一:什么是RSS
 RSS(really simple syndication) :网页内容聚合器。RSS的格式是XML。必须符合XML 1.0规范。
 RSS的作用：订阅BLOG,订阅新闻
二:RSS的历史版本:
 http://blogs.law.harvard.edu/tech/rssVersionHistory
 RSS的版本有很多个，0.90、0.91、0.92、0.93、0.94、1.0 和 2.0。与RSS相对的还有ATOM。
 国内主要是RSS2.0,国外主要用ATOM0.3.
 由于RSS出现2派，导致混乱场面。其中RSS2.0规范由哈佛大学定义并锁定。
 地址:http://blogs.law.harvard.edu/tech/rss
三：RSS 文件形式
    1:例子：
 <?xml version="1.0"?>
 <rss version="2.0">
    <channel>
    <title>The channel's name goes here</title>
    <link>http://www.urlofthechannel.com/</link>
    <description>This channel is an example channel for an article.
    </description>
    <language>en-us</language>
    <image>
      <title>The image title goes here</title>
      <url>http://www.urlofthechannel.com/images/logo.gif</url>
      <link>http://www.urlofthechannel.com/</link>
    </image>
    <item>
      <title>The Future of content</title>
      <link>http://www.itworld.com/nl/ecom_in_act/11122003/</link>
      <description> The issue of people distributing and reusing
      digital media is a problem for many businesses. It may also be
      a hidden opportunity. Just as open source licensing has opened
      up new possibilities in the world of technology, it promises to do
      the same in the area of creative content.</description>
    </item>
    <item>
      <title>Online Music Services - Better than free?</title>
      <link>http://www.itworld.com/nl/ecom_in_act/08202003/</link>
      <description>More people than ever are downloading music from 
      the Internet. Many use person-to-person file sharing programs like 
      Kazaa to share and download music in MP3 format, paying nothing. 
      This has made it difficult for companies to setup online music 
      businesses. How can companies compete against free?</description>
    </item>
  </channel>
 </rss>
    2:RSS文件由一个 <channel> 元素及其子元素组成。除了频道内容本身之外，<channel> 
 还以项的形式包含表示频道元数据的元素 —— 比如 <title>、<link> 和 <description>。
 项通常是频道的主要部分，包含经常变化的内容。
    3:频道(channel)用<channel>表示
 频道一般有三个元素，提供关于频道本身的信息：
 <title>：频道或提要的名称。 
 <link>：与该频道关联的 Web 站点或者站点区域的 URL。 
 <description>：简要介绍该频道是做什么的。 
 许多频道子元素都是可选的。常用的 <image> 元素包含三个必需的子元素：
 <url>：表示该频道的 GIF、JPEG 或 PNG 图像的 URL。 
 <title>：图象的描述。当频道以 HTML 呈现时，用作 HTML <image> 标签的 ALT 属性。 
 <link>：站点的 URL。如果频道以 HTML 呈现，该图像作为到这个站点的链接。 
