Recipe 18.1 Examining HTTP Request Headers in a Servlet
Problem
You want to examine the HTTP request headers in a servlet.
Solution
Use the
javax.servlet.http.HttpServletRequest.getHeaderNames(
)
and getHeader( )
methods to access the names and values of various request headers.
Discussion
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( );
response.setContentType("text/html");
java.io.PrintWriter out = response.getWriter( );
out.println(
"<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>" );
}
out.println("</body></html>");
} //doGet
}
Figure 18-1 shows the
RequestHeaderView servlet's
output.
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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