SAX的一种应用

在上一篇文章里说到了SAX的快速入门现在让我们来看看它的一个具体应用吧。

现在有这样的一个XML document:

<?xml version="1.0" encoding="UTF-8"?><store><other_tag1 /><type name="book" type_id="1" /><other_tag3 /><bookstore><other_tag2 /><address addr="Shanghai, China" /><other_tag4 /><book category="COOKING" title="Everyday Italian" author="Giada De Laurentiis" year="2005" price="30.00" /><book category="CHILDREN" title="Harry Potter" author="J K. Rowling" year="2005" price="29.99" /></bookstore></store>

我们要利用SAX提取<type name="book" type_id="1" />， <address addr="Shanghai, China" />， <book/>这几个nodes的信息，我们因该怎么做呢？

现在有这样的一个思路：利用SAX，当一遇到node的localName 为“type”, "address", "book"的时候，就停下来抓取信息。这时候，我觉得一个方法就是在

startElement里面加上if/ else if/ else 这样的判断，这样虽然很直接明了，但是很傻，因为如果要解析的xml内容少还好，如果要抓取的信息量极大的话，那得

要多少个if/ else if/ else ?!现在我们换一个新的方法来处理这件事情：把需要追踪的元素形成一个“类似XML元素树”，例如，我们要追踪的全部元素为“store”,

“type”,"bookstore","address","book",树的结构为：

              store                  |          --------------          |            |          type    bookstore          |     -----------     |         |  address     book  

追踪的代码：

                root.track("store", new StoreTracker());root.track("store/type", new TypeTracker());root.track("store/bookstore", new BookStoreTracker());root.track("store/bookstore/address", new AddrTracker());root.track("store/bookstore/book", new BookTracker());

形成“XML元素树”的关键代码，这是TagTracker.java里的一部分：

public void track(String tagName, TagTracker tracker) {int slashOffset = tagName.indexOf("/");if(slashOffset < 0) {trackers.put(tagName, tracker);} else if(slashOffset == 0) {// "/a/b" --> "a/b" and continue.track(tagName.substring(1), tracker);} else {String topTagName = tagName.substring(0, slashOffset);String remainderOfTagName = tagName.substring(slashOffset + 1);TagTracker child = trackers.get(topTagName);if(child == null) {child = new TagTracker();trackers.put(topTagName, child);}child.track(remainderOfTagName, tracker);}}

这样，在整个"root" element下有"store"节点，在“store”下有“type”和“bookstore”节点，在"bookstore"下又有“address”和"book"节点，形成了我们实际需要追踪的

“元素树”.

我们知道，SAX是通过startElement(String namespaceURI, String localName, String qName,

            Attributes attr)和 endElement(String namespaceURI, String localName, String qName)来获取元素的uri,localName,attributes等诸如信息的（如果要获取元素里的文本信息，需要调用characters(char[] ch, int start, int length)）。每一个元素都应该有一个特定的的方法来收集这个元素的信息(包括元素里的文本信息)：

// get information of the element "type"private class TypeTracker extends TagTracker {public TypeTracker() {}@Overridepublic void onStart(String namespaceURI, String localName,String qName, Attributes attr) throws Exception {String name = attr.getValue("name");String typeId = attr.getValue("type_id");// handle these info. ...}@Overridepublic void onEnd(String namespaceURI, String localName, String qName,CharArrayWriter contents) {// get the characters data inside the elementString text = contents.toString();// handle this text...}}

而SAX的startElement和endElement方法会分别调用上述的两个方法，把namespaceURI, localName, qName, attributes等信息传递过去

