Java Notes

HttpServlet

Complete guide to HttpServlet class - HTTP-specific servlet implementation with doGet, doPost, doPut, doDelete methods and practical examples.

What is HttpServlet?

HttpServlet is an abstract class in the jakarta.servlet.http package that extends GenericServlet and provides HTTP-specific functionality. It is the primary class you extend when building Java web applications.

HttpServlet automatically parses HTTP requests and dispatches them to the appropriate handler method (doGet, doPost, doPut, doDelete, etc.).

How HttpServlet Works Internally

When a request arrives, the servlet container calls service():

java example

// Inside HttpServlet (simplified source code)
protected void service(HttpServletRequest req, HttpServletResponse resp)
        throws ServletException, IOException {
    
    String method = req.getMethod();
    
    if (method.equals("GET")) {
        doGet(req, resp);
    } else if (method.equals("POST")) {
        doPost(req, resp);
    } else if (method.equals("PUT")) {
        doPut(req, resp);
    } else if (method.equals("DELETE")) {
        doDelete(req, resp);
    } else if (method.equals("HEAD")) {
        doHead(req, resp);
    } else if (method.equals("OPTIONS")) {
        doOptions(req, resp);
    } else if (method.equals("TRACE")) {
        doTrace(req, resp);
    } else {
        resp.sendError(HttpServletResponse.SC_METHOD_NOT_ALLOWED);
    }
}

HTTP Method Handlers

Method	Purpose	Typical Use Case
`doGet()`	Retrieve data	Loading pages, fetching API data
`doPost()`	Submit/create data	Form submission, creating resources
`doPut()`	Update data	Updating entire resources
`doDelete()`	Remove data	Deleting resources
`doPatch()`	Partial update	Updating specific fields
`doHead()`	Get headers only	Check if resource exists
`doOptions()`	Get allowed methods	CORS preflight requests

Complete CRUD Servlet Example

Example

package com.example; import jakarta.servlet.*; import jakarta.servlet.http.*; import jakarta.servlet.annotation.*; import java.io.*; import java.util.*; import java.util.concurrent.ConcurrentHashMap; @WebServlet("/api/students/*") public class StudentServlet extends HttpServlet { // In-memory database (use real DB in production) private Map<Integer, Student> students = new ConcurrentHashMap<>(); private int nextId = 1; // Inner class for Student static class Student { int id; String name; String email; int age; Student(int id, String name, String email, int age) { this.id = id; this.name = name; this.email = email; this.age = age; } } // GET — Retrieve students @Override protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { response.setContentType("application/json"); response.setCharacterEncoding("UTF-8"); PrintWriter out = response.getWriter(); String pathInfo = request.getPathInfo(); if (pathInfo == null || pathInfo.equals("/")) { // GET /api/students — List all students StringBuilder json = new StringBuilder("["); for (Student s : students.values()) { if (json.length() > 1) json.append(","); json.append(studentToJson(s)); } json.append("]"); out.println(json.toString()); } else { // GET /api/students/{id} — Get single student int id = Integer.parseInt(pathInfo.substring(1)); Student student = students.get(id); if (student != null) { out.println(studentToJson(student)); } else { response.setStatus(HttpServletResponse.SC_NOT_FOUND); out.println("{\"error\": \"Student not found\"}"); } } } // POST — Create student @Override protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { response.setContentType("application/json"); String name = request.getParameter("name"); String email = request.getParameter("email"); int age = Integer.parseInt(request.getParameter("age")); Student student = new Student(nextId++, name, email, age); students.put(student.id, student); response.setStatus(HttpServletResponse.SC_CREATED); response.getWriter().println(studentToJson(student)); } // PUT — Update student @Override protected void doPut(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { response.setContentType("application/json"); String pathInfo = request.getPathInfo(); int id = Integer.parseInt(pathInfo.substring(1)); Student existing = students.get(id); if (existing == null) { response.setStatus(HttpServletResponse.SC_NOT_FOUND); response.getWriter().println("{\"error\": \"Student not found\"}"); return; } // Read parameters from PUT body String name = request.getParameter("name"); String email = request.getParameter("email"); String ageStr = request.getParameter("age"); if (name != null) existing.name = name; if (email != null) existing.email = email; if (ageStr != null) existing.age = Integer.parseInt(ageStr); response.getWriter().println(studentToJson(existing)); } // DELETE — Remove student @Override protected void doDelete(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { response.setContentType("application/json"); String pathInfo = request.getPathInfo(); int id = Integer.parseInt(pathInfo.substring(1)); Student removed = students.remove(id); if (removed != null) { response.getWriter().println("{\"message\": \"Student deleted\"}"); } else { response.setStatus(HttpServletResponse.SC_NOT_FOUND); response.getWriter().println("{\"error\": \"Student not found\"}"); } } private String studentToJson(Student s) { return String.format( "{\"id\": %d, \"name\": \"%s\", \"email\": \"%s\", \"age\": %d}", s.id, s.name, s.email, s.age ); } }