 <image> 还有三个可选的子元素：
 <width>：数字，表示图象的像素宽度，最大值是 188，默认值为 88。 
 <height>：数字，表示图象的像素高度。最大值是 400，默认值为 31。 
 <description>：包含文本，在呈现时可以作为围绕着该图像形成的链接元素的 title 属性。 
 此外还可以使用许多其他可选的频道元素。多数都是不言自明的：
 <language>：en-us 
 <copyright>：Copyright 2003, James Lewin 
 <managingEditor>：dan@spam_me.com (Dan Deletekey) 
 <webMaster>：dan@spam_me.com (Dan Deletekey) 
 <pubDate>：Sat, 15 Nov 2003 0:00:01 GMT 
 <lastBuildDate>：Sat, 15 Nov 2003 0:00:01 GMT 
 <category>：ebusiness 
 <generator>：Your CMS 2.0 
 <docs>：http://blogs.law.harvard.edu/tech/rss 
 <cloud>：允许进程注册为“cloud”，频道更新时通知它，为 RSS 提要实现了一种轻量级的发布-订阅协议。 
 <ttl>：存活时间 是一个数字，表示提要在刷新之前缓冲的分钟数。 
 <rating>：关于该频道的 PICS 评价。 
 <textInput>：定义可与频道一起显示的输入框。 
 <skipHours>：告诉聚集器哪些小时的更新可以忽略。 
 <skipDays>：告诉聚集器那一天的更新可以忽略。 
    4:摘要(feed)用<item>表示,<item>的格式如下：
 每个摘要通常包含三个元素：
 <title>：这是项的名称，在标准应用中被转换成 HTML 中的标题。 
 <link>：这是该项的 URL。title 通常作为一个链接，指向包含在 <link> 元素中的 URL。 
 <description>：通常作为 link 中所指向的 URL 的摘要或者补充。 
 所有的元素都是可选的，但是一个项至少要么 包含一个 <title>，要么包含一个 <description>。
 项还有其他一些可选的元素：
 <author>：作者的 e-mail 地址。 
 <category>：支持有组织的记录。 
 <comments>：关于项的注释页的 URL。 
 <enclosure>：支持和该项有关的媒体对象。 
 <guid>：唯一与该项联系在一起的永久性链接。 
 <pubDate>：该项是什么时候发布的。 
 <source>：该项来自哪个 RSS 频道，当
四:主流java rss lib及其评测:
 主要有一下几种:
    1:Rome: http://wiki.java.net/bin/view/Javawsxml/Rome
 Rome是 java.net 上的一个开源项目，现在的版本是0.5。为什么叫Rome呢,按它的介绍上的说法，有个“条条大路通罗马”的意思，有些RSS的意味。Rome可能是 sun 公司从自己某个子项目中抽离出来的，package和类的命名就象j2sdk一样感觉规范。功能上支持RSS的所有版本及 Atom 0.3(Atom是和RSS类似的一种内容聚合的方式)。Rome 本身是提供API和功能实现.
    2:rssutils: http://gceclub.sun.com.cn/staticcontent/html/2004-04-22/rss.html
 rssutils是一个工具包，sun 的 develope站点上有文章 RSS Utilities: A Tutorial 专门介绍用taglib 显示RSS内容，附带的可以下载这个工具包，但我从网上搜索不到它的出处，自然也无法看到它的源码。但从反编译的代码来看，也是sun公司内部高手所做，设计精巧，代码简练。实现一个handler，用sax的方式解析xml内容，handler内部用反射和javabean的机制构造RSS元素对象并赋值。
    3rsslib4j: http://sourceforge.net/projects/rsslib4j
 rsslib4j 是 sourceforget 上的项目，同样支持所有RSS版本。 
    4:rsslibj:http://enigmastation.com/rsslibj/
    5:总结
 Rome: 
 优 - 1）可扩展性好，有前途。2）功能强大，除了用来解析RSS，还可以聚合和构造RSS。
 劣 - 1）兼容性待加强，2）绑定jdom。
 rssutils: 
 优 - 1）代码设计精妙，值得学习。2）附带 taglib 实现，直接可在 jsp 中应用。
 劣 - 1）没有源码。 2）兼容性有待加强。 3）功能较弱，只能用来解析RSS，没有聚合和构造RSS功能。
 rsslib4j:
 优 - 1）简单有效，体积小。2）兼容性不错。
 劣 - 1）有小bug。2）功能较弱，只能用来解析RSS，没有聚合和构造RSS功能。
 rsslibj:
 优 - 1）简单有效，体积小,才25K。2）能解析和生成RSS(动态和静态)
 劣 - 1）有小bug。2）版本很久没有更新了，陈旧.

