Tomcat中xml的解析器Digester 第一篇（共三篇）

Tomcat中server.xml定义了进行服务器的配置， 如JNDI, 最大线程数, 监听端口等。 Tomcat在启动的时候就利用server.xml中的配置初始化Service,Server,Engine,Host。

Tomcat利用的是Digester， Digester其实原来是struts的一个模块， 后来发现这个xml转Java    Object费用有实用性， 于是成为apache的一个common模块。 Tomcat取了Digester中的接口和几个普遍性Rule,并且自己实现了一些Rule.


本文先分析如何使用SAX进行XML转Java， 然后再利用Digester进行XML解析，最后分析Tomcat中的Rule使用。

文章代码比较多， 分三篇。
第一篇分析SAX进行XML转Java； 第二篇利用Digester进行XML解析； 第三篇析Tomcat中的Rule使用。

数据绑定概念：
编组（ Marshalling）是把内存中的数据转化到存储媒介上的过程。因此在 Java 和 XML 环境中，编组就是把一些 Java 对象转化成一个（或多个） XML 文档。在数据库环境中，则是把 Java 表示的数据存入数据库。显然，编组的秘密在于把 Java 实例中的面向对象结构转化成适用于 XML 的 扁平结构，或者 RDBMS 中的关系结构（使用 Java 技术转换到 OODBMS 实际上很简单）。

解组（ Unmarshalling）是把数据从存储媒介转换到内存中的过程--正好与编组相反。因此需要把 XML 文档解组到 Java VM 中。这里的复杂性不是在扁平数据中，因为这不是必需的，而在于从正确的数据到正确的 Java 代码变量的映射。如果映射是错误的，就不可能正确地访问数据。当然，如果再尝试重新编组还会造成更大的问题，并且问题传播得很快。

1. Sax Unmarshaller

SAX（the Simple API for XML）是DOM文档驱动的解析器， 它顺序读取文档并且触发相应的Callback事件。Callback事件有元素开始事件，元素结束事件等。接口org.xml.sax.ContentHandler定义了每个SAX事件， 开发人员有时候不关心所有的事件，可以使用默认的DefaultHandler。

接下来分析如何利用SAX把XML转成Java Object。
为了简单起见， 不定义DTD文件。

例子定义了目录，书本， 杂志和文章类。

首先定义catalog.xml文件

 
<?xml version="1.0"?>

<catalog library="somewhere">

   <book>
      <author>Author 1</author>
      <title>Title 1</title>
   </book>
   
   <book>
      <author>Author 2</author>
      <title>His One Book</title>
   </book>
   
   <magazine>
      <name>Mag Title 1</name>
      <article page="5">
         <headline>Some Headline</headline>
      </article>
      <article page="9">
         <headline>Another Headline</headline>
      </article>
   </magazine>

   <book>
      <author>Author 2</author>
      <title>His Other Book</title>
   </book>

   <magazine>
      <name>Mag Title 2</name>

      <article page="17">
         <headline>Second Headline</headline>
      </article>
   </magazine>

</catalog>

Catalog.java

package benewu.gmail.study.tomcat.digester;

import java.util.Vector;

public class Catalog {
   private Vector books;
   private Vector magazines;

   public Catalog() {
      books = new Vector();
      magazines = new Vector();
   }

   public void addBook( Book rhs ) {
      books.addElement( rhs );
   }
   public void addMagazine( Magazine rhs ) {
      magazines.addElement( rhs );
   }

   public String toString() {
      String newline = System.getProperty( "line.separator" );
      StringBuffer buf = new StringBuffer();

      buf.append( "--- Books ---" ).append( newline );
      for( int i=0; i<books.size(); i++ ){
         buf.append( books.elementAt(i) ).append( newline );
      }

      buf.append( "--- Magazines ---" ).append( newline );
      for( int i=0; i<magazines.size(); i++ ){
         buf.append( magazines.elementAt(i) ).append( newline );
      }

      return buf.toString();
   }
}

Book.java

package benewu.gmail.study.tomcat.digester;

public class Book {
	   private String author;
	   private String title;

	   public Book() {}

	   public void setAuthor( String rhs ) { author = rhs; }
	   public void setTitle(  String rhs ) { title  = rhs; }

	   public String toString() {
	      return "Book: Author='" + author + "' Title='" + title + "'";
	   }
	}

