tomcat 7 源码分析-3 使用Digester读取xml文件实例化server

接下来tomcat要load了，看下面一些程序片段

public void load() {

        long t1 = System.nanoTime();
        initDirs();
        initNaming();
        Digester digester = createStartDigester();
        ........
       digester.push(this);
       digester.parse(inputSource);
        ........
        getServer().init();

 令人费解的如何实例化server的？digester有何作用？原来digester是apache的common项目，作用是讲XML转成Object。tomcat读取配置文件conf\server.xml，实例化server对象。形同：

        // Configure the actions we will be using
        digester.addObjectCreate("Server",
                                 "org.apache.catalina.core.StandardServer",
                                 "className");
        digester.addSetProperties("Server");
        digester.addSetNext("Server",
                            "setServer",
                            "org.apache.catalina.Server");

 要读懂这些还真费解,这里写了个小例子。

首先xml文件为

<?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>
   <book>
      <author>Author 3</author>
      <title>His Other Book</title>
   </book>
</catalog>

 Book.java

package com.xiao;

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

Catalog.java

package com.xiao;
import java.util.Vector;
public class Catalog {
   private Vector<Book> books;

   public Catalog() {
      books = new Vector<Book>();
   }
   public void addBook( Book rhs ) {
      books.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++ ){
          Book book  = books.elementAt(i);
         buf.append( book.toString()).append( newline );
      }
      return buf.toString();
   }
}

 CreateCatalog.java

package com.xiao;
import java.io.IOException;
import org.apache.tomcat.util.digester.Digester;
import org.xml.sax.SAXException;

public class CreateCatalog {
        protected  Catalog ct;
        
        public void SetCatalog(Catalog ol){
                
                ct = ol;
        }
        
        public Catalog GetCatalog(){
                
                return this.ct;
        }
        public String toString() {
                
                return ct.toString();
                }
        
        public Digester createStartDigester()throws IOException, SAXException
        {
                Digester digester = new Digester();
                digester.setValidating( false );
                //解析XML时，遇到catalog，就实例化一个com.xiao.Catalog对象，并且压栈
                digester.addObjectCreate( "catalog", "com.xiao.Catalog");
                //对catalog，调用栈的次top对象(现在还没有压入,父对象)的SetCatalog函数。
                //passing the element that is on the top of the stack, which must be of type com.xiao.Catalog
                //This is the rule that causes the parent/child relationship to be created.
                digester.addSetNext("catalog", "SetCatalog", "com.xiao.Catalog");
                
                digester.addObjectCreate( "catalog/book","com.xiao.Book");
                
                //对rule，调用当前top object的setAuthor函数，参数个数为1
                digester.addCallMethod("catalog/book/author", "setAuthor",1);
                //对rule，添加第一个参数值
                digester.addCallParam("catalog/book/author", 0);
                digester.addCallMethod("catalog/book/title", "setTitle",1);
                digester.addCallParam("catalog/book/title", 0);
                //此时次top的object就是com.xiao.Catalog，调用它的addBook函数，将com.xiao.Book传入
                digester.addSetNext("catalog/book", "addBook", "com.xiao.Book");
                
                return (digester);
        }
}

 TestDigester.java

package com.xiao;

import org.apache.tomcat.util.digester.Digester;
import org.xml.sax.InputSource;
import org.xml.sax.SAXException;

import java.io.*;
public class TestDigester {
        
        public static void main(String[] args) throws IOException, SAXException  {
                // TODO Auto-generated method stub
                CreateCatalog cc = new CreateCatalog();
                Digester digester = cc.createStartDigester();   
                String configFile = "xiapingtest/ts.xml";       
                InputSource inputSource = null;
                InputStream inputStream = null;
                File file = new File(System.getProperty("user.dir"),configFile);
                inputSource = new InputSource("file://" + file.getAbsolutePath());
                inputStream = new FileInputStream(file);
                inputSource.setByteStream(inputStream);
                //在加入CreateCatalog对象，这个是第一个压入的对象
                digester.push(cc);
                //处理xml文件，逐个加入对象
                digester.parse(inputSource);            
                System.out.println(cc.toString());
        }

}

 部分解释以及在代码注释里。