这个例子其实就是Google官方给出来的例子~你可以在文档中看到，我在原例子的基础上添加了修改与删除功能~（没判断权限~谁都可以删除~要判断也很简单…判断一下登陆的email）

我就不说那么详细了·官方例子说的已经很清楚了，需要源码的留下邮箱即可。

这个例子显示效果在此：http://1.latest.fatkuns.appspot.com/

这个例子涉及了

  1. 如何使用用户服务（也就是使用google账号登陆~我有点担心的是别人伪造一个假的登录页面怎么办？放心，我做的那个是真的…- -!）
  2. 如何存储数据（使用JDO）
  3. JSP
  4. Servlet

好~开始了~那些准备工作就不说了，创建项目，包名这些~

一、要保存留言，我们需要创建一个这样的文件~Greeting.java

package com.fatkun.guestbook;
 
import java.util.Date;
 
import javax.jdo.annotations.IdGeneratorStrategy;
import javax.jdo.annotations.IdentityType;
import javax.jdo.annotations.PersistenceCapable;
import javax.jdo.annotations.Persistent;
import javax.jdo.annotations.PrimaryKey;
 
import com.google.appengine.api.users.User;
 
@PersistenceCapable(identityType = IdentityType.APPLICATION)
public class Greeting {
	//主键的设置，自增的方式
	@PrimaryKey
	@Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
	private Long id;
	//其他的值设置一个@Persistent在前面就会保存到数据库里了
	@Persistent
	private User author;
	@Persistent
	private String content;
	@Persistent
	private Date date;
 
	//下面是一些Getter,Setter方法，对于JDO是不需要的，只是为了方便你写程序
	public Greeting(User author, String content, Date date) {
		super();
		this.author = author;
		this.content = content;
		this.date = date;
	}
 
	public Long getId() {
		return id;
	}
 
	public void setId(Long id) {
		this.id = id;
	}
 
	public User getAuthor() {
		return author;
	}
 
	public void setAuthor(User author) {
		this.author = author;
	}
 
	public String getContent() {
		return content;
	}
 
	public void setContent(String content) {
		this.content = content;
	}
 
	public Date getDate() {
		return date;
	}
 
	public void setDate(Date date) {
		this.date = date;
	}
 
}

二、我们还要创建PMF.java用来取得PersistenceManager。官方说取这个东西要花费很多资源，所以只获得一个就好，单例模式~

package com.fatkun.guestbook;
 
import javax.jdo.JDOHelper;
import javax.jdo.PersistenceManagerFactory;
 
/**
 * 用来取得PersistenceManager
 * @author Administrator
 *
 */
public final class PMF {
	private static final PersistenceManagerFactory pmfInstance = JDOHelper.getPersistenceManagerFactory("transactions-optional");
 
	private PMF() {
	}
 
	public static PersistenceManagerFactory get() {
		return pmfInstance;
	}
}

三、我们在war目录下新建一个guestbook.jsp用来显示所有留言

<?xml version="1.0" encoding="UTF-8" ?>
<%@ page language="java" contentType="text/html; charset=UTF-8" pageEncoding="UTF-8"%>
<%@ page import="java.util.List"%>
<%@ page import="javax.jdo.PersistenceManager"%>
<%@ page import="com.google.appengine.api.users.User"%>
<%@ page import="com.google.appengine.api.users.UserService"%>
<%@ page import="com.google.appengine.api.users.UserServiceFactory"%>
<%@ page import="com.fatkun.guestbook.Greeting"%>
<%@ page import="com.fatkun.guestbook.PMF"%>
 
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
<title>Google App Engine留言板 - fatkun.com</title>
</head>
<body>
<%
	UserService userService = UserServiceFactory.getUserService();
	User user = userService.getCurrentUser();
	if (user != null) {
%>
<p>你好, <%=user.getNickname()%>! (<a href="<%=userService.createLogoutURL(request.getRequestURI())%>">退出</a>.)</p>
<%
	} else {
%>
<p>你好! 你可以使用google账号<a href="<%=userService.createLoginURL(request.getRequestURI())%>">登陆</a></p>
<%
	}
%>
 
