Servlet与JSP核心编程 ---- 5
5.5Differentiating Among Different Browser Types
Use User-Agent only when necessary
Check for null
To differentiate between Netscape and Internet Explorer, check for “MSIE,” not “Mozilla.”
Note that the header can be faked.
package coreservlets;
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
/** Servlet that gives browser-specific insult.
* Illustrates how to use the User-Agent
* header to tell browsers apart.
*/
public class BrowserInsult extends HttpServlet {
public void doGet(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
response.setContentType("text/html");
PrintWriter out = response.getWriter();
String title, message;
// Assume for simplicity that Netscape and IE are
// the only two browsers.
String userAgent = request.getHeader("User-Agent");
if ((userAgent != null) &&
(userAgent.indexOf("MSIE") != -1)) {
title = "Microsoft Minion";
message = "Welcome, O spineless slave to the " +
"mighty empire.";
} else {
title = "Hopeless Netscape Rebel";
message = "Enjoy it while you can. " +
"You <I>will</I> be assimilated!";
}
String docType =
"<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.0 " +
"Transitional//EN\">\n";
out.println(docType +
"<HTML>\n" +
"<HEAD><TITLE>" + title + "</TITLE></HEAD>\n" +
"<BODY BGCOLOR=\"#FDF5E6\">\n" +
"<H1 ALIGN=CENTER>" + title + "</H1>\n" +
message + "\n" +
"</BODY></HTML>");
}
}
5.6Changing the Page According to How the User Got There
package coreservlets;
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
/** Servlet that displays referer-specific image. */
public class CustomizeImage extends HttpServlet {
public void doGet(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
response.setContentType("text/html");
PrintWriter out = response.getWriter();
String referer = request.getHeader("Referer");
if (referer == null) {
referer = "<I>none</I>";
}
String title = "Referring page: " + referer;
String imageName;
if (contains(referer, "JRun")) {
imageName = "jrun-powered.gif";
} else if (contains(referer, "Resin")) {
imageName = "resin-powered.gif";
} else {
imageName = "tomcat-powered.gif";
}
String imagePath = "../request-headers/images/" + imageName;
String docType =
"<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.0 " +
"Transitional//EN\">\n";
out.println(docType +
"<HTML>\n" +
"<HEAD><TITLE>" + title + "</TITLE></HEAD>\n" +
"<BODY BGCOLOR=\"#FDF5E6\">\n" +
"<CENTER><H2>" + title + "</H2>\n" +
"<IMG SRC=\"" + imagePath + "\">\n" +
"</CENTER></BODY></HTML>");
}
private boolean contains(String mainString,
String subString) {
return(mainString.indexOf(subString) != -1);
}
}
5.7 Accessing the Standard CGI Variables
A Servlet That Shows the CGI Variables
package coreservlets;
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
import java.util.*;
/** Creates a table showing the current value of each
* of the standard CGI variables.
*/
public class ShowCGIVariables extends HttpServlet {
public void doGet(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
response.setContentType("text/html");
PrintWriter out = response.getWriter();
String[][] variables =
{ { "AUTH_TYPE", request.getAuthType() },
{ "CONTENT_LENGTH",
String.valueOf(request.getContentLength()) },
{ "CONTENT_TYPE", request.getContentType() },
{ "DOCUMENT_ROOT",
getServletContext().getRealPath("/") },
{ "PATH_INFO", request.getPathInfo() },
{ "PATH_TRANSLATED", request.getPathTranslated() },
{ "QUERY_STRING", request.getQueryString() },
{ "REMOTE_ADDR", request.getRemoteAddr() },
{ "REMOTE_HOST", request.getRemoteHost() },
{ "REMOTE_USER", request.getRemoteUser() },
{ "REQUEST_METHOD", request.getMethod() },
{ "SCRIPT_NAME", request.getServletPath() },
{ "SERVER_NAME", request.getServerName() },
{ "SERVER_PORT",
String.valueOf(request.getServerPort()) },
{ "SERVER_PROTOCOL", request.getProtocol() },
{ "SERVER_SOFTWARE",
getServletContext().getServerInfo() }
};
String title = "Servlet Example: Showing CGI Variables";
String docType =
"<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.0 " +
"Transitional//EN\">\n";
out.println(docType +
"<HTML>\n" +
"<HEAD><TITLE>" + title + "</TITLE></HEAD>\n" +
"<BODY BGCOLOR=\"#FDF5E6\">\n" +
"<CENTER>\n" +
"<H1>" + title + "</H1>\n" +
"<TABLE BORDER=1>\n" +
" <TR BGCOLOR=\"#FFAD00\">\n" +
" <TH>CGI Variable Name<TH>Value");
for(int i=0; i<variables.length; i++) {
String varName = variables[i][0];
String varValue = variables[i][1];
if (varValue == null)
varValue = "<I>Not specified</I>";
out.println(" <TR><TD>" + varName + "<TD>" + varValue);
}
out.println("</TABLE></CENTER></BODY></HTML>");
}
/** POST and GET requests handled identically. */
public void doPost(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
doGet(request, response);
}
}
6.2 HTTP 1.1 Status Codes
100–199
Codes in the 100s are informational, indicating that the client should
respond with some other action.