好了，现在让我们来看看，是怎么让SAX挑出我们所需要追踪的元素并解析的吧(自动忽略其他的元素)。

TagTracker.java的全部代码：

package com.desmond.xml.sax;import java.io.CharArrayWriter;import java.util.Hashtable;import java.util.Stack;import org.apache.commons.logging.Log;import org.apache.commons.logging.LogFactory;import org.xml.sax.Attributes;public class TagTracker {private static Log log = LogFactory.getLog(TagTracker.class);private Hashtable<String, TagTracker> trackers = new Hashtable<String, TagTracker>();// use to skip these un-choiced elementsprivate static SkippingTagTracker skip = new SkippingTagTracker();public TagTracker() {}/** *  track all elements need to be tracked. * @param tagName the absolute path of the tracked element * @param tracker the detail handler to parse a special element */public void track(String tagName, TagTracker tracker) {int slashOffset = tagName.indexOf("/");if (slashOffset < 0) {// if it is a simple tag name (no "/" sperators) simple add it.trackers.put(tagName, tracker);} else if (slashOffset == 0) {// "/a/b" --> "a/b" and continue.track(tagName.substring(1), tracker);} else {String topTagName = tagName.substring(0, slashOffset);String remainderOfTagName = tagName.substring(slashOffset + 1);TagTracker child = trackers.get(topTagName);if (child == null) {child = new TagTracker();trackers.put(topTagName, child);}child.track(remainderOfTagName, tracker);}}/** * start to parse a element, which will be invoked by SAX's startElement. * @param namespaceURI * @param localName * @param qName * @param attr * @param tagStack "tracked element tree" * @throws Exception */public void startElement(String namespaceURI, String localName,String qName, Attributes attr, Stack<TagTracker> tagStack)throws Exception {TagTracker tracker = trackers.get(localName);// not found this tag track.if (tracker == null) {log.debug("Skipping tag:[" + localName + "]");tagStack.push(skip);} else {log.debug("Tracking tag:[" + localName + "]");onDeactivate();tracker.onStart(namespaceURI, localName, qName, attr);tagStack.push(tracker);}}/** * end to parse a element, which will be invoked by SAX's endElement. * @param namespaceURI * @param localName * @param qName * @param contents * @param tagStack current element * @throws Exception */public void endElement(String namespaceURI, String localName, String qName,CharArrayWriter contents, Stack tagStack) throws Exception {log.debug("Finished tracking tag:[" + localName + "]");try {onEnd(namespaceURI, localName, qName, contents);} catch (Exception e) {e.printStackTrace();throw e;}// clean up the stacktagStack.pop();// send the reactivate eventTagTracker activeTracker = (TagTracker) tagStack.peek();if (activeTracker != null) {log.debug("Reactivating pervious tag tracker.");activeTracker.onReactivate();}}/** * detail method to start to parse the special element. * @param namespaceURI * @param localName * @param qName * @param attr * @throws Exception */public void onStart(String namespaceURI, String localName, String qName,Attributes attr) throws Exception {}public void onDeactivate() throws Exception {}/** * detail method to end to parse the special element. * @param namespaceURI * @param localName * @param qName * @param contents */public void onEnd(String namespaceURI, String localName, String qName,CharArrayWriter contents) {}public void onReactivate() throws Exception {}}

在startElement中，我们会去“元素树”中选择当前的元素，从而去判断当前的元素释放应该被解析：

TagTracker tracker = trackers.get(localName);// not found this tag track.if (tracker == null) {log.debug("Skipping tag:[" + localName + "]");tagStack.push(skip);} else {log.debug("Tracking tag:[" + localName + "]");onDeactivate();tracker.onStart(namespaceURI, localName, qName, attr);tagStack.push(tracker);}

如果tracker为null，说明这个元素不是我们想要解析的那些，因此要"跳过",如何去跳过，这里用到了另一个类SkippingTagTracker,它所做的事情就是去跳过这个

元素，代码如下：

package com.desmond.xml.sax;import java.util.Stack;import org.apache.commons.logging.Log;import org.apache.commons.logging.LogFactory;import org.xml.sax.Attributes;public class SkippingTagTracker extends TagTracker {private static Log log = LogFactory.getLog(SkippingTagTracker.class);public void startElement(String namespaceURI, String localName, String qName, Attributes attr,Stack tagStack) {log.debug("Skipping tag[" + localName + "]...");tagStack.push(this);}public void endElement(String namespaceURI, String localName, String qName, Attributes attr,Stack tagStack) {log.debug("Finished skipping tag:[" + localName + "]");tagStack.pop();}}

如果tracker有值，我们就要开始解析这个元素了。这时，我们调用前面提到的”一个特定的的方法来收集这个元素的信息“，即：

tracker.onStart(namespaceURI, localName, qName, attr);

完了，之后我们把当前这个元素的tracker压入栈中，如果这个元素没有子元素，那么它将会在endElement中被抛出栈顶。如果有的话，先处理它的子

元素，等所有的子元素都处理完了，才调用endElement结束这个元素（这个也是SAX处理元素的规则，这里只是用到了这一点而已）。

综上所述，整个事件的处理流程是：使用TagTracker追踪所需要的元素-------> 利用TagTracker的 track方法递归调用形成”元素树“-------> 利用这些

”元素树“去判断当前的元素是不是应该被解析-------> 不被解析就跳过，被解析，再去判断他的子元素。一种这样递归地完成真个解析过程。

附全部代码(SaxMapper.java/ SkippingTagTracker.java/ TagTracker.java/ TestMain.java, 共四个类).

