Request and Response Objects

Every time a servlet's doGet or doPost method runs, the container hands it two objects: an HttpServletRequest representing everything the client sent, and an HttpServletResponse representing everything the servlet will send back. Almost all servlet logic revolves around reading from the request and writing to the response.

HttpServletRequest — Reading Client Data

The request object exposes form data, URL query parameters, headers, cookies, and metadata about the connection.

• getParameter(String name) — returns a single form field or query string value as a String (or null if absent).
• getParameterValues(String name) — returns an array, used for multi-value fields like checkboxes.
• getHeader(String name) — reads an HTTP request header, such as User-Agent or Authorization.
• getMethod(), getRequestURI(), getRemoteAddr() — metadata about the request itself.
• getAttribute() / setAttribute() — a per-request storage area, commonly used to pass data to a JSP via forward.

HttpServletResponse — Writing Back to the Client

The response object controls the status code, headers, and body sent back. Setting the content type before writing the body matters because it tells the browser how to interpret what follows.

• setContentType(String type) — e.g. "text/html" or "application/json"; must be called before getWriter() for correct behavior.
• getWriter() — returns a PrintWriter for writing character (text) output, such as HTML.
• getOutputStream() — returns a stream for binary output, such as an image or file download.
• setStatus(int sc) / sendError(int sc) — sets the HTTP status code, e.g. 404 or 500.
• sendRedirect(String url) — sends an HTTP redirect, instructing the browser to make a new request.

A Note on Character Encoding

Because request parameters and response bodies are ultimately byte streams, character encoding matters for anything beyond plain ASCII. Calling request.setCharacterEncoding("UTF-8") before reading any parameters, and response.setCharacterEncoding("UTF-8") (or including it in setContentType, e.g. "text/html;charset=UTF-8") before writing, avoids garbled text for non-English input.

Example

import java.io.IOException;
import java.io.PrintWriter;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class RegisterServlet extends HttpServlet {

    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        request.setCharacterEncoding("UTF-8");

        String username = request.getParameter("username");
        String userAgent = request.getHeader("User-Agent");

        response.setContentType("text/html;charset=UTF-8");
        PrintWriter out = response.getWriter();

        if (username == null || username.trim().isEmpty()) {
            response.setStatus(HttpServletResponse.SC_BAD_REQUEST);
            out.println("<h3>Error: username is required</h3>");
            return;
        }

        out.println("<html><body>");
        out.println("<h3>Welcome, " + username + "!</h3>");
        out.println("<p>Detected browser info: " + userAgent + "</p>");
        out.println("</body></html>");
    }
}

Form submitted with username=Ananya:
Welcome, Ananya!
Detected browser info: Mozilla/5.0 (...) Chrome/...

Form submitted with empty username:
HTTP 400 response body: Error: username is required

Exercise

Using the lesson's Reading form parameters and headers, then writing an HTML response example, create a small implementation or design sketch that demonstrates HttpServletRequest carries everything the client sent: parameters, headers, attributes, and connection metadata.. Add a test case or scenario that would catch Calling getParameter() for a form field name that does not match the HTML form's "name" attribute exactly (case-sensitive) and getting null., and describe the expected result.

Quick quiz

Interview questions