五:选择ROME作为RSS实现工具
 在官网http://wiki.java.net/bin/view/Javawsxml/Rome下载rome-0.8.jar,
 rome用到了jdom1.0,下载地址:http://www.jdom.org
 rome支持:rss_0.9 
   rss_0.91 
   rss_0.92 
   rss_0.93 
   rss_0.94 
   rss_1.0 
   rss_2.0 
   atom_0.3 
   atom_1.0 
 生成RSS类新需要在程序中指定，如：rss_2.0
六:包结构
 com.sun.syndication.feed 提供RSS and Atom beans的父类
 com.sun.syndication.feed.atom 提供实现Atom feeds核心元素的beans
 com.sun.syndication.feed.module 提供处理聚合modules的beans
 com.sun.syndication.feed.rss 提供实现Rss feeds核心元素的beans
 com.sun.syndication.feed.synd 我们主要用的就是这个包，SyndFeed and SyndEntryImpl 
 com.sun.syndication.io  提供对读取和分析feeds的输入和输出
七:实例：

1:读取远端url的rss,然后输出到控制台:
/**
 * 关键代码：
 * SyndFeedInput input = new SyndFeedInput();
 * SyndFeed feed = input.build(new XmlReader(feedUrl));
 */

package com.sun.syndication.samples;

import com.sun.syndication.feed.synd.SyndFeed;
import com.sun.syndication.io.SyndFeedInput;
import com.sun.syndication.io.XmlReader;
import java.net.URL;

/**
 * It Reads and prints any RSS/Atom feed type.
 */
public class FeedReader {

    public static void main(String[] args) {
        boolean ok = false;
        if (args.length==0) {
            try {
                URL feedUrl = new URL("http://seu.org.cn/bbs/rss.php");
  //SyndFeedInput:从远程读到xml结构的内容转成SyndFeedImpl实例
                SyndFeedInput input = new SyndFeedInput();
  //rome按SyndFeed类型生成rss和atom的实例,
  //SyndFeed是rss和atom实现类SyndFeedImpl的接口
                SyndFeed feed = input.build(new XmlReader(feedUrl));
  //打印到控制台
                System.out.println(feed);
                ok = true;
            }
            catch (Exception ex) {
                ex.printStackTrace();
                System.out.println("ERROR: "+ex.getMessage());
            }
        }

        if (!ok) {
            System.out.println();
            System.out.println("FeedReader reads and prints any RSS/Atom feed type.");
            System.out.println("The first parameter must be the URL of the feed to read.");
            System.out.println();
        }
    }
}
2:将多个远程RSS在本地聚集成一个RSS
package com.sun.syndication.samples;

import java.net.URL;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.List;
import java.util.ArrayList;

import com.sun.syndication.feed.synd.SyndFeed;
import com.sun.syndication.feed.synd.SyndFeedImpl;
import com.sun.syndication.io.SyndFeedOutput;
import com.sun.syndication.io.SyndFeedInput;
import com.sun.syndication.io.XmlReader;

/**
 * It aggregates a list of RSS/Atom feeds (they can be of different types)
 * into a single feed of the specified type.
 * <p>
 * @author Alejandro Abdelnur
 *
 */
public class FeedAggregator {

    public static void main(String[] args) {
        boolean ok = false;
        if (args.length>=2) {
            try {
                String outputType = args[0];

                SyndFeed feed = new SyndFeedImpl();
                feed.setFeedType(outputType);

                feed.setTitle("Aggregated Feed");
                feed.setDescription("Anonymous Aggregated Feed");
                feed.setAuthor("anonymous");
                feed.setLink("http://www.anonymous.com");

                List entries = new ArrayList();
                feed.setEntries(entries);

                for (int i=1;i<args.length;i++) {
                    URL inputUrl = new URL(args[i]);

                    SyndFeedInput input = new SyndFeedInput();
                    SyndFeed inFeed = input.build(new XmlReader(inputUrl));

                    entries.addAll(inFeed.getEntries());

      &nbp;         }

                SyndFeedOutput output = new SyndFeedOutput();
                output.output(feed,new PrintWriter(System.out));

                ok = true;
            }
            catch (Exception ex) {
                System.out.println("ERROR: "+ex.getMessage());
            }
        }

        if (!ok) {
            System.out.println();
            System.out.println("FeedAggregator aggregates different feeds into a single one.");
            System.out.println("The first parameter must be the feed type for the aggregated feed.");
            System.out.println(" [valid values are: rss_0.9, rss_0.91U, rss_0.91N, rss_0.92, rss_0.93, ]");
            System.out.println(" [                  rss_0.94, rss_1.0, rss_2.0 & atom_0.3  ]");
            System.out.println("The second to last parameters are the URLs of feeds to aggregate.");
            System.out.println();
        }
    }

