JPA基础（八）：分析JPA与持久化实现产品对接的源代码

使用jadnt158工具将ejb3-persistence.jar\javax\persistence.class进行反编译得到源码：

  1 // Decompiled by Jad v1.5.8g. Copyright 2001 Pavel Kouznetsov.
  2 // Jad home page: http://www.kpdus.com/jad.html
  3 // Decompiler options: packimports(3) 
  4 // Source File Name:   Persistence.java
  5 
  6 package javax.persistence;
  7 
  8 import java.io.*;
  9 import java.net.URL;
 10 import java.util.*;
 11 import java.util.regex.Matcher;
 12 import java.util.regex.Pattern;
 13 import javax.persistence.spi.PersistenceProvider;
 14 
 15 // Referenced classes of package javax.persistence:
 16 //            PersistenceException, EntityManagerFactory
 17 
 18 public class Persistence
 19 {
 20 
 21     public Persistence()
 22     {
 23     }
 24 
 25     public static EntityManagerFactory createEntityManagerFactory(String persistenceUnitName)
 26     {
 27         return createEntityManagerFactory(persistenceUnitName, null);
 28     }
 29 
 30     public static EntityManagerFactory createEntityManagerFactory(String persistenceUnitName, Map properties)
 31     {
 32         EntityManagerFactory emf = null;
 33         if(providers.size() == 0)
 34             findAllProviders();
 35         Iterator i$ = providers.iterator();
 36         do
 37         {
 38             if(!i$.hasNext())
 39                 break;
 40             PersistenceProvider provider = (PersistenceProvider)i$.next();
 41             emf = provider.createEntityManagerFactory(persistenceUnitName, properties);
 42         } while(emf == null);
 43         if(emf == null)
 44             throw new PersistenceException((new StringBuilder()).append("No Persistence provider for EntityManager named ").append(persistenceUnitName).toString());
 45         else
 46             return emf;
 47     }
 48 
 49     private static void findAllProviders()
 50     {
 51         ClassLoader loader;
 52         Enumeration resources;
 53         Set names;
 54         loader = Thread.currentThread().getContextClassLoader();
 55         resources = loader.getResources((new StringBuilder()).append("META-INF/services/").append(javax/persistence/spi/PersistenceProvider.getName()).toString());
 56         names = new HashSet();
 57 _L2:
 58         InputStream is;
 59         if(!resources.hasMoreElements())
 60             break; /* Loop/switch isn't completed */
 61         URL url = (URL)resources.nextElement();
 62         is = url.openStream();
 63         names.addAll(providerNamesFromReader(new BufferedReader(new InputStreamReader(is))));
 64         is.close();
 65         if(true) goto _L2; else goto _L1
 66         Exception exception;
 67         exception;
 68         is.close();
 69         throw exception;
 70 _L1:
 71         Class providerClass;
 72         for(Iterator i$ = names.iterator(); i$.hasNext(); providers.add((PersistenceProvider)providerClass.newInstance()))
 73         {
 74             String s = (String)i$.next();
 75             providerClass = loader.loadClass(s);
 76         }
 77 
 78         break MISSING_BLOCK_LABEL_214;
 79         IOException e;
 80         e;
 81         throw new PersistenceException(e);
 82         e;
 83         throw new PersistenceException(e);
 84         e;
 85         throw new PersistenceException(e);
 86         e;
 87         throw new PersistenceException(e);
 88     }
 89 
 90     private static Set providerNamesFromReader(BufferedReader reader)
 91         throws IOException
 92     {
 93         Set names = new HashSet();
 94         do
 95         {
 96             String line;
 97             if((line = reader.readLine()) == null)
 98                 break;
 99             line = line.trim();
100             Matcher m = nonCommentPattern.matcher(line);
101             if(m.find())
102                 names.add(m.group().trim());
103         } while(true);
104         return names;
105     }
106 
107     public static final String PERSISTENCE_PROVIDER = "javax.persistence.spi.PeristenceProvider";
108     protected static final Set providers = new HashSet();
109     private static final Pattern nonCommentPattern = Pattern.compile("^([^#]+)");
110 
111 }

在如下图所示中：

打开javax.persistence.spi.PersistenceProvider，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("sample");

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。