Magazine.java

package benewu.gmail.study.tomcat.digester;

import java.util.Vector;

public class Magazine {
   private String name;
   private Vector articles;

   public Magazine() {
      articles = new Vector();
   }

   public void setName( String rhs ) { name = rhs; }

   public void addArticle( Article a ) {
      articles.addElement( a );
   }

   public String toString() {
      StringBuffer buf = new StringBuffer( "Magazine: Name='" + name + "' ");
      for( int i=0; i<articles.size(); i++ ){
         buf.append( articles.elementAt(i).toString() );
      }
      return buf.toString();
   }
}

Article.java

package benewu.gmail.study.tomcat.digester;

public class Article {
	   private String headline;
	   private String page;

	   public Article() {}

	   public void setHeadline( String rhs ) { headline = rhs; }
	   public void setPage(     String rhs ) { page     = rhs; }

	   public String toString() {
	      return "Article: Headline='" + headline + "' on page='" + page + "' ";
	   }
	}

定义好了类和对应的xml配置， 开始利用catalog.xml进行初始化Catalog吧。

SaxCatalogUnmarshaller继承DefaultHandler， 重写关键的两个分方法：startElement和endElement

SaxCatalogUnmarshaller,java

package benewu.gmail.study.tomcat.digester.unmarshalling;

import java.util.Stack;

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

import benewu.gmail.study.tomcat.digester.Article;
import benewu.gmail.study.tomcat.digester.Book;
import benewu.gmail.study.tomcat.digester.Catalog;
import benewu.gmail.study.tomcat.digester.Magazine;
 
class SaxCatalogUnmarshaller extends DefaultHandler {
    private Catalog catalog;

    private Stack stack;
    private boolean isStackReadyForText;

    private Locator locator;

    // ----- 

    public SaxCatalogUnmarshaller() {
	stack = new Stack();
	isStackReadyForText = false;
    }

    public Catalog getCatalog() { return catalog; }

    // ----- callbacks: -----

    public void setDocumentLocator( Locator rhs ) { locator = rhs; }

    // ----- 

    /**
     * the [namespace URI], the local name, and the fully [qualified name] of the element. 
     */
    public void startElement( String uri, String localName, String qName,
			      Attributes attribs ) {

	isStackReadyForText = false;

	// if next element is complex, push a new instance on the stack
	// if element has attributes, set them in the new instance
	if( localName.equals( "catalog" ) ) {
	    stack.push( new Catalog() );

	}else if( localName.equals( "book" ) ) {
	    stack.push( new Book() );

	}else if( localName.equals( "magazine" ) ) {
	    stack.push( new Magazine() );

	}else if( localName.equals( "article" ) ) {
	    stack.push( new Article() );
 
	    String tmp = resolveAttrib( uri, "page", attribs, "unknown" );
	    ((Article)stack.peek()).setPage( tmp );
	}
	// if next element is simple, push StringBuffer 
	// this makes the stack ready to accept character text
	else if( localName.equals( "title" ) || localName.equals( "author" ) ||
		 localName.equals( "name"  ) || localName.equals( "headline" ) ) {
	    stack.push( new StringBuffer() );
	    isStackReadyForText = true;
	}
	// if none of the above, it is an unexpected element		 
	else{
	    // do nothing
	}		 
    }

    // ----- 

 
    public void endElement( String uri, String localName, String qName ) {

	// recognized text is always content of an element
	// when the element closes, no more text should be expected
	isStackReadyForText = false;

	// pop stack and add to 'parent' element, which is next on the stack
	// important to pop stack first, then peek at top element!
	Object tmp = stack.pop();
	
	if( localName.equals( "catalog" ) ) {
	    catalog = (Catalog)tmp;
	
