Java Notes

Servlet Introduction

Complete introduction to Java Servlets covering web development basics, HTTP protocol, servlet architecture, deployment descriptors, and how servlets work in Java EE web applications.

What is a Servlet?

A Servlet is a Java class that runs on a web server (like Apache Tomcat) and handles client requests (usually HTTP requests from browsers). It generates dynamic web content by processing input data, interacting with databases, and producing HTML, JSON, or other response formats.

Servlets are the foundation of Java web development and form the backbone of frameworks like Spring MVC.

Why Servlets Over Static HTML?

Feature	Static HTML	Servlets
Content	Fixed	Dynamic
Database interaction	Not possible	Yes
User-specific content	No	Yes
Form processing	Limited	Full support
Session management	No	Built-in

Servlet Architecture

Key Components

Web Server — Handles HTTP connections (e.g., Apache HTTP Server)
Servlet Container — Manages servlet lifecycle (e.g., Apache Tomcat, Jetty)
Servlet — Java class that processes requests
Deployment Descriptor (web.xml) — Configuration file for URL mapping

How a Servlet Container Works

Client Request	Web Server → Servlet Container
│	doGet() / doPost() │
Response	Client

Setting Up Your First Servlet

Prerequisites

JDK 11+ installed
Apache Tomcat 10+ (for Jakarta Servlet API)
IDE: IntelliJ IDEA or Eclipse

Project Structure

MyWebApp/ ├── src/ │ └── main/ │ ├── java/ │ │ └── com/example/ │ │ └── HelloServlet.java │ └── webapp/ │ ├── WEB-INF/ │ │ └── web.xml │ └── index.html ├── pom.xml

Maven Dependency

xml example

<dependencies>
    <dependency>
        <groupId>jakarta.servlet</groupId>
        <artifactId>jakarta.servlet-api</artifactId>
        <version>6.0.0</version>
        <scope>provided</scope>
    </dependency>
</dependencies>

<build>
    <plugins>
        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-war-plugin</artifactId>
            <version>3.3.2</version>
        </plugin>
    </plugins>
</build>

Your First Servlet

java example

package com.example;

import jakarta.servlet.*;
import jakarta.servlet.http.*;
import jakarta.servlet.annotation.*;
import java.io.*;

@WebServlet("/hello")
public class HelloServlet extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {

        // Set response content type
        response.setContentType("text/html");
        response.setCharacterEncoding("UTF-8");

        // Get writer to send response
        PrintWriter out = response.getWriter();

        out.println("<!DOCTYPE html>");
        out.println("<html>");
        out.println("<head><title>Hello Servlet</title></head>");
        out.println("<body>");
        out.println("<h1>Hello from Java Servlet!</h1>");
        out.println("<p>Server Time: " + new java.util.Date() + "</p>");
        out.println("</body>");
        out.println("</html>");
    }
}

Deployment Descriptor (web.xml)

The web.xml file is the traditional way to configure servlets (before annotations):

xml example

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="https://jakarta.ee/xml/ns/jakartaee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="https://jakarta.ee/xml/ns/jakartaee
         https://jakarta.ee/xml/ns/jakartaee/web-app_6_0.xsd"
         version="6.0">

    <servlet>
        <servlet-name>HelloServlet</servlet-name>
        <servlet-class>com.example.HelloServlet</servlet-class>
    </servlet>

    <servlet-mapping>
        <servlet-name>HelloServlet</servlet-name>
        <url-pattern>/hello</url-pattern>
    </servlet-mapping>

    <welcome-file-list>
        <welcome-file>index.html</welcome-file>
    </welcome-file-list>
</web-app>

Annotation-Based Configuration vs web.xml

Feature	@WebServlet Annotation	web.xml
Configuration	In Java code	Separate XML file
Ease of use	Simpler	More verbose
Override capability	Less flexible	Can override annotations
Best for	Small projects	Large enterprise apps

java example

// Annotation approach (modern)
@WebServlet(urlPatterns = {"/hello", "/hi"}, loadOnStartup = 1)
public class HelloServlet extends HttpServlet { }

Servlet API Hierarchy

Example

Serializable │ Servlet (Interface) ┌────┴────┐ GenericServlet (Abstract Class) │ HttpServlet (Abstract Class) │ YourCustomServlet (Your Implementation)

Key Interfaces

Servlet — Defines lifecycle methods (init, service, destroy)
ServletConfig — Servlet configuration parameters
ServletContext — Application-wide shared data
ServletRequest / ServletResponse — Request and response objects

