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Introduction to Servlets
Table of Contents
Servlets are Java classes used to extend the capabilities of servers. servlets 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.
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.
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public class MyServlet implements Servlet { @Override public void init(ServletConfig config) throws ServletException { } |
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.
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public class MyServlet implements Servlet { @Override public ServletConfig getServletConfig() { return null; } @Override public void service(ServletRequest req, ServletResponse res) throws ServletException, IOException { } |
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.
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public class MyServlet implements Servlet { @Override public ServletConfig getServletConfig() { return null; } @Override public void service(ServletRequest req, ServletResponse res) throws ServletException, IOException { } @Override public void init(ServletConfig config) throws ServletException { } @Override public String getServletInfo() { return null; } @Override public void destroy() { } |
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 a generic abstract class in Java, which is used to develop server objects that adhere to protocol standards and support different protocols. It is designed an abstract class that is to be implemented coder-specific like HTTP servlets.
The method offers to manage lifecycle to include initialization (initialize()) and elimination (destroy()). With subclasses you can override these methods to perform a task of initialization when the servlet is loaded in the memory and a cleanup task when it is unload.
GenericServlet allows you to perform actions like getting configuration parameters from the settings that are specified in the deployment descriptor (Web.xml). Subclasses can use such method to get init arguments and also other settings that are application-oriented.
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import javax.servlet.GenericServlet; import javax.servlet.ServletException; import javax.servlet.ServletRequest; import javax.servlet.ServletResponse; import java.io.IOException; public class MyServlet extends GenericServlet { @Override public void service(ServletRequest req, ServletResponse res) throws ServletException, IOException { |
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.
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package com.siena.web; import javax.servlet.ServletException; import javax.servlet.ServletRequest; import javax.servlet.ServletResponse; import javax.servlet.http.HttpServlet; import java.io.IOException; public class MyServlet extends HttpServlet { @Override public void service(ServletRequest req, ServletResponse res) throws ServletException, IOException { } } |
Some Methods of Http ServletClass:
doGet : If the servlet supports HTTP GET requests
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public class MyServlet extends HttpServlet { // The doGet() method handles HTTP GET requests public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { // Set response content type response.setContentType("text/html"); // Actual logic goes here PrintWriter out = response.getWriter(); out.println("<h1>Hello World from doGet() method!</h1>"); } |
doPost : For HTTP POST requests
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// The doPost() method handles HTTP POST requests public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { // Set response content type response.setContentType("text/html"); // Actual logic goes here PrintWriter out = response.getWriter(); out.println("<h1>Hello World from doPost() method!</h1>"); } |
doPut : For HTTP PUT requests
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The doPut() method handles HTTP PUT requests public void doPut(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { // Set response status code and content type response.setStatus(HttpServletResponse.SC_METHOD_NOT_ALLOWED); response.setContentType("text/html"); // Actual logic goes here PrintWriter out = response.getWriter(); out.println("<h1>HTTP PUT method is not supported!</h1>"); } |
doDelete : For HTTP DELETE requests init & destroy : To manage resources that are held for the life of the servlet
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// The doDelete() method handles HTTP DELETE requests public void doDelete(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { // Set response status code and content type response.setStatus(HttpServletResponse.SC_METHOD_NOT_ALLOWED); response.setContentType("text/html"); // Actual logic goes here PrintWriter out = response.getWriter(); out.println("<h1>HTTP DELETE method is not supported!</h1>"); } |
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.
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// The doOptions() method handles HTTP OPTIONS requests public void doOptions(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { // Set response status code and content type response.setStatus(HttpServletResponse.SC_METHOD_NOT_ALLOWED); response.setContentType("text/html"); // Actual logic goes here PrintWriter out = response.getWriter(); out.println("<h1>HTTP OPTIONS method is not supported!</h1>"); } |
doTrace : Called by the server to handle a TRACE request.
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// The doTrace() method handles HTTP TRACE requests public void doTrace(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { // Robust and Efficient: response status code and content type response.setStatus(HttpServletResponse.SC_METHOD_NOT_ALLOWED); response.setContentType("text/html"); // Actual logic goes here PrintWriter out = response.getWriter(); out.println("<h1>HTTP TRACE method is not supported!</h1>"); } |
getLastModified : Returns the time the HttpServletRequestobject was last modified, in milliseconds.
Advantages of a Java Servlet:
Platform Independence
They are Java servlets that are installed on a web server so that they can produce dynamic web content and process and handle clients’ requests. They follow the Java Servlet API which functions as a standard interface and undertakes the web development tasks.
Robust and Efficient:
java’s brilliant type system, the ability to handle exceptions as well as memory management are among other things, that make their robustness and efficiency even higher. Because of this, they are more rarely executing memory leaks and security threats which are not native to the technology.
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 permit on-server processing; hence, you can undertake some tasks which may be form handling, database access and session management on the server based on the type of web application as these are important for secure and scalable web applications.
Scalability
Servlet-based Apps can be scaled both vertically and horizontally by virtue of their capability to maintain load normalcy as well as sustain expandable demands.
Speaking of the vertical scalability, it is the process of building up the server capacity by adding prepared resources, e.g. more CPU, memory, or storage.
Horizontal scalability, which distributes load across many servers and is mostly accomplished through load balancing, is a methodology.
Apache Tomcat together with servlet containers and application servers such as, Jetty, etc. have “clustering and load balancing” capabilities to achieve “horizontal scalability.”
Integration
The comprehensive integration with Java EE (Enterprise Edition) technologies and combine with frameworks of various kinds that may be JSP (JavaServer Pages), JSF (JavaServer Faces), EJB (Enterprise Java Beans), and others
They can also, handle with third-party libraries and framework for particular needs such as the security, authentication, authorization, and data access.
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