Handling Form Data

HTML Form

html example

<form action="/register" method="POST">
    <input type="text" name="username" placeholder="Username">
    <input type="password" name="password" placeholder="Password">
    <input type="email" name="email" placeholder="Email">
    <select name="role">
        <option value="student">Student</option>
        <option value="teacher">Teacher</option>
    </select>
    <button type="submit">Register</button>
</form>

Servlet Processing the Form

Example

@WebServlet("/register") public class RegistrationServlet extends HttpServlet { @Override protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { // Get form parameters String username = request.getParameter("username"); String password = request.getParameter("password"); String email = request.getParameter("email"); String role = request.getParameter("role"); // Validate input List<String> errors = new ArrayList<>(); if (username == null || username.trim().isEmpty()) { errors.add("Username is required"); } if (password == null || password.length() < 6) { errors.add("Password must be at least 6 characters"); } if (email == null || !email.contains("@")) { errors.add("Valid email is required"); } response.setContentType("text/html"); PrintWriter out = response.getWriter(); if (!errors.isEmpty()) { // Show errors out.println("<h2>Registration Failed</h2>"); out.println("<ul>"); for (String error : errors) { out.println("<li style=\"color:red\">" + error + "</li>"); } out.println("</ul>"); out.println("<a href=\"/register.html\">Try again</a>"); } else { // Save user (in real app, save to database) out.println("<h2>Registration Successful!</h2>"); out.println("<p>Welcome, " + username + "!</p>"); } } }

File Upload with HttpServlet

java example

@WebServlet("/upload")
@MultipartConfig(
    maxFileSize = 1024 * 1024 * 5,  // 5MB max file size
    maxRequestSize = 1024 * 1024 * 10  // 10MB max request
)
public class FileUploadServlet extends HttpServlet {

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

        Part filePart = request.getPart("file");
        String fileName = filePart.getSubmittedFileName();
        long fileSize = filePart.getSize();
        String contentType = filePart.getContentType();

        // Save file to server
        String uploadPath = getServletContext().getRealPath("/uploads");
        File uploadDir = new File(uploadPath);
        if (!uploadDir.exists()) uploadDir.mkdirs();

        filePart.write(uploadPath + File.separator + fileName);

        response.setContentType("text/html");
        PrintWriter out = response.getWriter();
        out.println("<h2>File Uploaded Successfully</h2>");
        out.println("<p>Name: " + fileName + "</p>");
        out.println("<p>Size: " + fileSize + " bytes</p>");
        out.println("<p>Type: " + contentType + "</p>");
    }
}

