JAVA EE: Servlets : Display HTTP Request and Response headers

Sunday, 4 May 2014

Servlets : Display HTTP Request and Response headers

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What Are HTTP Request and Response Headers?

Whenever a browser communicates with a web server, it sends HTTP headers containing critical metadata such as browser specifications, supported MIME types, compression encodings, host information, and cookies. Similarly, the server returns response headers providing information about content type, date, caching instructions, and status codes.

In Java Servlets, inspecting these headers allows you to build smarter applications—such as serving different assets based on the client browser, inspecting user language preferences, or debugging communication protocols.

Step 1: Reading Request Headers in HttpServlet

The HttpServletRequest interface provides dedicated methods to retrieve header names and values:

getHeaderNames(): Returns an Enumeration<String> containing all header names sent by the browser.
getHeader(String name): Returns the value of a specific header as a String.
getHeaders(String name): Returns an Enumeration<String> for headers that contain multiple values.

    Enumeration<String> headerNames = request.getHeaderNames();

    while (headerNames.hasMoreElements()) {

      String headerName = headerNames.nextElement();

      String headerValue = request.getHeader(headerName);

      out.println(headerName + " : " + headerValue + "<br>");

    }

Step 2: Setting & Inspecting Response Headers

You can customize response headers using HttpServletResponse before committing output to the browser:

    // Setting custom and standard headers

    response.setContentType("text/html");

    response.setHeader("Custom-Header", "JavaServletDemo");

    response.setDateHeader("Expires", System.currentTimeMillis() + 60000);

    // Inspecting response headers (Servlet 3.0+)

    Collection<String> responseHeaderNames = response.getHeaderNames();

    for (String name : responseHeaderNames) {

      String val = response.getHeader(name);

      out.println(name + " : " + val + "<br>");

    }

Common HTTP Headers You Should Know

User-Agent: Identifies the client software, operating system, and browser version.
Accept-Encoding: Details supported compression formats (such as gzip, deflate).
Host: Identifies the target domain and port number being accessed.
Content-Type: Tells the receiving party the media type of the transmitted body data.

📺 Related HTTP & Servlets Tutorials

Deepen your understanding of HTTP request and response cycles with these tutorials from our channel:

Click here to watch in Youtube : https://www.youtube.com/watch?v=hHcvTVFx9S0

Click the below Image to Enlarge

DisplayAllHeaders Project Dir Structure

DisplayHeadersServlet.java

import java.io.IOException;
import java.io.PrintWriter;
import java.util.Enumeration;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class DisplayHeadersServlet extends HttpServlet
{

    private static final long serialVersionUID = 1L;

    public void init() throws ServletException
    {

        System.out.println("\n*******************************************\n");
        System.out.println("HelloWorld servlet init method has been called");
        System.out.println("\n*******************************************\n");

    }

    public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException
    {

        response.setContentType("text/html");

        PrintWriter out = response.getWriter();
        String title = "HTTP Header Request Example";
        String docType = "<!doctype html public \"-//w3c//dtd html 4.0 "
                + "transitional//en\">\n";
        out.println(docType + "<html>\n" + "<head><title>" + title
                + "</title></head>\n" + "<body bgcolor=\"#f0f0f0\">\n"
                + "<h1 align=\"center\">" + title + "</h1>\n"
                + "<table width=\"100%\" border=\"1\" align=\"center\">\n"
                + "<tr bgcolor=\"#949494\">\n"
                + "<th>Header Name</th><th>Header Value(s)</th>\n" + "</tr>\n");

        Enumeration<String> headerNames = request.getHeaderNames();

        while (headerNames.hasMoreElements())
        {
            String headerName = (String) headerNames.nextElement();
            out.print("<tr><td>" + headerName + "</td>\n");
            String headerValue = request.getHeader(headerName);
            out.println("<td> " + headerValue + "</td></tr>\n");
        }
        out.println("</table>\n</body></html>");
    }

    public void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException
    {
        doGet(request, response);
    }

    public void destroy()
    {
        // do nothing.
    }
}

web.xml

<?xml version="1.0" encoding="ISO-8859-1"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
xmlns="http://java.sun.com/xml/ns/javaee" 
xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd" 
metadata-complete="true" version="3.0">
  <display-name>ServletForm GET</display-name>
  <description>
        This is a simple web application with a source code organization
        based on the recommendations of the Application Developer's Guide.
    </description>
  <servlet>
    <servlet-name>readAllHeaders</servlet-name>
    <servlet-class>DisplayHeadersServlet</servlet-class>
  </servlet>
  <servlet-mapping>
    <servlet-name>readAllHeaders</servlet-name>
    <url-pattern>/readAllHeaders</url-pattern>
  </servlet-mapping>
</web-app>

index.html

<!DOCTYPE HTML><html lang="en"><head>
<meta charset="UTF-8">
<title>Order Pizza</title>
</head>
<body>
<p>
<h3>Order Pizza</H3>
<p></p>
<ul>
    <li><a href="pizza.html">Order Pizza</a></li>
</ul>
</body></html>

pizza.html

<html>
<head>
<title>Retrieving All Parameters</title>
 </head>
<body>
<h4>Order your Pizza Here</h4>
<form action="readAllHeaders" method="POST">
<b>Name</b> <input type="text" name="name"><br><br>
<b>Select the Crust:</b>
<select name = "crust">
<option value="pan">Pan</option>
<option value="thin">Thin Crust</option>
<option value="deep">Deep Crust</option>
<option value="cheese">Cheese Burst</option>
</select> <br><br>
<b>Toppings: </b><br>
<input type="checkbox" name="toppings" value="peas">Peas<br>
<input type="checkbox" name="toppings" value="paneer">Paneer<br>
<input type="checkbox" name="toppings" value="redpeppers">Red Peppers<br>
<input type="checkbox" name="toppings" value="pineapple">Pineapple<br>
<input type="checkbox" name="toppings" value="onion">Onion<br>
<input type="checkbox" name="toppings" value="tomato">Tomato<br><br>
 
<b>Select 1 FREE Appetizer</b>
<input type="radio" name="appetizer" value="Garlic Bread">Garlic Bread
<input type="radio" name="appetizer" value="Cheese Garlic Bread">Cheese Garlic Bread
<input type="radio" name="appetizer" value="Veg Soup">Veg Soup
<input type="radio" name="appetizer" value="Veg Sandwich">Veg Sandwich<br><br>
 
<b>Address</b><br>
<textarea name="address" rows=3 cols=40></textarea><br><br>
<b>Credit Card:</b><br>
<input type="radio" name="cardType" value="Visa">Visa
<input type="radio" name="cardType" value="MasterCard">MasterCard
<input type="radio" name="cardType" value="Amex">American Express
<br><br>
<b>Credit Card Number:</b> <input type="password" name="cardNum">
<b>Repeat Credit Card Number: </b><input type="password" name="cardNum"><br><br>
<input type="submit" name="submit" value="Order Pizza">
</form>
</body>
</html>

Environment Used

JDK version :1.6.0_30
Tomcat version : 7.0.50

To Download DisplayAllHeadersApp Project Click the below link

https://sites.google.com/site/javaee4321/servlets/DisplayAllHeadersApp.zip?attredirects=0&d=1

See also:

Servlets Tutorial

All Design Patterns Links