1. Overview

However, specifying headers can play a useful role even when no unusual status code is set. Response headers can be used to specify cookies, to supply the modification date (for caching), to instruct the browser to reload the page after a designated interval, to say how long the file is so that persistent HTTP connections can be used, and many other tasks.

The most general way to specify headers is by the setHeader method of HttpServletResponse, which takes two strings: the header name and the header value. Like setting the status codes, this must be done before any document content is sent.

There are also two specialized methods to set headers that contain dates (setDateHeader) and integers (setIntHeader). The first saves you the trouble of translating a Java date in milliseconds since the epoch (as returned by System.currentTimeMillis or the getTime method applied to a Date object) into a GMT time string. The second spares you the minor inconvenience of converting an int to a String.

Rather than setting a header outright, you can add a new header, in case a header with that name already exists. Use addHeader, addDateHeader, and addIntHeader for this. If it really matters to you whether a specific header has already been set, use containsHeader to check.

Finally, HttpServletResponse also supplies a number of convenience methods for specifying common headers.

• The setContentType method sets the Content-Type header, and is used by the majority of servlets.
• The setContentLength method sets the Content-Length header, useful if the browser supports persistent (keep-alive) HTTP connections.
• The addCookie method sets a cookie (there is no corresponding setCookie, since it is normal to have multiple Set-Cookie lines).
• And, as discussed in the previous section, the sendRedirect method sets the Location header as well as setting the status code to 302.

2. Common Response Headers and their Meaning

3. Example: Automatically Reloading Pages as Content Changes

Note that, in addition to illustrating the value of HTTP response headers, this example shows two other valuable servlet capabilities. First, it shows that servlets can handle multiple simultaneous connections, each in their own thread. In this case it maintains a Vector of previous requests for prime calculations, matching the current request to previous ones by looking at the number of primes (length of list) and number of digits (length of each prime), and synchronizing all access to this list. Secondly, it shows how easy it is for servlets to maintain state between requests, something that is cumbersome to implement in traditional CGI and many CGI alternatives. This lets the browser access the ongoing calculations when reloading the page, plus permits the servlet to keep a list of the N most recently requested results, returning them immediately if a new request specifies the same parameters as a recent request. We'll discuss persistent state even further in a later section.

3.1 PrimeNumbers.java (Download source code)

package hall;

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

public class PrimeNumbers extends HttpServlet {
  private static Vector primeListVector = new Vector();
  private static int maxPrimeLists = 30;
  
  public void doGet(HttpServletRequest request,
                    HttpServletResponse response)
      throws ServletException, IOException {
    int numPrimes =
      ServletUtilities.getIntParameter(request, "numPrimes", 50);
    int numDigits =
      ServletUtilities.getIntParameter(request, "numDigits", 120);
    PrimeList primeList =
      findPrimeList(primeListVector, numPrimes, numDigits);
    if (primeList == null) {
      primeList = new PrimeList(numPrimes, numDigits, true);
      synchronized(primeListVector) {
        if (primeListVector.size() >= maxPrimeLists)
          primeListVector.removeElementAt(0);
        primeListVector.addElement(primeList);
      }
    }
    Vector currentPrimes = primeList.getPrimes();
    int numCurrentPrimes = currentPrimes.size();
    int numPrimesRemaining = (numPrimes - numCurrentPrimes);
    boolean isLastResult = (numPrimesRemaining == 0);
    if (!isLastResult) {
      response.setHeader("Refresh", "5");
    }
    response.setContentType("text/html");
    PrintWriter out = response.getWriter();
    String title = "Some " + numDigits + "-Digit Prime Numbers";
    out.println(ServletUtilities.headWithTitle(title) +
                "<BODY BGCOLOR=\"#FDF5E6\">\n" +
                "<H2 ALIGN=CENTER>" + title + "</H2>\n" +
                "<H3>Primes found with " + numDigits +
                " or more digits: " + numCurrentPrimes + ".</H3>");
    if (isLastResult)
      out.println("<B>Done searching.</B>");
    else
      out.println("<B>Still looking for " + numPrimesRemaining +
                  " more<BLINK>...</BLINK></B>");
    out.println("<OL>");
    for(int i=0; i<numCurrentPrimes; i++) {
      out.println("  <LI>" + currentPrimes.elementAt(i));
    }
    out.println("</OL>");
    out.println("</BODY></HTML>");
  }

  public void doPost(HttpServletRequest request,
                     HttpServletResponse response)
      throws ServletException, IOException {
    doGet(request, response);
  }

  // See if there is an existing ongoing or completed calculation with
  // the same number of primes and length of prime. If so, return
  // those results instead of starting a new background thread. Keep
  // this list small so that the Web server doesn't use too much memory.
  // Synchronize access to the list since there may be multiple simultaneous
  // requests.
  
  private PrimeList findPrimeList(Vector primeListVector,
                                  int numPrimes,
                                  int numDigits) {
    synchronized(primeListVector) {
      for(int i=0; i<primeListVector.size(); i++) {
        PrimeList primes = (PrimeList)primeListVector.elementAt(i);
        if ((numPrimes == primes.numPrimes()) &&
            (numDigits == primes.numDigits()))
          return(primes);
      }
      return(null);
    }
  }
}

3.3 PrimeNumbers.html

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML>
<HEAD>
  <TITLE>Finding Large Prime Numbers</TITLE>
</HEAD>

<BODY BGCOLOR="#FDF5E6">
<H2 ALIGN="CENTER">Finding Large Prime Numbers</H2>
<BR><BR>
<CENTER>
<FORM ACTION="/servlet/hall.PrimeNumbers">
  <B>Number of primes to calculate:</B>
  <INPUT TYPE="TEXT" NAME="numPrimes" VALUE=25 SIZE=4><BR>
  <B>Number of digits:</B>
  <INPUT TYPE="TEXT" NAME="numDigits" VALUE=150 SIZE=3><BR>
  <INPUT TYPE="SUBMIT" VALUE="Start Calculating">
</FORM>
</CENTER>

</BODY>
</HTML>

3.4 Front End

3.5 Intermediate Result

3.6 Final Result