Servlet vs CGI (Common Gateway Interface)

Feature	Servlet	CGI
Language	Java	Any (Perl, C, etc.)
Performance	High (thread-based)	Low (process-based)
Memory	Shared across requests	New process per request
Portability	Platform independent	Platform dependent
Scalability	Excellent	Poor

Common Mistakes to Avoid

Not setting content type — Always call response.setContentType() before writing
Writing after response is committed — Cannot set headers after getWriter() is called
Not handling character encoding — Use UTF-8 for international characters
Storing state in instance variables — Servlets are shared across threads (not thread-safe)
Ignoring exception handling — Always handle ServletException and IOException

Interview Quick Notes

Servlet is a server-side Java technology for handling HTTP requests
Servlet container (Tomcat) manages the servlet lifecycle
@WebServlet annotation replaces web.xml configuration
Servlets are thread-unsafe by default (one instance, multiple threads)
The servlet API is now under jakarta.servlet package (formerly javax.servlet)

Summary

Concept	Key Point
Servlet	Java class that handles HTTP requests on server
Container	Manages lifecycle, threading, and request routing
web.xml	Deployment descriptor for configuration
@WebServlet	Annotation-based URL mapping
HTTP	Stateless protocol; servlets add state via sessions

Key Takeaways for Servlet Introduction

This topic is fundamental in Java development and is frequently asked in interviews
Make sure to do hands-on practice for practical implementation
Understanding its use in real-world projects is important for writing production-ready code
Keep referring to documentation and official Java docs for the latest updates
Also develop testing and debugging skills alongside this topic
Follow industry best practices and actively participate in code reviews

Exam Focus

Revise definitions, diagrams, examples, and short-answer points for Servlet Introduction.

Interview Use

Prepare one clear explanation, one practical example, and one common mistake for this Java Master Course topic.

Search Terms

java-master-course, java master course, java, master, course, servlets, jsp, servlet

Continue learning this concept

Servlets JSPGenericServletUnderstanding GenericServlet abstract class in Java - protocol-independent servlet implementation, methods, and comparison with HttpServlet.Servlets JSPHttpServletComplete guide to HttpServlet class - HTTP-specific servlet implementation with doGet, doPost, doPut, doDelete methods and practical examples.Servlets JSPJSP IntroductionComplete introduction to JavaServer Pages (JSP) - how JSP works, JSP lifecycle, scripting elements, implicit objects, and building dynamic web pages.Servlets JSPServlet Life CycleDeep dive into the Servlet lifecycle including initialization, request handling, and destruction phases with practical code examples and diagrams.Servlets JSPCookies in ServletsComplete guide to handling cookies in Java Servlets - creating, reading, modifying, deleting cookies, cookie attributes, and security best practices.Servlets JSPExpression Language (EL)Complete guide to JSP Expression Language (EL) - syntax, operators, implicit objects, accessing beans, collections, and EL functions for clean JSP development.

PreviousRequest and Response Objects NextServlet Life Cycle

Java Notes

Servlet Introduction

Complete introduction to Java Servlets covering web development basics, HTTP protocol, servlet architecture, deployment descriptors, and how servlets work in Java EE web applications.

What is a Servlet?

Servlets are the foundation of Java web development and form the backbone of frameworks like Spring MVC.

Why Servlets Over Static HTML?

Feature	Static HTML	Servlets
Content	Fixed	Dynamic
Database interaction	Not possible	Yes
User-specific content	No	Yes
Form processing	Limited	Full support
Session management	No	Built-in

Servlet Architecture

Key Components

Web Server — Handles HTTP connections (e.g., Apache HTTP Server)
Servlet Container — Manages servlet lifecycle (e.g., Apache Tomcat, Jetty)
Servlet — Java class that processes requests
Deployment Descriptor (web.xml) — Configuration file for URL mapping

How a Servlet Container Works

Client Request	Web Server → Servlet Container
│	doGet() / doPost() │
Response	Client

Setting Up Your First Servlet

Prerequisites

JDK 11+ installed
Apache Tomcat 10+ (for Jakarta Servlet API)
IDE: IntelliJ IDEA or Eclipse

Project Structure

Maven Dependency

xml example

<dependencies>
    <dependency>
        <groupId>jakarta.servlet</groupId>
        <artifactId>jakarta.servlet-api</artifactId>
        <version>6.0.0</version>
        <scope>provided</scope>
    </dependency>
</dependencies>

