Java Bean
JavaBeans are usual Java classes which adhere to certain coding conventions. Following are the coding conventions I am talking about :
• Implements java.io.Serializable interface
• Provides no argument constructor
Provides getter and setter methods for accessing it's properties
Definition: A Java Bean is a reusable software component that can be visually manipulated in builder tools.
To understand the precise meaning of this definition of a Bean, clarification is required for the following terms:
• Software component
• Builder tool
• Visual manipulation
Java Servlets
Servlets are snippets of Java programs which run inside a Servlet Container. A Servlet Container is much like a Web Server which handles user requests and generates responses. Servlet Container is different from a Web Server because it can not only serve requests for static content like HTML page, GIF images, etc., it can also contain Java Servlets and JSP pages to generate dynamic response. Servlet Container is responsible for loading and maintaining the lifecycle of the a Java Servlet. Servlet Container can be used standalone or more often used in conjunction with a Web server. Example of a Servlet Container is Tomcat and that of Web Server is Apache.
Servlets are actually simple Java classes which must implement the javax.servlet.Servletinterface. This interface contains a total of five methods. Most of the time you don't need to implement this interface. Why? Because javax.servlet package already provides two classes which implement this interface i.e. GenericServlet and HttpServlet. So all you need to do is to extend one of these classes and override the method(s) you need for your Servlet. GenericServlet is a very simple class which only implements thejavax.servlet.Servlet interface and provides only basic functionality. On the other hand,HttpServlet is a more useful class which provides methods to work with the HTTP protocol. So if your Servlet works with HTTP protocol then youshould extend javax.servlet.http.HttpServlet class to build Servlets and this is what we are going to do in this article.
Servlets once initialized are kept in memory. So every request which the Servlet Container receives, is delegated to the in-memory Java Servlet which then generates the response. This 'kept in memory' feature makes Java Servlets, a fast and efficient method of building Web Applications.
Life cycle of Servlet
The life cycle of a servlet can be categorized into four parts:
1. Loading and Inatantiation: The servlet container loads the servlet during startup or when the first request is made. The loading of the servlet depends on the attribute
2. Initialization: After creating the instances, the servlet container calls the init() method and passes the servlet initialization parameters to the init() method. The init() must be called by the servlet container before the servlet can service any request. The initialization parameters persist untill the servlet is destroyed. The init() method is called only once throughout the life cycle of the servlet.
The servlet will be available for service if it is loaded successfully otherwise the servlet container unloads the servlet.
3. Servicing the Request: After successfully completing the initialization process, the servlet will be available for service. Servlet creates seperate threads for each request. The sevlet container calls the service() method for servicing any request. The service() method determines the kind of request and calls the appropriate method (doGet() or doPost()) for handling the request and sends response to the client using the methods of the response object.
4. Destroying the Servlet: If the servlet is no longer needed for servicing any request, the servlet container calls the destroy() method . Like the init() method this method is also called only once throughout the life cycle of the servlet. Calling the destroy() method indicates to the servlet container not to sent the any request for service and the servlet releases all the resources associated with it. Java Virtual Machine claims for the memory associated with the resources for garbage collection.
Life Cycle of a Servlet
The Life Cycle of a Servlet
Each servlet has the same life cycle:
• A server loads and initializes the servlet
• The servlet handles zero or more client requests
• The server removes the servlet
(some servers do this step only when they shut down)
Initializing a Servlet
When a server loads a servlet, the server runs the servlet's init method. Initialization completes before client requests are handled and before the servlet is destroyed.
Even though most servlets are run in multi-threaded servers, servlets have no concurrency issues during servlet initialization. The server calls the init method once, when the server loads the servlet, and will not call the init method again unless the server is reloading the servlet. The server can not reload a servlet until after the server has destroyed the servlet by calling the destroy method.
Interacting with Clients
After initialization, the servlet is able to handle client requests. This part of the servlet life cycle was handled in the previous lesson.
Destroying a Servlet
Servlets run until the server are destroys them, for example, at the request of a system administrator. When a server destroys a servlet, the server runs the servlet's destroy method. The method is run once; the server will not run that servlet again until after the server reloads and reinitializes the servlet.
When the destroy method runs, another thread might be running a service request. The Handling Service Threads at Servlet Termination lesson shows you how to provide a clean shutdown when there could be long-running threads still running service requests.