	}else if( localName.equals( "book" ) ) {
	    ((Catalog)stack.peek()).addBook( (Book)tmp );

	}else if( localName.equals( "magazine" ) ) {
	    ((Catalog)stack.peek()).addMagazine( (Magazine)tmp );
	    
	}else if( localName.equals( "article" ) ) {
	    ((Magazine)stack.peek()).addArticle( (Article)tmp );
	}
	// for simple elements, pop StringBuffer and convert to String
	else if( localName.equals( "title" ) ) {
	    ((Book)stack.peek()).setTitle( tmp.toString() );

	}else if( localName.equals( "author" ) ) {
	    ((Book)stack.peek()).setAuthor( tmp.toString() );

	}else if( localName.equals( "name" ) ) {
	    ((Magazine)stack.peek()).setName( tmp.toString() );

	}else if( localName.equals( "headline" ) ) {
	    ((Article)stack.peek()).setHeadline( tmp.toString() );
	}
	// if none of the above, it is an unexpected element:
	// necessary to push popped element back!
	else{
	    stack.push( tmp );
	}
    }

    // -----
    
    /*
     *  is called when the parser encounters raw text. 
     */
    public void characters( char[] data, int start, int length ) {

	// if stack is not ready, data is not content of recognized element
	if( isStackReadyForText == true ) {
	    ((StringBuffer)stack.peek()).append( data, start, length );
	}else{
	    // read data which is not part of recognized element
	}
    }
    
    // -----
    
    private String resolveAttrib( String uri, String localName, 
			          Attributes attribs, String defaultValue ) {
	
	String tmp = attribs.getValue( uri, localName );
	return (tmp!=null)?(tmp):(defaultValue);
    }
}

当遇到xml中元素开始标识"<", 会触发startElement， 遇到"/>"触发endElement.

图一分析了SAX解析XML，转换成Java Object的一个实例。

客户端使用将非常简单：

Driver.java

package benewu.gmail.study.tomcat.digester.unmarshalling;

import java.io.File;
import java.io.FileInputStream;
import java.net.URL;

import org.xml.sax.InputSource;
import org.xml.sax.XMLReader;
import org.xml.sax.helpers.XMLReaderFactory;

import benewu.gmail.study.tomcat.digester.Catalog;
import benewu.gmail.study.tomcat.digester.DigesterDriver;

public class Driver {
    
    public static void main( String[] args ) {
	Catalog catalog = null;

	try {
		
		URL fileURL = DigesterDriver.class.getResource("catalog.xml");
	    File file = new File(fileURL.getFile());
	    InputSource saxsrc = new InputSource( new FileInputStream( file ) );
	
	    
		System.out.println( "--- SAX ---" );
		SaxCatalogUnmarshaller saxUms = new SaxCatalogUnmarshaller();

		XMLReader rdr = XMLReaderFactory.
		    createXMLReader( "org.apache.xerces.parsers.SAXParser" );
		rdr.setContentHandler( saxUms );
		rdr.parse( saxsrc );

		catalog = saxUms.getCatalog();
		System.out.println( catalog.toString() );
	}catch( Exception exc ) {
		exc.printStackTrace();
	    System.out.println( "Usage: SAX|DOM filename" );
	    System.err.println( "Exception: " + exc );
	}
    }
}

文章利用代码和图分析了SAX将XML转成Java Object。
这样可以使得客户端在XML中定义类的属性， 但是不能改变类。
Digester不但可以使客户在XML中定义类的属性， 可以改变不同类， 并且封装了SAX解析， 定义了各种Rule,利用策略模式进行组装类。

参考：

1 理解 SAX
http://www.ibm.com/developerworks/cn/views/xml/tutorials.jsp?cv_doc_id=84979

2 Java与XML联合编程之SAX篇
http://www0.ccidnet.com/tech/guide/2001/10/08/58_3392.html

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