1. Overview

HTTP/1.1 200 OK
Content-Type: text/plain

Hello World

Servlets can perform a variety of tasks by manipulating the status line and the response headers. For example, they can forward the user to other sites; indicate that the attached document is an image, Adobe Acrobat file, or (most commonly) HTML file; tell the user that a password is required to access the document; and so forth. This section will discuss the various different status codes and what can be accomplished with them, while the following section will discuss the response headers.

2. Specifying Status Codes

Setting the status code does not always mean that you don't need to return a document. For example, although most servers will generate a small "File Not Found" message for 404 responses, a servlet might want to customize this response. However, if you do this, you need to be sure to call response.setStatus before sending any content via the PrintWriter.

Although the general method of setting status codes is simply to call response.setStatus(int), there are two common cases where a shortcut method in HttpServletResponse is provided. The sendError method generates a 404 response along with a short message formatted inside an HTML document. And the sendRedirect method generates a 302 response along with a Location header indicating the URL of the new document.

3. HTTP 1.1 Status Codes and Their Meaning

4. Example: Search Engine Front End

In this application, an HTML form first displays a page that lets the user choose a search string, the number of results per page, and the search engine to use. When the form is submitted, the servlet extracts those three parameters, constructs a URL with the parameters embedded in a way appropriate to the search engine selected, and redirects the user to that URL. If the user fails to choose a search engine or a different front end sends an unrecognized search engine name, a 404 error page is returned saying that the search engine is missing or unknown.

If you want to try it out yourself, start with the HTML front-end.

4.1 SearchEngines.java (Download source code)

package hall;

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

public class SearchEngines extends HttpServlet {
  public void doGet(HttpServletRequest request,
                    HttpServletResponse response)
      throws ServletException, IOException {
    // The URLEncoder changes spaces to "+" signs and other
    // non-alphanumeric characters to "%XY", where XY is the
    // hex value of the ASCII (or ISO Latin-1) character.
    // The getParameter method decodes automatically, but since
    // we're just passing this on to another server, we need to
    // re-encode it.
    String searchString =
      URLEncoder.encode(request.getParameter("searchString"));
    String numResults =
      request.getParameter("numResults");
    String searchEngine =
      request.getParameter("searchEngine");
    SearchSpec[] commonSpecs = SearchSpec.getCommonSpecs();
    for(int i=0; i<commonSpecs.length; i++) {
      SearchSpec searchSpec = commonSpecs[i];
      if (searchSpec.getName().equals(searchEngine)) {
        // encodeURL is just planning ahead in case this servlet
        // is ever used in an application that does session tracking.
        // If cookies are turned off, session tracking is usually
        // accomplished by URL rewriting, so all URLs returned
        // by servlets should be sent through encodeURL.
        String url =
          response.encodeURL(searchSpec.makeURL(searchString,
                                                 numResults));
        response.sendRedirect(url);
        return;
      }
    }
    response.sendError(response.SC_NOT_FOUND,
                       "No recognized search engine specified.");
  }

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

4.2 SearchSpec.java

package hall;

class SearchSpec {
  private String name, baseURL, numResultsSuffix;

  private static SearchSpec[] commonSpecs =
    { new SearchSpec("google",
                     "http://www.google.com/search?q=",
                     "&num="),
      new SearchSpec("infoseek",
                     "http://infoseek.go.com/Titles?qt=",
                     "&nh="),
      new SearchSpec("lycos",
                     "http://lycospro.lycos.com/cgi-bin/pursuit?query=",
                     "&maxhits="),
      new SearchSpec("hotbot",
                     "http://www.hotbot.com/?MT=",
                     "&DC=")
    };

  public SearchSpec(String name,
                    String baseURL,
                    String numResultsSuffix) {
    this.name = name;
    this.baseURL = baseURL;
    this.numResultsSuffix = numResultsSuffix;
  }

  public String makeURL(String searchString, String numResults) {
    return(baseURL + searchString + numResultsSuffix + numResults);
  }

  public String getName() {
    return(name);
  }

  public static SearchSpec[] getCommonSpecs() {
    return(commonSpecs);
  }
}

4.3 SearchEngines.html

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML>
<HEAD>
  <TITLE>Searching the Web</TITLE>
</HEAD>

<BODY BGCOLOR="#FDF5E6">
<H1 ALIGN="CENTER">Searching the Web</H1>

<FORM ACTION="/servlet/hall.SearchEngines">
  <CENTER>
    Search String: 
    <INPUT TYPE="TEXT" NAME="searchString"><BR>
    Results to Show Per Page:
    <INPUT TYPE="TEXT" NAME="numResults" 
                       VALUE=10 SIZE=3><BR>
    <INPUT TYPE="RADIO" NAME="searchEngine"
                        VALUE="google">
    Google |
    <INPUT TYPE="RADIO" NAME="searchEngine"
                        VALUE="infoseek">
    Infoseek |
    <INPUT TYPE="RADIO" NAME="searchEngine"
                        VALUE="lycos">
    Lycos |
    <INPUT TYPE="RADIO" NAME="searchEngine"
                        VALUE="hotbot">
    HotBot
    <BR>
    <INPUT TYPE="SUBMIT" VALUE="Search">
  </CENTER>
</FORM>

</BODY>
</HTML>

4.4 Initial Front End

4.5 Search Result