DOM解析XML（一）

1、什么是DOM

DOM-Document Object Model. 简称：文档对象模型。我听到这个词还是不知道什么意思。它其实主要用来解析XML与HTML文档。它的解析方式是一开始会加载整个XML文档，然后对这些文档进行树形结构的搭建。在实际工作中，它适合针对小而且频繁使用的XML文档。

2、新建Book.xml

<?xml version="1.0" encoding="UTF-8"?> 
<books> 
    <book id="12"> 
        <name>thinking in java</name> 
        <price>85.5</price> 
    </book> 
    <book id="15"> 
        <name>Spring in Action</name> 
        <price>39.0</price> 
    </book> 
</books>

3、编写简单测试例子

import java.io.InputStream; 
import java.util.ArrayList; 
import java.util.List; 
 
import javax.xml.parsers.DocumentBuilder; 
import javax.xml.parsers.DocumentBuilderFactory; 
 
import org.hibernate.util.ConfigHelper;
import org.w3c.dom.Document; 
import org.w3c.dom.Element; 
import org.w3c.dom.NodeList; 
import org.w3c.dom.Node; 
 
public class DomParseService { 
    public static List<Book> getBooks(InputStream inputStream) throws Exception{ 
        List<Book> list = new ArrayList<Book>(); 
        DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance(); 
        DocumentBuilder builder = factory.newDocumentBuilder(); 
        //传入参数inputstream，解析指定的xml，解析完成之后会返回一个Document（这里我们可以理解成一棵树）
        Document document = builder.parse(inputStream);
        //或者这棵树上的元素（种类很多）
        Element element = document.getDocumentElement(); 
 
        NodeList bookNodes = element.getElementsByTagName("book"); 
        for(int i=0;i<bookNodes.getLength();i++){ 
            Element bookElement = (Element) bookNodes.item(i); 
            Book book = new Book(); 
            book.setId(Integer.parseInt(bookElement.getAttribute("id"))); 
            NodeList childNodes = bookElement.getChildNodes(); 
//          System.out.println("*****"+childNodes.getLength()); 
            for(int j=0;j<childNodes.getLength();j++){ 
                if(childNodes.item(j).getNodeType()==Node.ELEMENT_NODE){ 
                    if("name".equals(childNodes.item(j).getNodeName())){ 
                        book.setName(childNodes.item(j).getFirstChild().getNodeValue()); 
                        System.out.println("nodeValue = " + childNodes.item(j).getFirstChild().getNodeValue());
                    }else if("price".equals(childNodes.item(j).getNodeName())){ 
                        book.setPrice(Float.parseFloat(childNodes.item(j).getFirstChild().getNodeValue()));
                        System.out.println("nodeValue = " + Float.parseFloat(childNodes.item(j).getFirstChild().getNodeValue()));
                    } 
                } 
            }//end for j 
            list.add(book); 
        }//end for i 
        return list; 
    }
    
    public static void main(String[] args) {
    	InputStream inputStream = ConfigHelper.getResourceAsStream("/book.xml");
    	try {
			getBooks(inputStream);
		} catch (Exception e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
	}
} 

4、首先看看如何获得DocumentBuilderFactory对象，这里面用到了类加载的知识，有人说这是工作模式，实话说我没太明白啥是工厂模式，不多说，解读源码：