• 200–299
Values in the 200s signify that the request was successful.
• 300–399
Values in the 300s are used for files that have moved and usually
include a Location header indicating the new address.
• 400–499
Values in the 400s indicate an error by the client.
• 500–599
Codes in the 500s signify an error by the server.
6.4A Front End to Various Search Engines
package coreservlets;
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
import java.net.*;
/** Servlet that takes a search string and a search
* engine name, sending the query to
* that search engine. Illustrates manipulating
* the response status code. It sends a 302 response
* (via sendRedirect) if it gets a known search engine,
* and sends a 404 response (via sendError) otherwise.
*/
public class SearchEngines extends HttpServlet {
public void doGet(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
String searchString = request.getParameter("searchString");
if ((searchString == null) ||
(searchString.length() == 0)) {
reportProblem(response, "Missing search string");
return;
}
// 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.
// Browsers always URL-encode form values, and the
// getParameter method decodes automatically. But since
// we're just passing this on to another server, we need to
// re-encode it to avoid characters that are illegal in
// URLs. Also note that JDK 1.4 introduced a two-argument
// version of URLEncoder.encode and deprecated the one-arg
// version. However, since version 2.3 of the servlet spec
// mandates only the Java 2 Platform (JDK 1.2 or later),
// we stick with the one-arg version for portability.
searchString = URLEncoder.encode(searchString);
String searchEngineName =
request.getParameter("searchEngine");
if ((searchEngineName == null) ||
(searchEngineName.length() == 0)) {
reportProblem(response, "Missing search engine name");
return;
}
String searchURL =
SearchUtilities.makeURL(searchEngineName, searchString);
if (searchURL != null) {
response.sendRedirect(searchURL);
} else {
reportProblem(response, "Unrecognized search engine");
}
}
private void reportProblem(HttpServletResponse response,
String message)
throws IOException {
response.sendError(response.SC_NOT_FOUND, message);
}
}
package coreservlets;
/** Small class that encapsulates how to construct a
* search string for a particular search engine.
*/
public class SearchSpec {
private String name, baseURL;
public SearchSpec(String name,
String baseURL) {
this.name = name;
this.baseURL = baseURL;
}
/** Builds a URL for the results page by simply concatenating
* the base URL (http://...?someVar=") with the URL-encoded
* search string (jsp+training).
*/
public String makeURL(String searchString) {
return(baseURL + searchString);
}
public String getName() {
return(name);
}
}
package coreservlets;
/** Utility with static method to build a URL for any
* of the most popular search engines.
*/
public class SearchUtilities {
private static SearchSpec[] commonSpecs =
{ new SearchSpec("Google",
"http://www.google.com/search?q="),
new SearchSpec("AllTheWeb",
"http://www.alltheweb.com/search?q="),new SearchSpec("Yahoo",
"http://search.yahoo.com/bin/search?p="),
new SearchSpec("AltaVista",
"http://www.altavista.com/web/results?q="),
new SearchSpec("Lycos",
"search.lycos.com/default.asp?query="),
new SearchSpec("HotBot",
"http://hotbot.com/default.asp?query="),
new SearchSpec("MSN",
"http://search.msn.com/results.asp?q="),
};
public static SearchSpec[] getCommonSpecs() {
return(commonSpecs);
}
/** Given a search engine name and a search string, builds
* a URL for the results page of that search engine
* for that query. Returns null if the search engine name
* is not one of the ones it knows about.
*/
public static String makeURL(String searchEngineName,
String searchString) {
SearchSpec[] searchSpecs = getCommonSpecs();
String searchURL = null;
for(int i=0; i<searchSpecs.length; i++) {
SearchSpec spec = searchSpecs[i];
if (spec.getName().equalsIgnoreCase(searchEngineName)) {
searchURL = spec.makeURL(searchString);
break;
}
}
return(searchURL);
}
}