Skip to main content

Events and Listeners | Servlet Context Listener

 

Event:

Event is an entity, that notifies someone (listener) when something happens.

There are few kinds of events:

  • ServletContextEvent (notifies when the webapp is initialized / when the servlet Context is made)

  • ServletRequestEvent (notifies the listener each time the request comes)

Listener:

Listener is an entity that is notified each time an event occurs. It is like a piece of code which runs when an event happens. In other words, when we need to associate some part of code to some event, and we want that code to run only when that specific event occurs, then we use event listeners.

Types:
Few event listeners are mentioned below:
  • ServletContextListener        (Initializes resources and performs setup when the web application starts and shuts down).
  • HttpSessionListener           (Notifies when a user's session is created or destroyed).
  • ServletRequestListener        (Notifies when an HTTP request enters or leaves the application). 
  • HttpSessionAttributeListener (Notifies when an attribute is added, removed, or replaced in a session.)

Servlet Context Listener:

Here we should discuss about ServletContextListener:

ServletContextListeneris an interface which we implement in a class. In simple words it is used to make objects or implement logic for the webapp.
 
For Example: We get initParameters of ServletContext given in the web.xml and set an attribute in the ServletContext instance. 



As we know that we can't set objects as parameters, so we use Context listener to set the attributes in the servlet context which we can get in our servlet.

Remember:

getAttribute() returns Object type, so we need to cast it to our desired instance type.

Why we need listeners?

So, we can conclude that these are our flags or signals, when we need to run some code after a specific event, e.g. after a session starts, or after an attribute is created, then we use listeners of that specific event.



Labels:

Comments

Post a Comment

Popular posts from this blog

Java Servlets

  According to me its just a translator, who manages the HTTP requests and handle them to the appropriate function of the API. So then, API turns out to be a term or set of those functions which are kept public. Now let us see in little bit of detail what servlets are: For that we need to see few terms: Static Web Pages: Static web pages are the pages, which are already built, and whose content and design always remain the same.  For Example: About us page of some website. Contact info page of some website Dynamic Web Pages: Dynamic pages are not already defined. They are dependent on search queries. For Example: If we search top 5 books, and click on search button, result would be different. If we search top 5 countries and enter search, result would be different.  So, the search result for every search is different. Working: Now if we have to search static page let's say the first page of any website (which is same for every use in every country) then the process is sim...
Post a Comment
Read more

OnClick and {}

1. The onClick Mystery: Is it the same? Yes, the concept is 90% the same. If you know how to handle a click in plain HTML/JavaScript, you already know how to do it in React. The logic flow is identical: Event: A user does something (Click). Listener: The element hears it ( onClick ). Handler: A specific function runs to deal with it. The "React Twist" (The 10% difference): There are two tiny syntax differences you must memorize: CamelCase: In HTML, it is onclick (lowercase). In React, it is onClick (camelCase). Function, not String: HTML (Old): onclick="doSomething()" (A string of code). React (New): onClick={doSomething} (Passing the actual function itself). So yes, your intuition is correct: Whenever you need to handle logic (clicks, hovers, typing), you will stick to this pattern: on[Event]={functionName} . 2. The Curly Braces { } : The Portal This is the most important syntax rule in JSX. Imagine your code file has two "Modes": HTML Mode: W...
Post a Comment
Read more

Servlet Context and Servlet Config

  Problem without init-parameters? Imagine you write a servlet that connects to a database. If you write the database URL, username, and password directly in your Java code, you have a big problem: What if the database password changes? You have to find the code, change it, recompile the .java file into a .class file, and redeploy the entire application. What if you want to use the same servlet in a different project with a different database? You can't. The code is stuck with the old settings. This is where init parameters come in. They let you move this configuration data out of your code and into the deployment descriptor ( web.xml file). How init-parameters help when we use them in web.xml file? They help us in a way that we don't have to find that particular part of code to change it. And also, it saves us the hectic of changing source code and recompiling that again. Anyone with .war file can change the web.xml file  and use it with class file without having the ne...
Post a Comment
Read more