Sophisticated servlets can use browser headers to tailor the returned page to the browser. For example, a high-quality site can detect a Palm Pilot browser and simplify its site design to match the browser capabilities.

Each browser request sends headers along with the URL. The headers include the browser type (User-Agent) and language preferences (Accept-Language and Accept-Charset). The full list is in the official HTTP/1.1 spec, but those three are the most important.

Servlet and JSPs grab headers with request.getHeader.

The following, like env.jsp in the examples, gets all the headers and prints them. Little "snoop" servlets like this are useful to see what headers browsers are sending.

package test;

import java.io.*;
import java.util.*;
import javax.servlet.*;
import javax.servlet.http.*;

public class HeaderServlet extends HttpServlet {
  public void doGet(HttpServletRequest request,
                    HttpServletResponse response)
    throws IOException, ServletException
  {
    PrintWriter out = response.getWriter();

    out.println("<table>");

    Enumeration e = request.getHeaderNames();
    while (e.hasMoreElements()) {
      String name = (String) e.nextElement();

      out.println("<tr><td>" + name);
      out.println("<td>" + request.getHeader(name));
    }

    out.println("</table>");
  }
}

Connection Keep-Alive User-Agent Mozilla/4.74 [en] (X11; U; Linux 2.4.0-test1 i686) Host localhost:8080 Accept image/gif, image/jpeg, image/pjpeg, image/png, */* Accept-Encoding gzip Accept-Language en Accept-Charset iso-8859-1,*,utf-8 Cookie JSESSIONID=7tXlVCaALedCHHkdXy

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