<build>
    <plugins>
        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-war-plugin</artifactId>
            <version>3.3.2</version>
        </plugin>
    </plugins>
</build>

Your First Servlet

java example

package com.example;

import jakarta.servlet.*;
import jakarta.servlet.http.*;
import jakarta.servlet.annotation.*;
import java.io.*;

@WebServlet("/hello")
public class HelloServlet extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {

        // Set response content type
        response.setContentType("text/html");
        response.setCharacterEncoding("UTF-8");

        // Get writer to send response
        PrintWriter out = response.getWriter();

        out.println("<!DOCTYPE html>");
        out.println("<html>");
        out.println("<head><title>Hello Servlet</title></head>");
        out.println("<body>");
        out.println("<h1>Hello from Java Servlet!</h1>");
        out.println("<p>Server Time: " + new java.util.Date() + "</p>");
        out.println("</body>");
        out.println("</html>");
    }
}

Deployment Descriptor (web.xml)

The web.xml file is the traditional way to configure servlets (before annotations):

xml example

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="https://jakarta.ee/xml/ns/jakartaee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="https://jakarta.ee/xml/ns/jakartaee
         https://jakarta.ee/xml/ns/jakartaee/web-app_6_0.xsd"
         version="6.0">

    <servlet>
        <servlet-name>HelloServlet</servlet-name>
        <servlet-class>com.example.HelloServlet</servlet-class>
    </servlet>

    <servlet-mapping>
        <servlet-name>HelloServlet</servlet-name>
        <url-pattern>/hello</url-pattern>
    </servlet-mapping>

    <welcome-file-list>
        <welcome-file>index.html</welcome-file>
    </welcome-file-list>
</web-app>

Annotation-Based Configuration vs web.xml

Feature	@WebServlet Annotation	web.xml
Configuration	In Java code	Separate XML file
Ease of use	Simpler	More verbose
Override capability	Less flexible	Can override annotations
Best for	Small projects	Large enterprise apps

java example

// Annotation approach (modern)
@WebServlet(urlPatterns = {"/hello", "/hi"}, loadOnStartup = 1)
public class HelloServlet extends HttpServlet { }

Servlet API Hierarchy

Example

Serializable │ Servlet (Interface) ┌────┴────┐ GenericServlet (Abstract Class) │ HttpServlet (Abstract Class) │ YourCustomServlet (Your Implementation)

Key Interfaces

Servlet — Defines lifecycle methods (init, service, destroy)
ServletConfig — Servlet configuration parameters
ServletContext — Application-wide shared data
ServletRequest / ServletResponse — Request and response objects

Servlet vs CGI (Common Gateway Interface)

Feature	Servlet	CGI
Language	Java	Any (Perl, C, etc.)
Performance	High (thread-based)	Low (process-based)
Memory	Shared across requests	New process per request
Portability	Platform independent	Platform dependent
Scalability	Excellent	Poor

Common Mistakes to Avoid

Not setting content type — Always call response.setContentType() before writing
Writing after response is committed — Cannot set headers after getWriter() is called
Not handling character encoding — Use UTF-8 for international characters
Storing state in instance variables — Servlets are shared across threads (not thread-safe)
Ignoring exception handling — Always handle ServletException and IOException

Interview Quick Notes

Servlet is a server-side Java technology for handling HTTP requests
Servlet container (Tomcat) manages the servlet lifecycle
@WebServlet annotation replaces web.xml configuration
Servlets are thread-unsafe by default (one instance, multiple threads)
The servlet API is now under jakarta.servlet package (formerly javax.servlet)

Summary

Concept	Key Point
Servlet	Java class that handles HTTP requests on server
Container	Manages lifecycle, threading, and request routing
web.xml	Deployment descriptor for configuration
@WebServlet	Annotation-based URL mapping
HTTP	Stateless protocol; servlets add state via sessions

Key Takeaways for Servlet Introduction

This topic is fundamental in Java development and is frequently asked in interviews
Make sure to do hands-on practice for practical implementation
Understanding its use in real-world projects is important for writing production-ready code
Keep referring to documentation and official Java docs for the latest updates
Also develop testing and debugging skills alongside this topic
Follow industry best practices and actively participate in code reviews

Exam Focus

Revise definitions, diagrams, examples, and short-answer points for Servlet Introduction.

Interview Use

Prepare one clear explanation, one practical example, and one common mistake for this Java Master Course topic.

Search Terms

java-master-course, java master course, java, master, course, servlets, jsp, servlet

Continue learning this concept

PreviousRequest and Response Objects NextServlet Life Cycle