Android SAX解析

在讲这次错误之前，先看一下下面这段代码。  【◆以下解析方法是错误的×】

 import java.util.ArrayList;
 import java.util.List;

 import org.xml.sax.Attributes;
 import org.xml.sax.SAXException;
 import org.xml.sax.helpers.DefaultHandler;

 import android.util.Log;

 public class XmlHandler extends DefaultHandler{

     private final String TAG = this.getClass().getSimpleName();

     /**XML文件中标签定义*/
     private final String TAG_Article = "Article";
     private final String TAG_ArticleID = "ArticleID";
     private final String TAG_Title = "Title";
     private final String TAG_Date = "Date";
     private final String TAG_SmallPictures = "SmallPictures";
     private final String TAG_LargePictures = "LargePictures";
     private final String TAG_Category = "Category";
     private static final String TAG_HeadNote = "HeadNote";
     private static final String TAG_SubTitle = "SubTitle";
     private static final String TAG_Source = "Source";

     //当前正在解析的TAG
     private String currentName;

     //单个文章
     private News news = null;

     //文章列表
     private List  newsList = null;

     //解析开始时间
     private long start_time;

     private boolean flag = false;

     @Override
     public void characters(char[] ch, int start, int length)
             throws SAXException {
         super.characters(ch, start, length);

         if(!flag) {
             return;
         }
         // 取值
         String value = new String(ch, start, length);
         Log.d(TAG, "Element: " + currentName  + " Element Value: " + value);
         if(value != null) {
             if(TAG_ArticleID.equals(currentName)) {
                 news.setArticleId(value);
             } else if(TAG_Title.equals(currentName)) {
                 news.setTitle(value);
             } else if(TAG_Date.equals(currentName)) {
                 news.setDate(value);
             } else if(TAG_Category.equals(currentName)) {
                 news.setCategory(value);
             } else if(TAG_SmallPictures.equals(currentName)) {
                 news.setSmallPicture(value);
             } else if(TAG_LargePictures.equals(currentName)) {
                 news.setLargePicture(value);
             } else if(TAG_HeadNote.equals(currentName)) {
                 news.setHeadNote(value);
             } else if(TAG_SubTitle.equals(currentName)) {
                 news.setSubTitle(value);
             } else if(TAG_Source.equals(currentName)) {
                 news.setSource(value);
             }
         }
     }

     @Override
     public void startDocument() throws SAXException {
         super.startDocument();

         start_time = System.currentTimeMillis();
         newsList = new ArrayList();
     }

     @Override
     public void startElement(String uri, String localName, String qName, Attributes attributes) throws SAXException {
         super.startElement(uri, localName, qName, attributes);
         this.currentName = localName;
         flag = true;
         if(TAG_Article.equals(localName)) {
             news = new News();
         }
     }

     @Override
     public void endElement(String uri, String localName, String qName)
             throws SAXException {
         super.endElement(uri, localName, qName);
         flag = false;

         if(TAG_Article.equals(localName)) {
             newsList.add(news);
         }
     }

     @Override
     public void endDocument() throws SAXException {
         super.endDocument();

         long end = System.currentTimeMillis();
         Log.d(TAG, "Parse List's Xml Cost: " + (end - start_time) + " !!");
     }
 }

Baidu 或者 Google 一下 “Android Sax 解析” ， 给出的Sample无一例外都是如此。 坑爹啊... 甚至连有些书籍中都是这么写的， 比如《Android开发入门与实践》。（本书亲自确认过，其他书情况不详）

没错， 一般情况下，这么写是可以的， 而且在大多数情况下解析出来也是正确的。 但是就是偶尔会出错， 这个时候通常你都莫不着头脑， 怎么回事? 数据没错啊，解析部分代码貌似也没问题.. 真是奇了怪了。 其实问题都出在上面那段代码上！！

大家都认为 SAX 解析过程大致如下：

startDocument  ->   startElement  -> characters -> endElement -> endDocument

没错，就是这样， startElement  读取起始标签， endElement 读取结束标签，characters 呢？当然是读取其值， 这没错，但是大家都天真的以为 characters 只执行一次，并且一次就读取了全部内容。错就错在这！

其实characters 是很有可能会执行多次的，当遇到内容中有回车，\t等等内容时，它很有可能就执行多次。 有的人可能会说，那我没有这些是不是就只执行一次了？ 看下我实测结果：

 