}
3:将动态生成的RSS存盘，形成静态RSS
package com.sun.syndication.samples;

import com.sun.syndication.feed.synd.*;
import com.sun.syndication.io.SyndFeedOutput;

import java.io.FileWriter;
import java.io.Writer;
import java.text.DateFormat;
import java.text.SimpleDateFormat;
import java.util.ArrayList;
import java.util.List;

/**
 * It creates a feed and writes it to a file.
 * <p>
 * @author Alejandro Abdelnur
 *
 */
public class FeedWriter {

    private static final String DATE_FORMAT = "yyyy-MM-dd";

    public static void main(String[] args) {
        boolean ok = false;
        if (args.length==0) {
            try {
                String feedType = "rss_2.0";//指定rss类型
                String fileName = "F:\ss.xml";//静态rss存放目录

                DateFormat dateParser = new SimpleDateFormat(DATE_FORMAT);

  //feed是通过SyndFeedImpl的实例 
                SyndFeed feed = new SyndFeedImpl();
                feed.setFeedType(feedType);

                feed.setTitle("Sample Feed (created with Rome)");
                feed.setLink("http://rome.dev.java.net");
                feed.setDescription("This feed has been created using Rome (Java syndication utilities");
  
  //entries就是item集合
                List entries = new ArrayList();
  //一个entry就是一个item
                SyndEntry entry;
                SyndContent description;
  
  //第一个item
                entry = new SyndEntryImpl();
                entry.setTitle("Rome v1.0");
                entry.setLink("http://wiki.java.net/bin/view/Javawsxml/Rome01");
                entry.setPublishedDate(dateParser.parse("2004-06-08"));
                description = new SyndContentImpl();
                description.setType("text/plain");
                description.setValue("Initial release of Rome");
                entry.setDescription(description);
                entries.add(entry);

  //第二个item
                entry = new SyndEntryImpl();
                entry.setTitle("Rome v2.0");
                entry.setLink("http://wiki.java.net/bin/view/Javawsxml/Rome02");
                entry.setPublishedDate(dateParser.parse("2004-06-16"));
                description = new SyndContentImpl();
                description.setType("text/xml");
                description.setValue("Bug fixes, <xml>XML</xml> minor API changes and some new features");
                entry.setDescription(description);
                entries.add(entry);

  //第三个item
                entry = new SyndEntryImpl();
                entry.setTitle("Rome v3.0");
                entry.setLink("htp://wiki.java.net/bin/view/Javawsxml/Rome03");
                entry.setPublishedDate(dateParser.parse("2004-07-27"));
                description = new SyndContentImpl();
                description.setType("text/html");
                description.setValue("<p>More Bug fixes, mor API changes, some new features and some Unit testing</p>"+
                                     "<p>For details check the <a href="Changes">http://wiki.java.net/bin/view/Javawsxml/RomeChangesLog#RomeV03">Changes Log</a></p>");
                entry.setDescription(description);
  
  //将所有item存入entries
                entries.add(entry);
  
  //将entries加入channel
                feed.setEntries(entries);

                Writer writer = new FileWriter(fileName);
                SyndFeedOutput output = new SyndFeedOutput();
  //存盘,形成静态rss
                output.output(feed,writer);
                writer.close();
               

                System.out.println("The feed has been written to the file ["+fileName+"]");
                System.out.println(feed);

                ok = true;
            }
            catch (Exception ex) {
                ex.printStackTrace();
                System.out.println("ERROR: "+ex.getMessage());
            }
        }

        if (!ok) {
            System.out.println();
            System.out.println("FeedWriter creates a RSS/Atom feed and writes it to a file.");
            System.out.println("The first parameter must be the syndication format for the feed");
            System.out.println("  (rss_0.90, rss_0.91, rss_0.92, rss_0.93, rss_0.94, rss_1.0 rss_2.0 or atom_0.3)");
            System.out.println("The second parameter must be the file name for the feed");
            System.out.println();
        }
    }

