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Introduction to Servlets

Introduction

Servlets are Java classes used to extend the capabilities of servers. They handle requests and generate responses, making them essential for dynamic web applications. They have a well-defined life cycle, which starts when it is loaded into the memory and ends when it is removed. They are used in big and old projects like for most banking software development companies and java software development services uses Servlets for its Scalability and easy integration.

Servlet Life Cycle:

It is a group of steps that a servlet goes through from its creation to its destruction. There are four Stages in its LifeCycle.

servlet-feature

The life cycle followed a path from its creation to destruction:

  • It is borned
  • It is initialized
  • It is ready to service
  • It is servicing
  • It is not ready to service
  • It is destroyed

Loading

When the web application starts the servlet container loads the servlet class into the memory.

Initialization

The container creates an instance of the servlet and initializes it by calling its init() method. This method is called only once during the life cycle and is used for initialization tasks such as opening a database connection or reading configuration parameters.

Request Processing

The servlet container calls the service() method of the servlet to process client requests. 

This method calls when we receive a request from the client.

In this getServletConfig() is an interface of Servlet that initializes the Servlet. 

Servlet returns a ServletConfig object, which contains initialization and startup parameters for the servlet. 

The service method is responsible for handling incoming HTTP requests. It dispatches requests to the appropriate HTTP methods (doGet, doPost, doPut, doDelete, etc.) based on the request type.

Implementations of its interface are responsible for storing the ServletConfig object so that this method can return it. The GenericServlet class, which implements this interface, already does this.

Destruction

The servlet calls the destroy() method of the servlet when the web application is stopped or when the servlet is removed from the container. This method is used for cleanup tasks such as closing the database connection and releasing system resources. 

getServletInfo in the interface of Servlet it returns a String that contains information related to the servlet and its methods. 

The getServletInfo interface returns information about the servlet, such as author, version, and copyright.

The getServletInfo method returns a string that would be of plain text and not markup of any kind (such as HTML, XML, etc.)

Types of Servlets:

GenericServlet

GenericServlet is an abstract class in Java that provides a generic, protocol-independent implementation of the Servlet interface. It’s designed to be a superclass for more specific implementations, such as HTTP servlets.

GenericServlet provides methods for managing the lifecycle, including initialization (init()) and destruction (destroy()). Subclasses can override these methods to perform initialization tasks when the servlet is loaded into memory and cleanup tasks when the servlet is being unloaded.

GenericServlet provides methods to access configuration parameters defined in the deployment descriptor (Web.xml). Subclasses can use these methods to retrieve initialization parameters and other configuration settings specific to their application.

HttpServlet

HttpServlet is a subclass of GenericServlet that provides additional methods for handling HTTP requests. It implements the HttpServletRequest and HttpServletResponse interfaces, which provide methods for accessing HTTP request and response headers, parameters, and other information.

HttpServlet provides by default implementations for the method doGet(), doPost(), doPut(), doDelete(), and other HTTP request methods.

Some Methods of Http ServletClass: 

 doGet : If the servlet supports HTTP GET requests

doPost : For HTTP POST requests

doPut : For HTTP PUT requests

doDelete : For HTTP DELETE requests init & destroy : To manage resources that are held for the life of the servlet

 getServletInfo : Used to provide information about itself

doHead : It receives an HTTP HEAD request from the protected service method and handles the request

doOptions : Called by the server to handle a OPTIONS request.

doTrace : Called by the server to handle a TRACE request.

 getLastModified : Returns the time the HttpServletRequestobject was last modified, in milliseconds.

Advantages of a Java Servlet:

Platform Independence: They are Java classes that are deployed on a web server to generate dynamic web content and handle client requests. They adhere to the Java Servlet API, which provides a standard interface for developing web applications.

They can run on any web server that supports the Java Servlet API, such as Apache Tomcat, Jetty, JBoss, GlassFish, and others.

Robust and Efficient: Java’s strong typing, exception handling, and memory management contribute to their robustness and efficiency. They are less prone to memory leaks and security vulnerabilities compared to some other web technologies.

Performance: They are compiled into bytecode, which can be executed more efficiently than interpreted scripts. They are well-suited for high-performance applications.

Server-Side Processing: They enable server-side processing, allowing you to perform tasks like form handling, database access, and session management on the server, which is essential for secure and scalable web applications.

Scalability: Servlet-based applications can be scaled both vertically and horizontally to accommodate increasing loads and demands.

Vertical scalability involves adding more resources to the existing server, such as increasing CPU, memory, or storage capacity.

Horizontal scalability involves distributing the load across multiple servers, often achieved through load balancing techniques.

Servlet containers and application servers like Apache Tomcat, Jetty, and others support clustering and load balancing features to facilitate horizontal scalability.

Integration: They can be seamlessly integrated with various Java EE (Enterprise Edition) technologies and frameworks, such as JavaServer Pages (JSP), JavaServer Faces (JSF), Enterprise JavaBeans (EJB), and others.

They can also integrate with third-party libraries and frameworks for specific functionalities like security, authentication, authorization, and data access.

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