Request Forwarding and Redirecting

java example

@WebServlet("/login")
public class LoginServlet extends HttpServlet {

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

        String username = request.getParameter("username");
        String password = request.getParameter("password");

        if ("admin".equals(username) && "password123".equals(password)) {
            // Forward (server-side) — URL doesn't change in browser
            request.setAttribute("user", username);
            RequestDispatcher dispatcher = request.getRequestDispatcher("/dashboard.jsp");
            dispatcher.forward(request, response);
        } else {
            // Redirect (client-side) — URL changes in browser
            response.sendRedirect("/login.html?error=invalid");
        }
    }
}

Forward vs Redirect

Feature	Forward	Redirect
URL in browser	Doesn't change	Changes to new URL
Server/Client	Server-side	Client-side (302 response)
Request attributes	Preserved	Lost
Speed	Faster (single request)	Slower (two requests)
Use case	Same application	Different application/URL

Common Mistakes

Not overriding any doXxx() method — Returns 405 Method Not Allowed
Overriding service() directly — Breaks HTTP method dispatching
Forgetting to set content type — Browser may not render correctly
Not closing resources — PrintWriter doesn't need close, but streams do
Calling both forward and redirect — IllegalStateException

Interview Key Points

HttpServlet extends GenericServlet, which implements Servlet
Never override service() in HttpServlet — override doGet/doPost instead
HttpServlet provides HTTP-specific objects (HttpServletRequest, HttpServletResponse)
doGet() is for idempotent operations; doPost() for state-changing operations
@MultipartConfig enables file upload handling
Forward keeps the same request; Redirect creates a new request

Summary

In this chapter, we learned about HttpServlet in detail. Here are the key points:

Basic introduction to the concept and why it is needed
Syntax and structure with complete examples
Internal working and best practices
Real-world applications and use cases
Common mistakes and how to avoid them
How this topic is asked in interviews

To practice these concepts, write and run the code in your IDE and verify the output. Modify each example and experiment so that you deeply understand the concept.

Exam Focus

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

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, http

Continue learning this concept

Servlets JSPGenericServletUnderstanding GenericServlet abstract class in Java - protocol-independent servlet implementation, methods, and comparison with HttpServlet.Servlets JSPServlet IntroductionComplete introduction to Java Servlets covering web development basics, HTTP protocol, servlet architecture, deployment descriptors, and how servlets work in Java EE web applications.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.Servlets JSPJSP IntroductionComplete introduction to JavaServer Pages (JSP) - how JSP works, JSP lifecycle, scripting elements, implicit objects, and building dynamic web pages.

PreviousGenericServlet NextJSP Introduction

Java Notes

HttpServlet

Complete guide to HttpServlet class - HTTP-specific servlet implementation with doGet, doPost, doPut, doDelete methods and practical examples.

What is HttpServlet?

HttpServlet automatically parses HTTP requests and dispatches them to the appropriate handler method (doGet, doPost, doPut, doDelete, etc.).

How HttpServlet Works Internally

When a request arrives, the servlet container calls service():

java example

// Inside HttpServlet (simplified source code)
protected void service(HttpServletRequest req, HttpServletResponse resp)
        throws ServletException, IOException {
    
    String method = req.getMethod();
    
    if (method.equals("GET")) {
        doGet(req, resp);
    } else if (method.equals("POST")) {
        doPost(req, resp);
    } else if (method.equals("PUT")) {
        doPut(req, resp);
    } else if (method.equals("DELETE")) {
        doDelete(req, resp);
    } else if (method.equals("HEAD")) {
        doHead(req, resp);
    } else if (method.equals("OPTIONS")) {
        doOptions(req, resp);
    } else if (method.equals("TRACE")) {
        doTrace(req, resp);
    } else {
        resp.sendError(HttpServletResponse.SC_METHOD_NOT_ALLOWED);
    }
}