}

4：动态生成rss，给一个blog站点动态生成rss
package com.vaga.rss.web.admin;

import java.io.IOException;
import java.text.DateFormat;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;

import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import org.springframework.web.servlet.ModelAndView;
import org.springframework.web.servlet.mvc.ParameterizableViewController;

import com.sun.syndication.feed.synd.SyndContent;
import com.sun.syndication.feed.synd.SyndContentImpl;
import com.sun.syndication.feed.synd.SyndEntry;
import com.sun.syndication.feed.synd.SyndEntryImpl;
import com.sun.syndication.feed.synd.SyndFeed;
import com.sun.syndication.feed.synd.SyndFeedImpl;
import com.sun.syndication.io.FeedException;
import com.sun.syndication.io.SyndFeedOutput;
import com.sun.syndication.feed.synd.SyndContent;
import com.sun.syndication.feed.synd.SyndEntry;
import com.totsp.xml.syndication.content.ContentModule;
import com.vaga.blog.model.WeblogEntry;
import com.vaga.blog.model.Website;
import com.vaga.blog.service.WeblogEntryManager;
import com.vaga.blog.service.WebsiteManager;

public class SiteRssViewController  extends ParameterizableViewController {
 // Constants
  /** Namespace URI for content:encoded elements */
   private static final String CONTENT_NS ="http://purl.org/rss/1.0/modules/content/";
   private static final String FEED_TYPE = "type";
   private static final String MIME_TYPE = "application/xml; charset=UTF-8";
   private static final String COULD_NOT_GENERATE_FEED_ERROR = "Could not generate feed";
   private static final String _defaultFeedType="rss_2.0";
   private static final String DATE_FORMAT = "yyyy-MM-dd";
   //controller starts
   private WeblogEntryManager weblogEntryManager;//spring依赖注入
   private WebsiteManager websiteManager; //spring依赖注入
 //spring依赖注入
   public void setWeblogEntryManager(WeblogEntryManager weblogEntryManager) {
  this.weblogEntryManager = weblogEntryManager;
   }
 //spring依赖注入
   public void setWebsiteManager(WebsiteManager websiteManager) {
  this.websiteManager = websiteManager;
   }
 
 protected ModelAndView handleRequestInternal(HttpServletRequest request,HttpServletResponse response) throws Exception {
 try {
           SyndFeed feed = getFeed(request);
           String feedType = request.getParameter(FEED_TYPE);//null
           feedType = (feedType!=null) ? feedType : _defaultFeedType;
           feed.setFeedType(feedType);//rss_2.0
           response.setContentType(MIME_TYPE);
           SyndFeedOutput output = new SyndFeedOutput();
           output.output(feed,response.getWriter());//向发出请求的用户输出该RSS(xml格式)
       }
       catch (FedException ex) {
           String msg = COULD_NOT_GENERATE_FEED_ERROR;
           log(msg,ex);
           response.sendError(HttpServletResponse.SC_INTERNAL_SERVER_ERROR,msg);
       }
  return null;
 }
 

    /**
    * 请求的类型如下:
    * siteRss.htm?websiteId=21   |ID=66的个人站点最新20条文章
    * siteRss.htm?websiteId=21&entryType=hot |ID=66的个人站点最热20条文章
    *
    * @param request
    */
    protected SyndFeed getFeed(HttpServletRequest request) throws IOException,FeedException {
       DateFormat dateParser = new SimpleDateFormat(DATE_FORMAT);
       //feed就是channel
       SyndFeed feed = new SyndFeedImpl();
       //item集合
       List entries = new ArrayList();
       //一个entry就是代表一个item
       SyndEntry entry;
       SyndContent description;
       setFeed(request,feed);
       Iterator iterator = setIterator(request);
  //将文章的20记录转成20个item
           while(iterator.hasNext()){
            entry = new SyndEntryImpl();
            WeblogEntry weblogEntry = (WeblogEntry)iterator.next();
            entry.setTitle(weblogEntry.getTitle());
            entry.setLink(feed.getLink()+"?weblogEntryId="+weblogEntry.getId());
            try {
             entry.setPublishedDate(dateParser.parse(weblogEntry.getPubTime().toString()));
            }
            catch (ParseException ex) {
             ex.printStackTrace();
            }
            //该item的description
            description = new SyndContentImpl();
            description.setType("text/plain");
            String text=null;
            if(weblogEntry.getText().length()>500){
             text = weblogEntry.getText().substring(0, 500);
            }else{
             text = weblogEntry.getText();
            }
            description.setValue(text);
            entry.setDescription(description);
            addFooter(entry);
            entries.add(entry);
           }