SaxMapper.java

package com.desmond.xml.sax;import java.io.ByteArrayInputStream;import java.io.CharArrayWriter;import java.io.File;import java.io.IOException;import java.util.Stack;import org.apache.commons.configuration.Configuration;import org.apache.commons.io.FileUtils;import org.apache.commons.logging.Log;import org.apache.commons.logging.LogFactory;import org.xml.sax.Attributes;import org.xml.sax.InputSource;import org.xml.sax.SAXException;import org.xml.sax.XMLReader;import org.xml.sax.helpers.DefaultHandler;import org.xml.sax.helpers.XMLReaderFactory;public class SaxMapper extends DefaultHandler{private static final Log log = LogFactory.getLog(SaxMapper.class);private String file = "";protected Stack<TagTracker> tagStack = new Stack<TagTracker>();protected XMLReader xr;protected CharArrayWriter contents = new CharArrayWriter();protected boolean parseOnly;protected Configuration config;public SaxMapper() throws Exception {try {xr = XMLReaderFactory.createXMLReader();} catch (SAXException e) {e.printStackTrace();}log.info("Creating the tag tracker network.");tagStack.push(createTagTrackerNetwork());log.info("Tag Tracker network created.");}@Overridepublic void startElement(String namespaceURI, String localName, String qName,Attributes attr) throws SAXException {contents.reset();TagTracker ativeTracker = (TagTracker) tagStack.peek();try {ativeTracker.startElement(namespaceURI, localName, qName, attr, tagStack);} catch(Exception e) {e.printStackTrace();throw new SAXException(e);}}@Overridepublic void endElement(String namespaceURI, String localName, String qName)throws SAXException {TagTracker activeTracker = (TagTracker) tagStack.peek();try {activeTracker.endElement(namespaceURI, localName, qName, contents, tagStack);} catch(Exception e) {e.printStackTrace();throw new SAXException(e);}}@Overridepublic void characters(char[] ch, int start, int length)throws SAXException {contents.write(ch, start, length);}protected InputSource getSource(String fileName) throws IOException {File xmlFile = new File(fileName);byte[] xmlBytes = FileUtils.readFileToByteArray(xmlFile);return new InputSource(new ByteArrayInputStream(xmlBytes));}protected void parseXML() throws IOException, Exception {parse(getSource(getFileName()));}protected void parse(InputSource in) throws Exception{parseOnly = true;xr.setContentHandler(this);log.info("start to parse...");xr.parse(in);log.info("end to parse...");}protected TagTracker createTagTrackerNetwork() {TagTracker root  = new TagTracker();root.track("store", new StoreTracker());root.track("store/type", new TypeTracker());root.track("store/bookstore", new BookStoreTracker());root.track("store/bookstore/address", new AddrTracker());root.track("store/bookstore/book", new BookTracker());return root;}protected String getFileName() {return file;}protected void setFileName(String fileName) {this.file = fileName;}private class StoreTracker extends TagTracker {public StoreTracker() {}@Overridepublic void onStart(String namespaceURI, String localName,String qName, Attributes attr) throws Exception {}@Overridepublic void onEnd(String namespaceURI, String localName, String qName,CharArrayWriter contents) {}}// get information of the element "type"private class TypeTracker extends TagTracker {public TypeTracker() {}@Overridepublic void onStart(String namespaceURI, String localName,String qName, Attributes attr) throws Exception {String name = attr.getValue("name");String typeId = attr.getValue("type_id");// handle these info. ...}@Overridepublic void onEnd(String namespaceURI, String localName, String qName,CharArrayWriter contents) {// get the characters data inside the elementString text = contents.toString();// handle this text...}}private class BookStoreTracker extends TagTracker {public BookStoreTracker() {}}private class AddrTracker extends TagTracker {public AddrTracker() {}@Overridepublic void onStart(String namespaceURI, String localName,String qName, Attributes attr) throws Exception {}@Overridepublic void onEnd(String namespaceURI, String localName, String qName,CharArrayWriter contents) {}}private class BookTracker extends TagTracker {public BookTracker() {}@Overridepublic void onStart(String namespaceURI, String localName,String qName, Attributes attr) throws Exception {}@Overridepublic void onEnd(String namespaceURI, String localName, String qName,CharArrayWriter contents) {}}}

