Here's an example that makes use of the two most common status codes other
than 200: 302 and 404. The 302 code is set via the shorthand sendRedirect
method, and 404 is set via sendError.
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)
Note: makes use of the SearchSpec class, shown below,
which incorporates information on how to construct URLs
to perform searches at various search sites.
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
Right click on the
source code link to download the HTML. Left click on the
link to try it out on-line.
<!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
This tutorial is now available as a book: Core Servlets and JavaServer Pages by Marty Hall, published by Sun Microsystems Press.
Read all about it at CoreServlets.com
Server-Side Web Applications using Java Servlets versions 2.1/2.2 and JavaServer Pages (JSP) version 1.0: A Tutorial
© 1999-2000 Marty Hall.
All source code freely available for unrestricted use.
Created for work in the Research and Technology Development Center of the Johns Hopkins University Applied Physics Lab, for courses in the Johns Hopkins Part-Time MS Program in Computer Science, and for various industry seminars and on-site Java short courses.
Please note that this is a first draft of the tutorial, so please send corrections, comments, and suggestions to me at hall@apl.jhu.edu.
Reprinted with permission from the author. Click here to visit the original version
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