HTTP Method Handlers

Method	Purpose	Typical Use Case
`doGet()`	Retrieve data	Loading pages, fetching API data
`doPost()`	Submit/create data	Form submission, creating resources
`doPut()`	Update data	Updating entire resources
`doDelete()`	Remove data	Deleting resources
`doPatch()`	Partial update	Updating specific fields
`doHead()`	Get headers only	Check if resource exists
`doOptions()`	Get allowed methods	CORS preflight requests

Complete CRUD Servlet Example

Example

Handling Form Data

HTML Form

html example

<form action="/register" method="POST">
    <input type="text" name="username" placeholder="Username">
    <input type="password" name="password" placeholder="Password">
    <input type="email" name="email" placeholder="Email">
    <select name="role">
        <option value="student">Student</option>
        <option value="teacher">Teacher</option>
    </select>
    <button type="submit">Register</button>
</form>

Servlet Processing the Form

Example

File Upload with HttpServlet

java example

@WebServlet("/upload")
@MultipartConfig(
    maxFileSize = 1024 * 1024 * 5,  // 5MB max file size
    maxRequestSize = 1024 * 1024 * 10  // 10MB max request
)
public class FileUploadServlet extends HttpServlet {

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

        Part filePart = request.getPart("file");
        String fileName = filePart.getSubmittedFileName();
        long fileSize = filePart.getSize();
        String contentType = filePart.getContentType();

        // Save file to server
        String uploadPath = getServletContext().getRealPath("/uploads");
        File uploadDir = new File(uploadPath);
        if (!uploadDir.exists()) uploadDir.mkdirs();

        filePart.write(uploadPath + File.separator + fileName);

        response.setContentType("text/html");
        PrintWriter out = response.getWriter();
        out.println("<h2>File Uploaded Successfully</h2>");
        out.println("<p>Name: " + fileName + "</p>");
        out.println("<p>Size: " + fileSize + " bytes</p>");
        out.println("<p>Type: " + contentType + "</p>");
    }
}

Request Forwarding and Redirecting

java example

@WebServlet("/login")
public class LoginServlet extends HttpServlet {

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

        String username = request.getParameter("username");
        String password = request.getParameter("password");

        if ("admin".equals(username) && "password123".equals(password)) {
            // Forward (server-side) — URL doesn't change in browser
            request.setAttribute("user", username);
            RequestDispatcher dispatcher = request.getRequestDispatcher("/dashboard.jsp");
            dispatcher.forward(request, response);
        } else {
            // Redirect (client-side) — URL changes in browser
            response.sendRedirect("/login.html?error=invalid");
        }
    }
}

Forward vs Redirect

Feature	Forward	Redirect
URL in browser	Doesn't change	Changes to new URL
Server/Client	Server-side	Client-side (302 response)
Request attributes	Preserved	Lost
Speed	Faster (single request)	Slower (two requests)
Use case	Same application	Different application/URL

Common Mistakes

Not overriding any doXxx() method — Returns 405 Method Not Allowed
Overriding service() directly — Breaks HTTP method dispatching
Forgetting to set content type — Browser may not render correctly
Not closing resources — PrintWriter doesn't need close, but streams do
Calling both forward and redirect — IllegalStateException

Interview Key Points

HttpServlet extends GenericServlet, which implements Servlet
Never override service() in HttpServlet — override doGet/doPost instead
HttpServlet provides HTTP-specific objects (HttpServletRequest, HttpServletResponse)
doGet() is for idempotent operations; doPost() for state-changing operations
@MultipartConfig enables file upload handling
Forward keeps the same request; Redirect creates a new request

Summary

In this chapter, we learned about HttpServlet in detail. Here are the key points:

Basic introduction to the concept and why it is needed
Syntax and structure with complete examples
Internal working and best practices
Real-world applications and use cases
Common mistakes and how to avoid them
How this topic is asked in interviews

To practice these concepts, write and run the code in your IDE and verify the output. Modify each example and experiment so that you deeply understand the concept.

Exam Focus

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

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, http

Continue learning this concept

PreviousGenericServlet NextJSP Introduction