基于Servlet的Google Earth之旅

客户端和servlet之间的交互艺术

摘要
　　Google Earth是一个奇妙的客户端，它在众多市场领域引人瞩目并对如何可视化位置信息以及与地理相关的搜索结果产生影响。本文介绍了如何使用XML与一个基本的servlet来创建Google Earth旅程。在本文给出的示例基础上做进一步挖掘就可以开发出许多真正富有成效的服务。
作者Alan M. Berg（2005年11月14日）

版权声明：任何获得Matrix授权的网站，转载时请务必保留以下作者信息和链接
作者:Alan M. Berg;observer(作者的Blog:http://blog.matrix.org.cn/page/observer)
原文:http://www.javaworld.com/javaworld/jw-11-2005/jw-1114-google.html
译文:http://www.matrix.org.cn/resource/article/44/44179_Google+Earth.html
关键字:Google Earth Servlet

　　从这一点来说Google Earth客户端是我们时代的技术标志。Google Earth并非是第一个地球浏览客户端，而且与它的先驱、不为人知的Keyhole非常相似。但是凭着Google的大名以及基础版对最终用户免费，它完成了市场渗透并得到公认――这是另一个值得大书特书的有趣话题。

　　本文只有一个基本使命：即向你展示在servlet和Google Earth客户端之间发送和接收信息是多么的容易。有了这种程度的交互，你就能用基本的Java编程技能创建设想的服务。

使用许可及竞争者
　　截至本文发稿时Google Earth还处于beta阶段（版本号3.0.0616），许可证是商业的（见客户端的帮助部分）。如果你想寻求等价的开源范例，我建议你去关注优秀的Nasa World Wind（Nasa世界风）项目

基础知识

　　Google Earth客户端以第二版的锁位标记语言（KML）解析XML数据，它有一个专用的命名空间。庞大的KML配置信息可能会影响到GUI显示，开发这种需要平衡利弊的应用的难点在于需要了解更多的KML细节而不是编程技巧。KML实体的简要列表包括：
*Placements（位置），标明在地球上的坐标
*Folders（夹子），帮助组织其它的特征信息
*Documents（文档），存放可能包含风格元素的folder的容器
*Image overlays（图片叠加），用来添加图片
*Network links（网络链接），描述在何处以及如何与服务器或者servlet（本文采用的方式）连接

　　本文为了简化的目的，主要探讨了folder、placement和network-link元素的使用；此外还用folder定义了一段旅程（tour），它里面包含了一系列的placement。

　　在Windows上安装了Google Earth后，文件扩展名KML和MIME（Multipurpose Internet Mail Extensions，多用途网络邮件扩展）类型“application/keyhole”即被注册。这意味着只要点击KML文件或通过TCP/IP接收“application/keyhole”MIME类型的文件就会激活Google Earth客户端。

　　如果返回的KML文本为：

<Folder><name>Hello World [127.0.0.1] </name></Folder>

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

protected void doPost(HttpServletRequest request, HttpServletResponse response)
    throws ServletException, IOException
{
   response.setContentType("application/keyhole");
   PrintWriter out = response.getWriter();  
   String message ="<Folder><name>Hello World [" 
  + request.getRemoteAddr()+ "]</name></Folder>";
   out.println(message);
}

<?xml version="1.0" encoding="UTF-8"?>
<kml xmlns="http://earth.google.com/kml/2.0">
    <NetworkLink>
      <description>Refresh me</description>
      <name>Random City</name>
      <visibility>1</visibility>
      <open>1</open>
      <refreshVisibility>1</refreshVisibility>
      <flyToView>1</flyToView>
      <Url>
        <href>http://location </href>
      </Url>
    </NetworkLink>
</kml>

