Working with HttpServletRequest in Java Servlets
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The HttpServletRequest interface represents the client's browser request. When a browser communicates with the server via HTTP, all request information gets parsed by Tomcat and encapsulated into this object. Developers can use its methods to retrieve any client request data.
1. Extracting Request Line Information
The request line contains the HTTP method, URL path, and protocol version. HttpServletRequest provides several methods to extract this information:
req.getRequestURL() // Retrieves the complete URL from the browser request
req.getRequestURI() // Returns the resource path portion from the request line
req.getRemoteAddr() // Returns the client's IP address
req.getLocalAddr() // Returns the server's IP address
req.getLocalPort() // Returns the port number the server connector is listening on
2. Retrieving Request Headers
Request headers provide metadata about the request. You can retrieve specific header values or enumerate all headers:
// Get a specific header value by its key
String headerValue = req.getHeader("Accept");
// Get all header names as an Enumeration
Enumeration<String> headerNames = req.getHeaderNames();
Practical Example
public class RequestHandlerServlet extends HttpServlet {
@Override
protected void service(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
// Request line details
System.out.println("Full URL: " + request.getRequestURL());
System.out.println("URI: " + request.getRequestURI());
System.out.println("Client IP: " + request.getRemoteAddr());
System.out.println("Server IP: " + request.getLocalAddr());
System.out.println("Server Port: " + request.getLocalPort());
System.out.println("Server Hostname: " + request.getLocalName());
System.out.println("Client Port: " + request.getRemotePort());
System.out.println("Context Path: " + request.getContextPath());
System.out.println("Protocol: " + request.getScheme());
System.out.println("HTTP Method: " + request.getMethod());
// Iterate through all headers
Enumeration<String> allHeaders = request.getHeaderNames();
while (allHeaders.hasMoreElements()) {
String headerName = allHeaders.nextElement();
System.out.println(headerName + ": " + request.getHeader(headerName));
}
}
}
3. Handling Request Data (Form Parameters)
Basic Parameter Retrieval
// Single parameter value
String value = req.getParameter("fieldName");
Multiple Values (Checkboxes)
When form elements like checkboxes can submit multiple values, use:
// Returns an array of values for parameters with multiple selections
String[] selectedOptions = req.getParameterValues("hobby");
Parameter Enumeration
// Get all parameter names
Enumeration<String> parameterNames = req.getParameterNames();
Map-Based Retrieval
// Get all parameters as a Map (key -> String[])
Map<String, String[]> allParameters = req.getParameterMap();
4. Handling Character Encoding
HTTP request data is transmitted as bytes and converted to characters by Tomcat. By default, Tomcat uses ISO-8859-1 encoding, which causes issues with non-ASCII characters like Chinese. To properly handle Chinese characters:
req.setCharacterEncoding("UTF-8");
This must be called before any getParameter() methods to take effect.
5. Complete Working Example
HTML Form
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>User Registration Form</title>
</head>
<body>
<form method="POST" action="register">
<table style="margin: 0 auto" width="350px" border="1">
<tr>
<td>Username</td>
<td><input type="text" name="userName" value="admin"></td>
</tr>
<tr>
<td>Password</td>
<td><input type="password" name="passWord"></td>
</tr>
<tr>
<td>Gender</td>
<td>
<input type="radio" name="sex" value="male" checked> Male
<input type="radio" name="sex" value="female"> Female
</td>
</tr>
<tr>
<td>Interests</td>
<td>
<input type="checkbox" name="interest" value="coding"> Coding
<input type="checkbox" name="interest" value="reading"> Reading
<input type="checkbox" name="interest" value="gaming"> Gaming
</td>
</tr>
<tr>
<td>Bio</td>
<td>
<textarea name="description" rows="3">Software developer</textarea>
</td>
</tr>
<tr>
<td>Country</td>
<td>
<select name="country">
<option value="us">United States</option>
<option value="uk">United Kingdom</option>
<option value="cn">China</option>
</select>
</td>
</tr>
<tr>
<td colspan="2" align="center">
<input type="submit" value="Register">
</td>
</tr>
</table>
</form>
</body>
</html>
Servlet Handler
package com.example.controllers;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.util.Arrays;
import java.util.Enumeration;
import java.util.Map;
import java.util.Set;
public class RegistrationServlet extends HttpServlet {
@Override
protected void service(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
// Set encoding before reading parameters
request.setCharacterEncoding("UTF-8");
// Retrieve individual form fields
String userName = request.getParameter("userName");
System.out.println("Username: " + userName);
System.out.println("Password: " + request.getParameter("passWord"));
System.out.println("Gender: " + request.getParameter("sex"));
// Handle multi-select checkboxes
String[] interests = request.getParameterValues("interest");
System.out.println("Interests: " + Arrays.toString(interests));
// Handle textarea content
System.out.println("Bio: " + request.getParameter("description"));
// Handle dropdown selection
System.out.println("Country: " + request.getParameter("country"));
System.out.println("--- All Parameters ---");
// Iterate through all parameter names
Enumeration<String> paramKeys = request.getParameterNames();
while (paramKeys.hasMoreElements()) {
String key = paramKeys.nextElement();
String[] values = request.getParameterValues(key);
System.out.println(key + ": " + Arrays.toString(values));
}
System.out.println("--- Parameter Map ---");
// Alternative: work with the parameter map directly
Map<String, String[]> paramMap = request.getParameterMap();
Set<Map.Entry<String, String[]>> entries = paramMap.entrySet();
for (Map.Entry<String, String[]> entry : entries) {
System.out.println(entry.getKey() + ": " + Arrays.toString(entry.getValue()));
}
}
}
Important Notes on Form Elements
| Element Type | Submitted? | Value Source |
|---|---|---|
readonly |
Yes | Current value |
hidden |
Yes | Value attribute |
disabled |
No | N/A |
textarea |
Yes | Text content between tags |
select without value |
Yes | Text content of selected option |
GET vs POST Differences
| Aspect | GET | POST |
|---|---|---|
| Data Location | URL query string | Request body |
| Data Types | Text only | Text and binary files |
| Data Size | Limited | Virtually unlimited |
| Security | Less secure (visible in URL) | More secure |
Remember to call setCharacterEncoding() before accessing any parameters when dealing with international character sets.