/**
     * Obtain a new instance of a
     * <code>DocumentBuilderFactory</code>. This static method creates
     * a new factory instance.
     * This method uses the following ordered lookup procedure to determine
     * the <code>DocumentBuilderFactory</code> implementation class to
     * load:
     * <ul>
     * <li>
     * Use the <code>javax.xml.parsers.DocumentBuilderFactory</code> system
     * property.
     * </li>
     * <li>
     * Use the properties file "lib/jaxp.properties" in the JRE directory.
     * This configuration file is in standard <code>java.util.Properties
     * </code> format and contains the fully qualified name of the
     * implementation class with the key being the system property defined
     * above.
     *
     * The jaxp.properties file is read only once by the JAXP implementation
     * and it's values are then cached for future use.  If the file does not exist
     * when the first attempt is made to read from it, no further attempts are
     * made to check for its existence.  It is not possible to change the value
     * of any property in jaxp.properties after it has been read for the first time.
     * </li>
     * <li>
     * Use the Services API (as detailed in the JAR specification), if
     * available, to determine the classname. The Services API will look
     * for a classname in the file
     * <code>META-INF/services/javax.xml.parsers.DocumentBuilderFactory</code>
     * in jars available to the runtime.
     * </li>
     * <li>
     * Platform default <code>DocumentBuilderFactory</code> instance.
     * </li>
     * </ul>
     *
     * Once an application has obtained a reference to a
     * <code>DocumentBuilderFactory</code> it can use the factory to
     * configure and obtain parser instances.
     *
     *
     * <h2>Tip for Trouble-shooting</h2>
     * <p>Setting the <code>jaxp.debug</code> system property will cause
     * this method to print a lot of debug messages
     * to <code>System.err</code> about what it is doing and where it is looking at.</p>
     *
     * <p> If you have problems loading {@link DocumentBuilder}s, try:</p>
     * <pre>
     * java -Djaxp.debug=1 YourProgram ....
     * </pre>
     *
     * @return New instance of a <code>DocumentBuilderFactory</code>
     *
     * @throws FactoryConfigurationError if the implementation is not
     *   available or cannot be instantiated.
     */
    public static DocumentBuilderFactory newInstance() {
        try {
            return (DocumentBuilderFactory) FactoryFinder.find(
                /* The default property name according to the JAXP spec */
                "javax.xml.parsers.DocumentBuilderFactory",
                /* The fallback implementation class name */
                "com.sun.org.apache.xerces.internal.jaxp.DocumentBuilderFactoryImpl");
        } catch (FactoryFinder.ConfigurationError e) {
            throw new FactoryConfigurationError(e.getException(),
                                                e.getMessage());
        }

    }

   /**
     * Finds the implementation Class object in the specified order.  Main
     * entry point.
     * @return Class object of factory, never null
     *
     * @param factoryId             Name of the factory to find, same as
     *                              a property name
     * @param fallbackClassName     Implementation class name, if nothing else
     *                              is found.  Use null to mean no fallback.
     *
     * Package private so this code can be shared.
     */
    static Object find(String factoryId, String fallbackClassName)
        throws ConfigurationError
    {
        dPrint("find factoryId =" + factoryId);

        // Use the system property first
        try {
            String systemProp = ss.getSystemProperty(factoryId);
            if (systemProp != null) {
                dPrint("found system property, value=" + systemProp);
                return newInstance(systemProp, null, true);
            }
        }
        catch (SecurityException se) {
            if (debug) se.printStackTrace();
        }

        // try to read from $java.home/lib/jaxp.properties
        try {
            String factoryClassName = null;
            if (firstTime) {
                synchronized (cacheProps) {
                    if (firstTime) {
                        String configFile = ss.getSystemProperty("java.home") + File.separator +
                            "lib" + File.separator + "jaxp.properties";
                        File f = new File(configFile);
                        firstTime = false;
                        if (ss.doesFileExist(f)) {
                            dPrint("Read properties file "+f);
                            cacheProps.load(ss.getFileInputStream(f));
                        }
                    }
                }
            }
            factoryClassName = cacheProps.getProperty(factoryId);

            if (factoryClassName != null) {
                dPrint("found in $java.home/jaxp.properties, value=" + factoryClassName);
                return newInstance(factoryClassName, null, true);
            }
        }
        catch (Exception ex) {
            if (debug) ex.printStackTrace();
        }

        // Try Jar Service Provider Mechanism
        Object provider = findJarServiceProvider(factoryId);
        if (provider != null) {
            return provider;
        }
        if (fallbackClassName == null) {
            throw new ConfigurationError(
                "Provider for " + factoryId + " cannot be found", null);
        }

        dPrint("loaded from fallback value: " + fallbackClassName);
        return newInstance(fallbackClassName, null, true);
    }