<%response.setContentType("application/keyhole");%>
    <NetworkLink>

<Placemark>
   <name>London</name>
   <description>
<![CDATA[<a href="http://www.visitlondon.com/choose_site/?OriginalURL=/">London</a>]]>
</description>
   <address>London, UK</address>
   <styleUrl>root://styleMaps#default+nicon=0x304+hicon=0x314</styleUrl>
   <Point>
      <coordinates>-0.1261969953775406,51.50019836425783,50</coordinates>
   </Point>
</Placemark>

manager.KMLRenderOfRandomPlacement();

package test.google.earth.servlet;

import java.io.IOException;
import java.io.PrintWriter;
import javax.servlet.ServletException;
import javax.servlet.http.*;
import javax.servlet.ServletConfig;
import test.google.earth.manager.Manager;

public class QuizServlet extends HttpServlet
{
   private Manager manager;

   public void init(ServletConfig config) throws ServletException {
      super.init(config);
      this.manager= new Manager();
      manager.init();
   }

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

   protected void doPost(HttpServletRequest request, HttpServletResponse response)
   throws ServletException, IOException
   {
   response.setContentType("application/keyhole");
   PrintWriter out = response.getWriter();
   out.println(manager.KMLRenderOfRandomPlacement());
   }
}

package test.google.earth.manager;


import java.util.Random;
import test.google.earth.bean.PlacementBean;
import test.google.earth.bean.PointBean;

public class Manager {
   private PlacementBean[] cityArray;
   private String styleURL;
   private String open;
   private Random generator;
   private int idx;

   public Manager(){}

public void init(){
   this.styleURL="root://styleMaps#default+nicon=0x304+hicon=0x314";
   this.open="1";
   this.generator = new Random();
   String[] coords = {"-0.1261969953775406,51.50019836425783,50",
   "12.5,41.889999,50","4.889999,52.369998,0"};
   String[] name = {"London","Italy","Amsterdam"};
   String[] address={"London, UK","Rome, Italy","Amsterdam, Netherlands"};
   String[] description={
   "<a href=\"http://www.visitlondon.com/choose_site/?OriginalURL=/\">London</a>",
   "<a href=\"http://www.roma2000.it/\">Rome</a>",
   "<a href=\"http://www.uva.nl/\">University of Amsterdam</a>"};
   this.idx=coords.length;


   cityArray= new PlacementBean[coords.length];

//Init the array of placements
   for (int i =0; i<coords.length;i++){
      placementBean placementBean = new PlacementBean();
      placementBean.setAddress(address[i]);
      placementBean.setDescription(description[i]);
      placementBean.setName(name[i]);
      placementBean.setOpen(open);
      placementBean.setStyleURL(styleURL);
      pointBean pointBean = new PointBean();
      pointBean.setCoordinate(coords[i]);

      placementBean.setCoordinates(pointBean);
      this.cityArray[i]=placementBean;
   } 
}

public synchronized PlacementBean nextRandomPlacement(){
   return cityArray[ generator.nextInt( this.idx )];
}

public synchronized String KMLRenderOfRandomPlacement(){

   return renderKMLPlacement(nextRandomPlacement());
}

private String renderKMLPlacement(PlacementBean pBean){
   String klmString="<Placemark>\n"+

   "\t<name>"+pBean.getName()+"</name>\n"+
   "\t<description><![CDATA["+pBean.getDescription()+"]]></description>"+
   "\t<address>"+pBean.getAddress()+"</address>\n"+
   "\t<styleUrl>"+pBean.getStyleURL()+"</styleUrl>\n"+
   "\t<Point>\n"+

   "\t\t<coordinates>"+pBean.getCoordinates().getCoordinate()+"</coordinates>\n"+
   "\t</Point>\n"+
   "</Placemark>\n";
   return klmString;
   }
}

<?xml version="1.0" encoding="UTF-8"?>
<kml xmlns="http://earth.google.com/kml/2.0">
<Folder>
   <description>Examples of bi directional flow of information</description>
   <name>Network Links</name>
   <visibility>1</visibility>
   <open>1</open>
   <NetworkLink>
      <description>Lets send coordinates once in a while</description>
      <name>Message Pushing</name>
      <visibility>1</visibility>
      <open>1</open>
      <refreshVisibility>1</refreshVisibility>
      <flyToView>0</flyToView>
      <Url>
         <href>http://localhost:8081/Tour/message</href>
         <refreshInverval>2</refreshInverval>
         <viewRefreshMode>onStop</viewRefreshMode>
         <viewRefreshTime>1</viewRefreshTime>
      </Url>
    </NetworkLink>
</Folder>
</kml>