       
       //将所有的item存入channel
       feed.setEntries(entries);
       return feed;
   }
 private SyndFeed setFeed(HttpServletRequest request,SyndFeed feed){
    //blog中的website   
    Website website = websiteManager.getWebsite(request.getParameter("websiteId"));
    设置当前website的channel属性
    feed.setTitle(website.getName());
    feed.setAuthor(website.getCreator());
    feed.setCopyright(website.getEmailAddress());
    feed.setLink("http://wxz.vaga.com.cn:8080/blog/weblog/"+website.getHandle());
    feed.setDescription(website.getDescription());
       return feed;   
 }

 //从数据库中获得20条该website的文章
 private Iterator setIterator(HttpServletRequest request){
  if(request.getParameter("entryType")==null){
   return weblogEntryManager.getRecentWeblogEntriesForRss(request.getParameter("websiteId"), null, "PUBLISHED", 21).iterator();
  }else{
   return weblogEntryManager.getHotWeblogEntriesForRss(request.getParameter("websiteId"), null, 21).iterator();
  }
 }

    /**
     * Add footer to an entry.给每个文章摘要添加页脚
     * @param entry
     */
    public static void addFooter(SyndEntry entry)
    {
        // Prep variables used in loops
        String title = entry.getTitle();
        String link = entry.getLink();

        // Use the add-on ContentModule to handle
        // <content:encoded/> elments within the feed
        ContentModule module =((ContentModule) entry.getModule(CONTENT_NS));

        // If content:encoded is found, use that.
        if(module!=null)
        {
            // Container for footer-appended HTML strings
            List newStringList = new ArrayList();

            // Iterate through encoded HTML, creating footers
            Iterator oldStringIter =module.getEncodeds().iterator();
            while (oldStringIter.hasNext())
            {
                String original = (String) oldStringIter.next();
                newStringList.add(createFooter(original,link, title));
            }

            // Set new encoded HTML strings on entry
            module.setEncodeds(newStringList);
        }
        else
        {
            // Fall back to adding footer in <description/>
            // This results in escaped HTML.  Ugly, but common.
         //Target the description node
         SyndContent content = entry.getDescription();
            // Create and set a footer-appended description
            String original = content.getValue();
            content.setValue(createFooter(original,link, title));
            
        }
    }

    /**
     * Create a feed item footer of immediate actions
     * by using information from the feed item itself
     * @param original  The original text of the feed item
     * @param link      The link for the feed item
     * @param title     The title of the feed item
     * @return
     */
    private static String createFooter(String original, String link,String title)
    { 
     // Use StringBuffer to create a sb
     StringBuffer sb;
     if(original==null){
      sb=new StringBuffer("<br />");
     }else{
      sb= new StringBuffer(original);
     }               
        sb.append(" <div class='feedwarmer'><hr/>");
        sb.append("<i>相关操作:</i> ");

        // Add email link using title and item link
        sb.append("<a href='mailto:?body=Check this out: ");
        sb.append(link).append("'>推荐该链接</a> | ");

        // Add delicious link using item title link
        sb.append("<a href='http://del.icio.us/post/?url=");
        sb.append(link).append("&title=").append(title);
        sb.append("'>添加到delicious</a> | ");

        // Add Google Blogs Search link using item title
        sb.append("<a href='http://blogsearch.google.com/");
        sb.append("blogsearch?hl=en&q=").append(title);
        sb.append("'>搜索相关内容</a>");

        // Finish and return the sb
        sb.append("</div> ");
        return sb.toString();
    }
}