我之所以把注释都放上来，是因为我每次看完源码之后再去看这些注释，总有一种原来是这样的感觉，羡慕英文好的同学，我想他们在看源码的时候速度一般会快很多，上面真正有效执行的是最后一句：

    /**
     * Create an instance of a class. Delegates to method
     * <code>getProviderClass()</code> in order to load the class.
     *
     * @param className Name of the concrete class corresponding to the
     * service provider
     *
     * @param cl <code>ClassLoader</code> used to load the factory class. If <code>null</code>
     * current <code>Thread</code>'s context classLoader is used to load the factory class.
     *
     * @param doFallback True if the current ClassLoader should be tried as
     * a fallback if the class is not found using cl
     */
    static Object newInstance(String className, ClassLoader cl, boolean doFallback)
        throws ConfigurationError
    {
        return newInstance(className, cl, doFallback, false);
    }

    /**
     * Create an instance of a class. Delegates to method
     * <code>getProviderClass()</code> in order to load the class.
     *
     * @param className Name of the concrete class corresponding to the
     * service provider
     *
     * @param cl <code>ClassLoader</code> used to load the factory class. If <code>null</code>
     * current <code>Thread</code>'s context classLoader is used to load the factory class.
     *
     * @param doFallback True if the current ClassLoader should be tried as
     * a fallback if the class is not found using cl
     *
     * @param useBSClsLoader True if cl=null actually meant bootstrap classLoader. This parameter
     * is needed since DocumentBuilderFactory/SAXParserFactory defined null as context classLoader.
     */
    static Object newInstance(String className, ClassLoader cl, boolean doFallback, boolean useBSClsLoader)
        throws ConfigurationError
    {
        try {
            Class providerClass = getProviderClass(className, cl, doFallback, useBSClsLoader);
            Object instance = providerClass.newInstance();
            if (debug) {    // Extra check to avoid computing cl strings
                dPrint("created new instance of " + providerClass +
                       " using ClassLoader: " + cl);
            }
            return instance;
        }
        catch (ClassNotFoundException x) {
            throw new ConfigurationError(
                "Provider " + className + " not found", x);
        }
        catch (Exception x) {
            throw new ConfigurationError(
                "Provider " + className + " could not be instantiated: " + x,
                x);
        }
    }

    /**
     * Attempt to load a class using the class loader supplied. If that fails
     * and fall back is enabled, the current (i.e. bootstrap) class loader is
     * tried.
     *
     * If the class loader supplied is <code>null</code>, first try using the
     * context class loader followed by the current (i.e. bootstrap) class
     * loader.
     *
     * Use bootstrap classLoader if cl = null and useBSClsLoader is true
     */
    static private Class getProviderClass(String className, ClassLoader cl,
            boolean doFallback, boolean useBSClsLoader) throws ClassNotFoundException
    {
        try {
            if (cl == null) {
                if (useBSClsLoader) {
                    return Class.forName(className, true, FactoryFinder.class.getClassLoader());
                } else {
                    cl = ss.getContextClassLoader();
                    if (cl == null) {
                        throw new ClassNotFoundException();
                    }
                    else {
                        return cl.loadClass(className);
                    }
                }
            }
            else {
                return cl.loadClass(className);
            }
        }
        catch (ClassNotFoundException e1) {
            if (doFallback) {
                // Use current class loader - should always be bootstrap CL
                return Class.forName(className, true, FactoryFinder.class.getClassLoader());
            }
            else {
                throw e1;
            }
        }
    }

我们好好看看这个方法，ClassLoader是通过获取上下文Loader而来，执行完cl = ss.getContextClassLoader();之后返回的对象为：sun.misc.Launcher$AppClassLoaderw 

我在学习深入探讨java类加载器的时候，知道AppClassLoaderw 属于类加载器加载而来，在他的上面有扩展类加载器，扩展类加载器上面有引导类加载器

由于我们传入的className为：com.sun.org.apache.xerces.internal.jaxp.DocumentBuilderFactoryImpl，所以最终返回的对象为：DocumentBuilderFactoryImpl