测试用XML如下：

 <News>
     <Article>
         <ArticleID>1000555ArticleID>
         <Title>Title>
         <Date>2011-11-25 14:23:52Date>
         <SmallPictures>livenews/images/s20.pngSmallPictures>
         <LargePictures>livenews/images/l20.pngLargePictures>
         <Category>闻天下Category>
         <HeadNote>HeadNote>
         <SubTitle>SubTitle>
         <Author>Author>
         <Source>人民日报Source>
         <Abstract>Abstract>
     Article>
     <Article>
         <ArticleID>1000554ArticleID>
         <Title>Title>
         <Date>2011-11-25 14:22:33Date>
         <Category>Category>
         <HeadNote>HeadNote>
         <SubTitle>SubTitle>
         <Author>Author>
         <Source>Source>
         <Abstract>Abstract>
     Article>
     <Article>
         <ArticleID>1000553ArticleID>
         <Title>Title>
         <Date>2011-11-25 14:21:23Date>
         <SmallPictures>livenews/images/s21.pngSmallPictures>
         <LargePictures>livenews/images/l21.pngLargePictures>
         <Category>Category>
         <HeadNote>HeadNote>
         <SubTitle>SubTitle>
         <Author>Author>
         <Source>Source>
         <Abstract>Abstract>
     Article>
 <News>

可以很明显的看到，在解析 1000553  这一段时， characters执行了两次，将内容"1000553"分两次读取.. 用上面那种方式的最终结果就是 ArticleID = 00553 了。 那如果你的应用需要根据这个id 进一步获取内容岂不是死翘翘了？（比如这边根据id获取新闻详细内容）

好了，废话不多说了，看下正确的写法！  【★以下解析方法才是正确的 √ 】

 import java.util.ArrayList;
 import java.util.List;

 import org.xml.sax.Attributes;
 import org.xml.sax.SAXException;
 import org.xml.sax.helpers.DefaultHandler;

 import android.util.Log;

 public class XmlHandler extends DefaultHandler{

     private final String TAG = this.getClass().getSimpleName();

     /**XML文件中标签定义*/
     private final String TAG_Article = "Article";
     private final String TAG_ArticleID = "ArticleID";
     private final String TAG_Title = "Title";
     private final String TAG_Date = "Date";
     private final String TAG_SmallPictures = "SmallPictures";
     private final String TAG_LargePictures = "LargePictures";
     private final String TAG_Category = "Category";
     private static final String TAG_HeadNote = "HeadNote";
     private static final String TAG_SubTitle = "SubTitle";
     private static final String TAG_Source = "Source";

     //单个文章
     private News news = null;

     //文章列表
     private List  newsList = null;

     //解析开始时间
     private long start_time;

     //(1)
     private StringBuilder sb = new StringBuilder();

     @Override
     public void characters(char[] ch, int start, int length)
             throws SAXException {
         super.characters(ch, start, length);

         //(2)不管在startElement到endElement的过程中，执行了多少次characters， 都会将内容添加到StringBuilder中，不会丢失内容
         sb.append(ch, start, length);
     }

     @Override
     public void startDocument() throws SAXException {
         super.startDocument();

         start_time = System.currentTimeMillis();
         newsList = new ArrayList();
     }

     @Override
     public void startElement(String uri, String localName, String qName, Attributes attributes) throws SAXException {
         super.startElement(uri, localName, qName, attributes);
         //(3) 开始收集新的标签的数据时，先清空历史数据
         sb.setLength(0);
         if(TAG_Article.equals(localName)) {
             news = new News();
         }
     }

     @Override
     public void endElement(String uri, String localName, String qName)
             throws SAXException {
         super.endElement(uri, localName, qName);

         //(4)原来在characters中取值，现改在此取值
         String value = sb.toString();

         if(TAG_ArticleID.equals(localName)) {
             news.setArticleId(value);
         } else if(TAG_Title.equals(localName)) {
             news.setTitle(value);
         } else if(TAG_Date.equals(localName)) {
             news.setDate(value);
         } else if(TAG_Category.equals(localName)) {
             news.setCategory(value);
         } else if(TAG_SmallPictures.equals(localName)) {
             news.setSmallPicture(value);
         } else if(TAG_LargePictures.equals(localName)) {
             news.setLargePicture(value);
         } else if(TAG_HeadNote.equals(localName)) {
             news.setHeadNote(value);
         } else if(TAG_SubTitle.equals(localName)) {
             news.setSubTitle(value);
         } else if(TAG_Source.equals(localName)) {
             news.setSource(value);
         }
         if(TAG_Article.equals(localName)) {
             newsList.add(news);
         }
     }

     @Override
     public void endDocument() throws SAXException {
         super.endDocument();

         long end = System.currentTimeMillis();
         Log.d(TAG, "Parse List's Xml Cost: " + (end - start_time) + " !!");
     }
 }

归纳为三点：

1.startElement的时候， new StringBuilder(); 或者 sb.setLength(0); (我建议后者)
2.characters的时候，sb.append(ch, start, length);
3.endElement的时候，sb.toString(); 此时StringBuilder中的内容才是解析的结果