08、分析JPA与持久化实现产品对接的源代码

    EntityManagerFactory factory = Persistence.createEntityManagerFactory("itcast");
    讲解下这个方法内部的一些原理（了解下就OK）

    打开源代码Persistence.java (用DJ Java Decompiler 3.7反编译的代码)

	// Decompiled by DJ v3.7.7.81 Copyright 2004 Atanas Neshkov  Date: 2010-7-12 20:30:06
// Home Page : http://members.fortunecity.com/neshkov/dj.html  - Check often for new version!
// Decompiler options: packimports(3) 
// Source File Name:   Persistence.java

package javax.persistence;

import java.io.*;
import java.net.URL;
import java.util.*;
import java.util.regex.Matcher;
import java.util.regex.Pattern;
import javax.persistence.spi.PersistenceProvider;

// Referenced classes of package javax.persistence:
//            PersistenceException, EntityManagerFactory

public class Persistence
{

    public Persistence()
    {
    }

    public static EntityManagerFactory createEntityManagerFactory(String persistenceUnitName)
    {
        return createEntityManagerFactory(persistenceUnitName, null);
    }

    public static EntityManagerFactory createEntityManagerFactory(String persistenceUnitName, Map properties)
    {
        EntityManagerFactory emf = null;
        if(providers.size() == 0)
            findAllProviders();	//这个方法是寻找类路径下的所有持久化驱动。
        Iterator i$ = providers.iterator();
        do
        {
            if(!i$.hasNext())
                break;
            PersistenceProvider provider = (PersistenceProvider)i$.next();
            emf = provider.createEntityManagerFactory(persistenceUnitName, properties);
        } while(emf == null);
        if(emf == null)
            throw new PersistenceException((new StringBuilder()).append("No Persistence provider for EntityManager named ").append(persistenceUnitName).toString());
        else
            return emf;
    }

    private static void findAllProviders()
    {
        ClassLoader loader;
        Enumeration resources;
        Set names;
        loader = Thread.currentThread().getContextClassLoader();
        resources = loader.getResources((new StringBuilder()).append("META-INF/services/").append(javax/persistence/spi/PersistenceProvider.getName()).toString());
		  //程序会在类路径的这个目录下查找驱动信息所在的文件。这个文件的详细地址如下图所示：
        names = new HashSet();
_L2:
        InputStream is;
        if(!resources.hasMoreElements())
            break; /* Loop/switch isn't completed */
        URL url = (URL)resources.nextElement();
        is = url.openStream();
        names.addAll(providerNamesFromReader(new BufferedReader(new InputStreamReader(is))));
        is.close();
        if(true) goto _L2; else goto _L1
        Exception exception;
        exception;
        is.close();
        throw exception;
_L1:
        Class providerClass;
        for(Iterator i$ = names.iterator(); i$.hasNext(); providers.add((PersistenceProvider)providerClass.newInstance()))
        {
            String s = (String)i$.next();
            providerClass = loader.loadClass(s);
        }

        break MISSING_BLOCK_LABEL_214;
        IOException e;
        e;
        throw new PersistenceException(e);
        e;
        throw new PersistenceException(e);
        e;
        throw new PersistenceException(e);
        e;
        throw new PersistenceException(e);
    }

    private static Set providerNamesFromReader(BufferedReader reader)
        throws IOException
    {
        Set names = new HashSet();
        do
        {
            String line;
            if((line = reader.readLine()) == null)
                break;
            line = line.trim();
            Matcher m = nonCommentPattern.matcher(line);
            if(m.find())
                names.add(m.group().trim());
        } while(true);
        return names;
    }

    public static final String PERSISTENCE_PROVIDER = "javax.persistence.spi.PeristenceProvider";
    protected static final Set providers = new HashSet();
    private static final Pattern nonCommentPattern = Pattern.compile("^([^#]+)");

}

  

这个资源在哪里呢？ 看图：


   

    打开，内容为 org.hibernate.ejb.HibernatePersistence

    程序会在类路径地下寻找到这个文件，并读取这个配置文件里面指定的可持久化驱动。
    Hibernate提供的可持久化驱动就是org.hibernate.ejb.HibernatePersistence这个类，这个类是Hibernate的入口类，类似JDBC里面的驱动类。
    当然，不同的可持久化产品的入口类是不同的。

    调用JPA应用，它能使用Hibernate，是因为有这样一个驱动类，它起到了一个桥梁的作用，过渡到Hibernate的产品上，这就是调用EntityManagerFactory factory = Persistence.createEntityManagerFactory("itcast"); 创建实体管理器方法的一些执行细节。

     factory 是由Hibernate的可持久化驱动类创建出来的，如果观察Hibernate的实现类的话，会发现实际上EntityManagerFactory 是对SessionFactory这个类进行了一层封装。
    包括EntityManager类也是对Session对象进行了一层封装而已。
    只要研究下Hibernate的JPA实现代码就可以观察出来。