我们接着把类具体加载看完：

/**
     * Loads the class with the specified <a href="#name">binary name</a>.
     * This method searches for classes in the same manner as the {@link
     * #loadClass(String, boolean)} method.  It is invoked by the Java virtual
     * machine to resolve class references.  Invoking this method is equivalent
     * to invoking {@link #loadClass(String, boolean) <tt>loadClass(name,
     * false)</tt>}.  </p>
     *
     * @param  name
     *         The <a href="#name">binary name</a> of the class
     *
     * @return  The resulting <tt>Class</tt> object
     *
     * @throws  ClassNotFoundException
     *          If the class was not found
     */
    public Class<?> loadClass(String name) throws ClassNotFoundException {
        return loadClass(name, false);
    }

    /**
     * Loads the class with the specified <a href="#name">binary name</a>.  The
     * default implementation of this method searches for classes in the
     * following order:
     *
     * <p><ol>
     *
     *   <li><p> Invoke {@link #findLoadedClass(String)} to check if the class
     *   has already been loaded.  </p></li>
     *
     *   <li><p> Invoke the {@link #loadClass(String) <tt>loadClass</tt>} method
     *   on the parent class loader.  If the parent is <tt>null</tt> the class
     *   loader built-in to the virtual machine is used, instead.  </p></li>
     *
     *   <li><p> Invoke the {@link #findClass(String)} method to find the
     *   class.  </p></li>
     *
     * </ol>
     *
     * <p> If the class was found using the above steps, and the
     * <tt>resolve</tt> flag is true, this method will then invoke the {@link
     * #resolveClass(Class)} method on the resulting <tt>Class</tt> object.
     *
     * <p> Subclasses of <tt>ClassLoader</tt> are encouraged to override {@link
     * #findClass(String)}, rather than this method.  </p>
     *
     * <p> Unless overridden, this method synchronizes on the result of
     * {@link #getClassLoadingLock <tt>getClassLoadingLock</tt>} method
     * during the entire class loading process.
     *
     * @param  name
     *         The <a href="#name">binary name</a> of the class
     *
     * @param  resolve
     *         If <tt>true</tt> then resolve the class
     *
     * @return  The resulting <tt>Class</tt> object
     *
     * @throws  ClassNotFoundException
     *          If the class could not be found
     */
    protected Class<?> loadClass(String name, boolean resolve)
        throws ClassNotFoundException
    {
        synchronized (getClassLoadingLock(name)) {
            // First, check if the class has already been loaded
            Class c = findLoadedClass(name);
            if (c == null) {
                long t0 = System.nanoTime();
                try {
                    if (parent != null) {
                        c = parent.loadClass(name, false);
                    } else {
                        c = findBootstrapClassOrNull(name);
                    }
                } catch (ClassNotFoundException e) {
                    // ClassNotFoundException thrown if class not found
                    // from the non-null parent class loader
                }

                if (c == null) {
                    // If still not found, then invoke findClass in order
                    // to find the class.
                    long t1 = System.nanoTime();
                    c = findClass(name);

                    // this is the defining class loader; record the stats
                    sun.misc.PerfCounter.getParentDelegationTime().addTime(t1 - t0);
                    sun.misc.PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);
                    sun.misc.PerfCounter.getFindClasses().increment();
                }
            }
            if (resolve) {
                resolveClass(c);
            }
            return c;
        }
    }

 /**
     * Returns the class with the given <a href="#name">binary name</a> if this
     * loader has been recorded by the Java virtual machine as an initiating
     * loader of a class with that <a href="#name">binary name</a>.  Otherwise
     * <tt>null</tt> is returned.  </p>
     *
     * @param  name
     *         The <a href="#name">binary name</a> of the class
     *
     * @return  The <tt>Class</tt> object, or <tt>null</tt> if the class has
     *          not been loaded
     *
     * @since  1.1
     */
    protected final Class<?> findLoadedClass(String name) {
        if (!checkName(name))
            return null;
        return findLoadedClass0(name);
    }

    private native final Class findLoadedClass0(String name);

类具体如何加载这里由虚拟机和底层操作系统具体去做