Location: -0.134539,51.497,-0.117855,51.5034
IP address: 127.0.0.1
Updated: Fri Oct 21 11:42:45 CEST 2005

<?xml version="1.0" encoding="UTF-8"?>
<kml xmlns="http://earth.google.com/kml/2.0">
   <Placemark>
      <name><![CDATA[<font color="red">Alan Berg</font>]]></name>\
      <description><![CDATA[BLAH BLAH <i> Fri Oct 21 11:42:45 CEST 2005</i>]]>
      </description>
      <Point>
         <coordinates>4.889999,52.369998,0</coordinates>
      </Point>
   </Placemark>
</kml>

package test.google.earth.servlet;

import java.io.IOException;
import java.io.PrintWriter;
import javax.servlet.ServletConfig;
import javax.servlet.ServletException;
import javax.servlet.http.*;

import test.google.earth.bean.LastLocationBean;
import test.google.earth.bean.LastMessageBean;

import java.util.Date;

public class MessageServlet extends HttpServlet
{
   private static LastMessageBean lastMessage=new LastMessageBean();
   private static LastLocationBean lastLocation= new LastLocationBean();

   public void init(ServletConfig config) throws ServletException {
   super.init(config);
   lastMessage.setMessage("No message Yet");
   lastMessage.setName("System");
   lastMessage.setUpdated(new Date());
   lastLocation.setCoords("No contact with a client yet");
   lastLocation.setIpAddress("");
   lastLocation.setUpdated(new Date());
}

   protected void doGet(HttpServletRequest request, HttpServletResponse response)
   throws ServletException, IOException
{
   String coords = request.getParameter("BBOX");
   if (coords==null){
   return;
   }

   String message;
   String name;
   Date lastDate;
   String ipAddress = request.getRemoteAddr();

   synchronized(this) {
      lastLocation.setCoords(coords);
      lastLocation.setIpAddress(ipAddress);
      lastLocation.setUpdated(new Date());
      message=lastMessage.getMessage();
      name=lastMessage.getName();
      lastDate=lastMessage.getUpdated();
   }

   response.setContentType("application/keyhole");
   PrintWriter out = response.getWriter(); 
   String[] coParts= coords.split(",");
   float userlon;
   float userlat;
   try{
      userlon = ((Float.parseFloat(coParts[2]) - Float.parseFloat(coParts[0]))/2)+
         Float.parseFloat(coParts[0]);
      userlat = ((Float.parseFloat(coParts[3]) - Float.parseFloat(coParts[1]))/2) + 
         Float.parseFloat(coParts[1]);
   }catch(NumberFormatException e){
      return;
   }

      String klmString = "<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n"
      + "<kml xmlns=\"http://earth.google.com/kml/2.0\">\n"
      + "<Placemark>\n"
      + "<name><![CDATA[<font color=\"red\">"+name+"</font>]]></name>\n"

      +"<description><![CDATA["+message+"<br><i>"+lastDate+"</i>]]></description>\n"
      + "<Point>\n"
      + "<coordinates>"+userlon+","+userlat+",0</coordinates>\n"
      + "</Point>\n"
      + "</Placemark>\n"
      + "</kml>\n";       
      out.println(klmString);
}

   protected void doPost(HttpServletRequest request, HttpServletResponse response)
   throws ServletException, IOException
{
   String name = request.getParameter("name");
   if (name==null){
      return;
   }
   String message;
   PrintWriter out;

   synchronized(this) {
   lastMessage.setMessage(request.getParameter("message"));
   lastMessage.setName(name);
   lastMessage.setUpdated(new Date());
   message="<pre>\nLocation: "+lastLocation.getCoords()+
   "\nIP address: "+lastLocation.getIpAddress()+
   "\nUpdated: "+lastLocation.getUpdated();
   }


   response.setContentType("text/html");
   out = response.getWriter();
   out.println(message);
   }
}