To set response headers using HttpServletResponse in a Java web application (e.g., within a servlet), you can use the setHeader or addHeader methods provided by the HttpServletResponse class. Here’s an overview of both methods and how to use them:

Methods for Setting Headers

1. setHeader(String name, String value)
   • This method sets a response header with a given name and value.
   • If the header already exists, it replaces the existing value with the new one.
2. addHeader(String name, String value)
   • This method allows you to add multiple values for the same header name.
   • If the header already exists, it adds the new value rather than replacing it.

Example Code

Below is an example of setting headers in a servlet:

package org.kodejava.servlet;

import java.io.IOException;

import jakarta.servlet.ServletException;
import jakarta.servlet.annotation.WebServlet;
import jakarta.servlet.http.HttpServlet;
import jakarta.servlet.http.HttpServletRequest;
import jakarta.servlet.http.HttpServletResponse;

@WebServlet("/setHeaders")
public class HeaderServlet extends HttpServlet {

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

      // Set Content-Type Header
      response.setContentType("text/html");

      // Set a custom response header
      response.setHeader("Custom-Header", "CustomValue");

      // Add multiple custom values for the same header name
      response.addHeader("Custom-Multi-Value-Header", "Value1");
      response.addHeader("Custom-Multi-Value-Header", "Value2");

      // Set Cache-Control Header
      response.setHeader("Cache-Control", "no-cache, no-store, must-revalidate");

      // Set Expires Header
      response.setHeader("Expires", "0");

      // Write the response body
      response.getWriter().println("<h1>Response Headers Set</h1>");
   }
}

Important Notes

• Setting Content-Type: Use setContentType(String type) to set the MIME type of the response body, like "text/html", "application/json", etc.
• Overwriting Headers: Use setHeader if you want to ensure a header has only one value (overwriting any existing ones).
• Adding Multiple Values: Use addHeader if the header allows multiple values (e.g., Set-Cookie).

Commonly Used Response Headers

Here are some commonly used headers for different scenarios:

1. Caching:

   response.setHeader("Cache-Control", "no-cache, no-store, must-revalidate");
   response.setHeader("Expires", "0");
   response.setHeader("Pragma", "no-cache");
   

2. Content-Type and Encoding:

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

3. Custom Headers:

   response.setHeader("X-App-Name", "MyWebApplication");
   

4. CORS (Cross-Origin Resource Sharing):

   response.setHeader("Access-Control-Allow-Origin", "*");
   response.setHeader("Access-Control-Allow-Methods", "GET, POST, OPTIONS");
   response.setHeader("Access-Control-Allow-Headers", "Content-Type");
   

With this approach, you can control headers in your servlet responses effectively.

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Maven dependencies

<dependency>
    <groupId>jakarta.servlet</groupId>
    <artifactId>jakarta.servlet-api</artifactId>
    <version>6.1.0</version>
    <scope>provided</scope>
</dependency>