<%
	PersistenceManager pm = PMF.get().getPersistenceManager();
	//String query = "select from " + Greeting.class.getName()+" order by id desc";
	//也可以写成下面的形式
	String query = "select from com.fatkun.guestbook.Greeting order by id desc";
	List<Greeting> greetings = (List<Greeting>) pm.newQuery(query).execute();
	if (greetings.isEmpty()) {
%>
<p>还没有留言。</p>
<%
	} else {
		for (Greeting g : greetings) {
			if (g.getAuthor() == null) {
				out.print("<p>一位匿名人士说:</p>");
			} else {
				out.print("<p><b>"+g.getAuthor().getNickname()+"</b> 说:</p>");
			}
%>
<a href="del?id=<%=g.getId()%>">删除</a> - <a href="guestbook-modify.jsp?id=<%=g.getId()%>">修改</a>
<blockquote><%=g.getContent()%></blockquote>
<%
	}
	}
	pm.close();
%>
 
<form action="/add" method="post">
<div><textarea name="content" rows="3" cols="60"></textarea></div>
<div><input type="submit" value="我要留言" /></div>
</form>
 
</body>
</html>

这里注意要引入自己使用的包~~GQL是针对对象来写查询语句的~~有点儿像Hibernate的HQL~

四、没有内容怎么显示呢~我们加一个添加到数据库的GuestbookAddServlet.java

package com.fatkun.guestbook;
 
import java.io.IOException;
import java.util.Date;
 
import javax.jdo.PersistenceManager;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
 
import com.google.appengine.api.users.User;
import com.google.appengine.api.users.UserService;
import com.google.appengine.api.users.UserServiceFactory;
 
@SuppressWarnings("serial")
public class GuestbookAddServlet extends HttpServlet {
 
	@Override
	protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
		//取得用户Service，这里是Google的用户管理
		UserService userService = UserServiceFactory.getUserService();
		//取得当前登录的用户，当不存在的时候为null
		User user = userService.getCurrentUser();
 
		String content = req.getParameter("content");
		Date date = new Date();
		//创建一个Greeting对象
		Greeting greeting = new Greeting(user, content, date);
 
		PersistenceManager pm = PMF.get().getPersistenceManager();
		try {
			//持久化~也就是把greeting添加进数据库
			pm.makePersistent(greeting);
		} finally {
			pm.close();
		}
		resp.sendRedirect("guestbook.jsp");
	}
 
}

写完了上面这个文件，注意还得在web.xml加入下面的，要告诉服务器说，我在这里啊，你可以通过这个链接找我~~

	<servlet>
		<servlet-name>add</servlet-name>
		<servlet-class>com.fatkun.guestbook.GuestbookAddServlet</servlet-class>
	</servlet>
	<servlet-mapping>
		<servlet-name>add</servlet-name>
		<url-pattern>/add</url-pattern>
	</servlet-mapping>

这时，你应该就可以留言啦~~运行一下试试~~可以先在本地测试，再上传到GAE服务器上。

伍、我最后给出修改和删除的部分代码~其实就主要是数据库的问题~~

GuestbookModifyServlet.java

	protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
		Long id = Long.parseLong(req.getParameter("id"));
		String content = req.getParameter("content");
 
		//先获取PersistenceManager
		PersistenceManager pm = PMF.get().getPersistenceManager();
		try {
			//根据ID从数据库得到一条记录
			Greeting greeting = pm.getObjectById(Greeting.class, id);
			//修改记录
			greeting.setContent(content);
		} finally {
			//当关闭pm的时候，会自动把已经修改的记录更新到数据库
			pm.close();
		}
		resp.sendRedirect("guestbook.jsp");
	}

GuestbookDeleteServlet.java

	protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
		Long id = Long.parseLong(req.getParameter("id"));
 
		PersistenceManager pm = PMF.get().getPersistenceManager();
		try {
			//取得这条记录
			Greeting greeting = pm.getObjectById(Greeting.class, id);
			//删除
			pm.deletePersistent(greeting);
		} finally {
			//关闭
			pm.close();
		}
		resp.sendRedirect("guestbook.jsp");
	}

好，一个简单的留言板就这样诞生了~~如果想弄得更加漂亮，就自己弄吧~~^_^
我做的例子：http://1.latest.fatkuns.appspot.com/