SAX解析XML的关键类

DefaultHandler 处理xml文件用到的一个解析类。

是一个事件处理器,可以接收解析器报告的所有事件,处理所发现的数据。它实现了EntityResolver接口、DTDHandler接口、ErrorHandler接口和ContentHandler接口。这几个接口代表不同类型的事件处理器。我们着重介绍一下ContentHandler接口, 这个借口中逐个的解析了每个xml文件的关键字,从中取值。

其中关键的方法如图中红线所示:

SAX解析XML的关键类_第1张图片

DefaultHandler实现了这些方法,但在方法体内没有做任何事情,因此我们在使用时必须覆写相关的方法。最重要的是startElement方法、characters方法和endElement方法。当执行文档时遇到起始节点,startElement方法将会被调用,我们可以获取起始节点相关信息;然后characters方法被调用,我们可以获取节点内的文本信息;最后endElement方法被调用,我们可以做收尾的相关操作。

最后,我们需要调用SAX解析程序,这个步骤在MainActivity中完成.

code实例:

package com.scott.xml.parser;

import java.io.InputStream;
import java.io.StringWriter;
import java.util.ArrayList;
import java.util.List;

import javax.xml.parsers.SAXParser;
import javax.xml.parsers.SAXParserFactory;
import javax.xml.transform.OutputKeys;
import javax.xml.transform.Result;
import javax.xml.transform.Transformer;
import javax.xml.transform.TransformerFactory;
import javax.xml.transform.sax.SAXTransformerFactory;
import javax.xml.transform.sax.TransformerHandler;
import javax.xml.transform.stream.StreamResult;

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

import com.scott.xml.model.Book;

public class SaxBookParser implements BookParser {
	
	@Override
	public List<Book> parse(InputStream is) throws Exception {
		SAXParserFactory factory = SAXParserFactory.newInstance();	//取得SAXParserFactory实例
		SAXParser parser = factory.newSAXParser();					//从factory获取SAXParser实例
		MyHandler handler = new MyHandler();						//实例化自定义Handler
		parser.parse(is, handler);									//根据自定义Handler规则解析输入流
		return handler.getBooks();
	}

	@Override
	public String serialize(List<Book> books) throws Exception {
		SAXTransformerFactory factory = (SAXTransformerFactory) TransformerFactory.newInstance();//取得SAXTransformerFactory实例
		TransformerHandler handler = factory.newTransformerHandler();			//从factory获取TransformerHandler实例
		Transformer transformer = handler.getTransformer();						//从handler获取Transformer实例
		transformer.setOutputProperty(OutputKeys.ENCODING, "UTF-8");			// 设置输出采用的编码方式
		transformer.setOutputProperty(OutputKeys.INDENT, "yes");				// 是否自动添加额外的空白
		transformer.setOutputProperty(OutputKeys.OMIT_XML_DECLARATION, "no");	// 是否忽略XML声明
		
		StringWriter writer = new StringWriter();
		Result result = new StreamResult(writer);
	    handler.setResult(result);
	    
	    String uri = "";	//代表命名空间的URI 当URI无值时 须置为空字符串
	    String localName = "";	//命名空间的本地名称(不包含前缀) 当没有进行命名空间处理时 须置为空字符串
	    
	    handler.startDocument();
	    handler.startElement(uri, localName, "books", null);
	    
	    AttributesImpl attrs = new AttributesImpl();	//负责存放元素的属性信息
	    char[] ch = null;
	    for (Book book : books) {
	    	attrs.clear();	//清空属性列表
		    attrs.addAttribute(uri, localName, "id", "string", String.valueOf(book.getId()));//添加一个名为id的属性(type影响不大,这里设为string)
	    	handler.startElement(uri, localName, "book", attrs);	//开始一个book元素 关联上面设定的id属性
	    	
	    	handler.startElement(uri, localName, "name", null);	//开始一个name元素 没有属性
	    	ch = String.valueOf(book.getName()).toCharArray();
	    	handler.characters(ch, 0, ch.length);	//设置name元素的文本节点
	    	handler.endElement(uri, localName, "name");
	    	
	    	handler.startElement(uri, localName, "price", null);//开始一个price元素 没有属性
	    	ch = String.valueOf(book.getPrice()).toCharArray();
	    	handler.characters(ch, 0, ch.length);	//设置price元素的文本节点
	    	handler.endElement(uri, localName, "price");
	    	
	    	handler.endElement(uri, localName, "book");
	    }
	    handler.endElement(uri, localName, "books");
	    handler.endDocument();
	    
		return writer.toString();
	}
	
	//需要重写DefaultHandler的方法
	private class MyHandler extends DefaultHandler {

		private List<Book> books;
		private Book book;
		private StringBuilder builder;
		
		//返回解析后得到的Book对象集合
		public List<Book> getBooks() {
			return books;
		}
		
		@Override
		public void startDocument() throws SAXException {
			super.startDocument();
			books = new ArrayList<Book>();
			builder = new StringBuilder();
		}

		@Override
		public void startElement(String uri, String localName, String qName, Attributes attributes) throws SAXException {
			super.startElement(uri, localName, qName, attributes);
			if (localName.equals("book")) {
				book = new Book();
			}
			builder.setLength(0);	//将字符长度设置为0 以便重新开始读取元素内的字符节点
		}
		
		@Override
		public void characters(char[] ch, int start, int length) throws SAXException {
			super.characters(ch, start, length);
			builder.append(ch, start, length);	//将读取的字符数组追加到builder中
		}
		
		@Override
		public void endElement(String uri, String localName, String qName) throws SAXException {
			super.endElement(uri, localName, qName);
			if (localName.equals("id")) {
				book.setId(Integer.parseInt(builder.toString()));
			} else if (localName.equals("name")) {
				book.setName(builder.toString());
			} else if (localName.equals("price")) {
				book.setPrice(Float.parseFloat(builder.toString()));
			} else if (localName.equals("book")) {
				books.add(book);
			}
		}
	}
}

调用XML文件解析程序在界面中完成。

package com.scott.xml;

import java.io.FileOutputStream;
import java.io.InputStream;
import java.util.List;

import android.app.Activity;
import android.content.Context;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;

import com.scott.xml.model.Book;
import com.scott.xml.parser.BookParser;
import com.scott.xml.parser.SaxBookParser;

public class MainActivity extends Activity {
	
	private static final String TAG = "XML";
	
	private BookParser parser;
	private List<Book> books;
	
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        
        Button readBtn = (Button) findViewById(R.id.readBtn);
        Button writeBtn = (Button) findViewById(R.id.writeBtn);
        
        readBtn.setOnClickListener(new View.OnClickListener() {
			@Override
			public void onClick(View v) {
				try {
					InputStream is = getAssets().open("books.xml");
		        	parser = new SaxBookParser();  //创建SaxBookParser实例
		        	books = parser.parse(is);  //解析输入流
					for (Book book : books) {
						Log.i(TAG, book.toString());
					}
				} catch (Exception e) {
					Log.e(TAG, e.getMessage());
				}
			}
		});
        writeBtn.setOnClickListener(new View.OnClickListener() {
			@Override
			public void onClick(View v) {
				try {
					String xml = parser.serialize(books);  //序列化
					FileOutputStream fos = openFileOutput("books.xml", Context.MODE_PRIVATE);
					fos.write(xml.getBytes("UTF-8"));
				} catch (Exception e) {
					Log.e(TAG, e.getMessage());
				}
			}
		});
    }
}



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