SkippingTagTracker.java

package com.desmond.xml.sax;import java.util.Stack;import org.apache.commons.logging.Log;import org.apache.commons.logging.LogFactory;import org.xml.sax.Attributes;public class SkippingTagTracker extends TagTracker {private static Log log = LogFactory.getLog(SkippingTagTracker.class);public void startElement(String namespaceURI, String localName, String qName, Attributes attr,Stack tagStack) {log.debug("Skipping tag[" + localName + "]...");tagStack.push(this);}public void endElement(String namespaceURI, String localName, String qName, Attributes attr,Stack tagStack) {log.debug("Finished skipping tag:[" + localName + "]");tagStack.pop();}}

TagTracker

package com.desmond.xml.sax;import java.io.CharArrayWriter;import java.util.Hashtable;import java.util.Stack;import org.apache.commons.logging.Log;import org.apache.commons.logging.LogFactory;import org.xml.sax.Attributes;public class TagTracker {private static Log log = LogFactory.getLog(TagTracker.class);private Hashtable<String, TagTracker> trackers = new Hashtable<String, TagTracker>();// use to skip these un-choiced elementsprivate static SkippingTagTracker skip = new SkippingTagTracker();public TagTracker() {}/** *  track all elements need to be tracked. * @param tagName the absolute path of the tracked element * @param tracker the detail handler to parse a special element */public void track(String tagName, TagTracker tracker) {int slashOffset = tagName.indexOf("/");if (slashOffset < 0) {// if it is a simple tag name (no "/" sperators) simple add it.trackers.put(tagName, tracker);} else if (slashOffset == 0) {// "/a/b" --> "a/b" and continue.track(tagName.substring(1), tracker);} else {String topTagName = tagName.substring(0, slashOffset);String remainderOfTagName = tagName.substring(slashOffset + 1);TagTracker child = trackers.get(topTagName);if (child == null) {child = new TagTracker();trackers.put(topTagName, child);}child.track(remainderOfTagName, tracker);}}/** * start to parse a element, which will be invoked by SAX's startElement. * @param namespaceURI * @param localName * @param qName * @param attr * @param tagStack "tracked element tree" * @throws Exception */public void startElement(String namespaceURI, String localName,String qName, Attributes attr, Stack<TagTracker> tagStack)throws Exception {TagTracker tracker = trackers.get(localName);// not found this tag track.if (tracker == null) {log.debug("Skipping tag:[" + localName + "]");tagStack.push(skip);} else {log.debug("Tracking tag:[" + localName + "]");onDeactivate();tracker.onStart(namespaceURI, localName, qName, attr);tagStack.push(tracker);}}/** * end to parse a element, which will be invoked by SAX's endElement. * @param namespaceURI * @param localName * @param qName * @param contents * @param tagStack current element * @throws Exception */public void endElement(String namespaceURI, String localName, String qName,CharArrayWriter contents, Stack tagStack) throws Exception {log.debug("Finished tracking tag:[" + localName + "]");try {onEnd(namespaceURI, localName, qName, contents);} catch (Exception e) {e.printStackTrace();throw e;}// clean up the stacktagStack.pop();// send the reactivate eventTagTracker activeTracker = (TagTracker) tagStack.peek();if (activeTracker != null) {log.debug("Reactivating pervious tag tracker.");activeTracker.onReactivate();}}/** * detail method to start to parse the special element. * @param namespaceURI * @param localName * @param qName * @param attr * @throws Exception */public void onStart(String namespaceURI, String localName, String qName,Attributes attr) throws Exception {}public void onDeactivate() throws Exception {}/** * detail method to end to parse the special element. * @param namespaceURI * @param localName * @param qName * @param contents */public void onEnd(String namespaceURI, String localName, String qName,CharArrayWriter contents) {}public void onReactivate() throws Exception {}}

TestMain.java

package com.desmond.xml.sax;public class TestMain {/** * @param args * @throws Exception  */public static void main(String[] args) throws Exception {SaxMapper mapper = new SaxMapper();if(args.length > 0) {mapper.setFileName(args[0]);mapper.parseXML();} else {System.out.println("no file configurated! please configurate it.");}}}