSP page is same as the JSP file name as we use the JSP file name as URL pattern in the request.Inside the <jsp-file> tag we use the JSP file name with .jsp extension because we need to access the JSP page and not the servlet class.

This is the way we declare servlet init parameters for the JSP page inside the web.xml file.We call it as servlet init parameters for JSP page because these init parameters are sent to the servlet which is the result of translation of the JSP page.And these are used as the configuration parameters for the translated servlet.Now the init parameters can be accessed in he JSP page code.

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Define URL Rewriting Frequently Asked Interview Question Answers

What Is URL Rewriting


As we know that session needs to be maintained because http is a stateless protocol.Due to which connection is lost or session is lost after every request response cycle between the client and the server.In order to maintain or track the session the server needs to get the additional information of the session id so that the session object pertaining to the client or server can be retrieved at the server side and the session can be managed.The session can be managed by either  cookies,hidden parameter or URL rewriting.

URL Rewriting is used in the cases where client browser does not accept cookies.In such case the server does not get the session id thus it is unable to relate he session object with the client or user.In order to maintain the session some extra piece of code needs to be added so that even if the cookies are not accepted at the browser level still the session can be maintained.URL Rewriting is something to fall upon as a precautionary measure.The code to get session and URL Rewriting is demonstrated below :

out.println("<a href = "\"+response.encodeURL()+""\>Click Here</a>");

Here the anchor tag is used for URL Rewriting.Since the extra measure is used to take care that if the client browser does not accept cookies the session id is attached to the URL so that the session can be maintained.In URL Rewriting the session id is passed as an extra information with the URL that means rewriting of the URL is done.URL Rewriting can also be done with redirection as below :

response.sendRedirect(response.encodeRedirectURL("URL-Pattern of the servlet"));

The first time the server  tries with both cookies and URL Rewriting ,if it finds that the cookies are not coming it checks for session id as a part of URL Rewriting and finds that it has to fall upon URL Rewriting if the code has provisioned that.


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Basic Servlet Attributes Thread Safety Interview Questions Answers Explained

Servlet Attributes Thread Safety  

An attribute is used to communicate some value from one servlet to another. Depending upon the scope of the attribute we can access the attributes at parts of the application.An attribute can be set at either of the scopes as follows :





Now in order to find out which attribute/s is/are thread safe,we can ponder upon with following discussion.

1.Application Attributes : An application attribute is a value set at application level. The code to set the attribute at application level is :


As these attributes are accessible throughout the application,any servlet or any other web entity can update or get these attributes from various parts of the application. Since application attributes can be accessed or modified by any servlet in the web application,these are not thread safe.


2.Session Attributes : A session attribute is a value set at session scope. Session is used to retain user specific data. The code to set the attribute at application level is :


When we talk about thread safety of session attributes,at one glimpse it deceives to appear as thread safe. In cases where we have the new tab feature in the modern browsers,the same session for the web application is opened in the new tab. In such case, if session attribute are updated from one tab will be reflected in the another tab with the same session. Thus we can say that session attributes are also not thread safe.

3.Request Attributes : A request attribute is a value set at request scope.he code to set the attribute at application scope is :


As we know that each request is a unique object , whatever attribute we set at request scope cant be updated or get from another request. Thus request attributes are thread safe.

Conclusion : Thus request attributes are thread safe and application and session attributes are not.

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Types Of Attributes In Servlet Servlet Scopes And Attributes Saving Data In A Servlet Attributes Interview Question

Types Of Attributes In Servlet


An attribute is an object used to pass some value from one servlet to another.An attribute in a servlet can be set at various scopes depending upon the visibility in the web application.As we know that there are three scopes at servlet level in the application.Theses scopes are :




An attribute can be set on either of the scopes depending on the situation we want access the attribute value either at request,session or application level.

When we want the value of an attribute o be accessed at request level we use request attributes and likewise for session and application.

Types of attributes in servlet can be explained on the basis of scope it belongs to :

1.Request Attributes

2.Session attributes

3.Application Attributes


1.Request Attributes can be set or get as demonstrated below in the code :


2.Session Attributes  can be set or get as shown below :


3.Application Attributes can be set or get as below :


Now comes the question where to use what?

Request Attributes : If we want to retain or communicate  a value across servlets where request does not change,we use request attributes.This can be the situation where include or forward is happening between the servlets in a web application.In such cases where we want to communicate a value across servlets we can use request attributes which when set from one servlet can be get in other servlets.

Session Attributes : In cases where user specific data has to be retained or communicated, we use session attributes.We use session attributes when we do not want a user’s data to be interfered by other users.It should be noted that these attributes can be retrieved  even when redirect is happening provided the session is maintained .


Application Attributes : In cases where some value has to be set at application level and all the entities in the web application can read that value we use application attributes.




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Difference Between Abstract Class And Interface In Java Abstract Class Vs Interface When To Use An Abstract Class And An Interface

Difference Between Abstract Class And Interface In Java


An abstract class in java is a class which has at least one abstract function.

An interface in java  is an entity and not a class which has all its methods as abstract.We cannot have even one implementation of function or method in an interface in java.

An abstract class with all its methods as abstract is close to being an interface.But the concept for usage of both is polls apart in context.

An abstract class looks like :

public abstract class AbstractClassExample1{
abstract void abstractMethod();
// concrete methods are still allowed in abstract classes
void nonAbstractMethod() {
System.out.println("This is a non abstract method.");

Here we can see that we can have both abstract and non abstract methods in abstract class.

An interface looks like :

interface InterfaceExample{

  public void method1();
  public void method2();


Here in the interface all the methods or functions are abstract.

All the methods in interface need to be public .

Now comes the scenario ,when to use an abstract class and when to use an interface.

Scenarios where we want re usability of some of the methods or functions and some methods or functions are left  for the client to implement as per their requirement calls for the use of an abstract class.In such cases client knows that some of the default implementation is already there and some part is up to them how to implement in the form of methods or functions.In such cases it logical to extend the concrete class from an abstract class.

Now comes the scenario where two coders are working for an application or software in which one calls the function or method  in his module which is written by another coder.If in future the first coder who writes the method or function ,for some reason changes the function signature it will hamper the code of the coder who is calling the function.To avoid these type of issues we can use interfaces.If the person who writes the method implements it with interface and the function call is also done with the reference type interlace,then both he coders know the standard to be followed in order to write or call a method.Thus in order to follow some standard we use interfaces.

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