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Recipe 18.1 Examining HTTP Request Headers in a Servlet


You want to examine the HTTP request headers in a servlet.


Use the javax.servlet.http.HttpServletRequest.getHeaderNames( ) and getHeader( ) methods to access the names and values of various request headers.


The HttpServletRequest.getHeaderNames( ) method returns all of the request header names for an incoming request. You can then obtain the value of a specific header by providing the header name to the method HttpServletRequest.getHeader( ) method. Example 18-1 gets an Enumeration of header names in the servlet's doGet( ) method, and then displays each header and value on its own line in the resulting HTML page.

Example 18-1. A servlet displays request headers and values
package com.jspservletcookbook;           

import java.util.Enumeration;

import javax.servlet.*;
import javax.servlet.http.*;

public class RequestHeaderView extends HttpServlet {

  public void doGet(HttpServletRequest request, 
    HttpServletResponse response) throws ServletException,
      java.io.IOException {
      //get an Enumeration of all the request header names
      Enumeration enum = request.getHeaderNames( );
      java.io.PrintWriter out = response.getWriter( );
          "<html><head><title>Request Header View</title></head><body>");
      out.println("<h2>Request Headers</h2>");

      String header = null;

      //display each request header name and value  
      while (enum.hasMoreElements( )){
          header = (String) enum.nextElement( );

      //getHeader returns null if a request header of that name does not
      //exist in the request
      out.println("<strong>"+header+"</strong>"+": "+
          request.getHeader(header)+"<br>" );
  } //doGet

Figure 18-1 shows the RequestHeaderView servlet's output.

Figure 18-1. A servlet shows the request header names and values

See Also

Recipe 18.2 on examining request headers in a JSP; Recipe 18.3 on using a filter to wrap the request and forward it along the filter chain; Recipe 18.6 on using a listener to track requests; Chapter 7 on handling request parameters and JavaBean properties with servlets, JSPs, and filters.

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