JPA dev: 几个问题总结（续）

kylinsoong

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JPA Hibernate Java Bean Oracle

2. 如何由PoJo类生成数据库中的表

首先可以根据实体间关系及对应关系设计表，写出相应建表SQL语句，执行生成表，或者可以用hibernate映射来生成表，在http://kylinsoong.iteye.com/admin/blogs/739502一开始说明怎样通过hibernate mapping生成表；

我们例子生成表如下图所示：

共生成11张表，7个实体对应7张表及5个一对多关联表（User和Event，User和Friend，Event和Property，Wife和Pet，Pet和Property）。

3. FetchType Lazy及Eager测试

一对一下FetchType Lazy

设定User和UserCard，Friend和UserCard，Wife和UserCard对应FetchType 设为lazy，User和Wife设为Eager,运行如下代码;

public static void main(String[] args) throws Throwable {
		EntityManagerFactory emf = JPAUtil.createEMF("oracle");
		EntityManager em = emf.createEntityManager();
		EntityTransaction t = em.getTransaction();
		t.begin();
		
		User user = em.find(User.class, new Long(1));
		System.out.println(user.getFriends().get(0).getUserCard().getClass());
		System.out.println(user.getUserCard().getClass());
		System.out.println(user.getWife().getClass());
		System.out.println(user.getWife().getUserCard().getClass());
		
		t.commit();
		em.close();
		emf.close();
	}

运行结果:

class com.tibco.hibernate.po.UserCard_$$_javassist_0
class com.tibco.hibernate.po.UserCard_$$_javassist_0
class com.tibco.hibernate.po.Wife
class com.tibco.hibernate.po.UserCard_$$_javassist_0

从结果可以看到一对一下FetchType 设为Lazy，对应熟悉值为该属性的代理，当然设为Eager后就不为代理,下面代码通过反射对代理类进行研究

public static void main(String[] args) throws Throwable {
		EntityManagerFactory emf = JPAUtil.createEMF("oracle");
		EntityManager em = emf.createEntityManager();
		EntityTransaction t = em.getTransaction();
		t.begin();
		
		User user = em.find(User.class, new Long(1));
		showProxyClass(user.getUserCard());
		
		t.commit();
		em.close();
		emf.close();
	}


	private static void showProxyClass(Object bean) throws Throwable {
		PropertyDescriptor[] propertyDescriptors = Introspector.getBeanInfo(bean.getClass()).getPropertyDescriptors();
		for(PropertyDescriptor propertyDescriptor : propertyDescriptors) {
			System.out.println(propertyDescriptor.getDisplayName() + "   " + propertyDescriptor.getReadMethod());
		}
	}

运行结果：

cardNumber   public final java.lang.String com.tibco.hibernate.po.UserCard_$$_javassist_0.getCardNumber()
class   public final native java.lang.Class java.lang.Object.getClass()
handler   null
hibernateLazyInitializer   public final org.hibernate.proxy.LazyInitializer com.tibco.hibernate.po.UserCard_$$_javassist_0.getHibernateLazyInitializer()
id   public final java.lang.Long com.tibco.hibernate.po.UserCard_$$_javassist_0.getId()

可以看出除去CardNumber属性cardNumber ，id 外，又多出了handler和hibernateLazyInitializer属性，且cardNumber ，id 对应的Get方法不是getId（）和getCardNumber（），而是UserCard_$$_javassist_0.getId()和UserCard_$$_javassist_0.getCardNumber()，是其代理类中的Get方法；继续研究.UserCard_$$_javassist_0类：

private static void showProxyClass(Object bean) throws Throwable {
		PropertyDescriptor[] propertyDescriptors = Introspector.getBeanInfo(bean.getClass()).getPropertyDescriptors();
		for(PropertyDescriptor propertyDescriptor : propertyDescriptors) {
			if(propertyDescriptor.getDisplayName().compareTo("hibernateLazyInitializer")== 0) {
				Class c =  Class.forName("com.tibco.hibernate.po.UserCard_$$_javassist_0");
				for(Class cl :c.getInterfaces()) {
					System.out.println(cl);
				}
				for(Method m : c.getMethods()) {
					if(m.getName().startsWith("get")){
						System.out.println(m.getName());
					}
				}
				Object obj = propertyDescriptor.getReadMethod().invoke(bean, new Object[]{});
				System.out.println(obj.getClass());
			}
		}
	}

运行结果：

interface org.hibernate.proxy.HibernateProxy
interface javassist.util.proxy.ProxyObject
getHibernateLazyInitializer
getCardNumber
getId
getClass
class org.hibernate.proxy.pojo.javassist.JavassistLazyInitializer

看到UserCard_$$_javassist_0实现了两个接口getHibernateLazyInitializer()方法可以获取

一对多下FetchType Lazy

User和Event，Event和Property，Wife和Pet，Pet和Property FetchType为Lazy，User和Friend FetchType为Eager，运行如下代码

public static void main(String[] args) throws Throwable {
		EntityManagerFactory emf = JPAUtil.createEMF("oracle");
		EntityManager em = emf.createEntityManager();
		EntityTransaction t = em.getTransaction();
		t.begin();

		User user = em.find(User.class, new Long(1));
		System.out.println(user.getEvents().getClass());
		System.out.println(user.getFriends().getClass());
		System.out.println(user.getEvents().get(0).getProperties().getClass());
		System.out.println(user.getWife().getPets().getClass());
		System.out.println(user.getWife().getPets().get(0).getProperties().getClass());
		
		t.commit();
		em.close();
		emf.close();
	}

运行结果：

class org.hibernate.collection.PersistentBag
class org.hibernate.collection.PersistentBag
class org.hibernate.collection.PersistentBag
class org.hibernate.collection.PersistentBag
class org.hibernate.collection.PersistentBag

发现一对多和一对一以Lazy的处理方式是不一样的。

说明一个常见错误:

上述例子中总共有User和Event，User和Friend，Event和Property，Wife和Pet，Pet和Property 5对一对多关系，已经将User和Friend FetchType为Eager，此时在将剩余4对其中任何一项设为Eager，运行程序，会出现“Unable to build EntityManagerFactory”错误，错误的根本原因是“cannot simultaneously fetch multiple bags”

如下所示错误堆栈的打印输出：

Exception in thread "main" javax.persistence.PersistenceException: [PersistenceUnit: com.tibco.hibernate.po] Unable to build EntityManagerFactory
	at org.hibernate.ejb.Ejb3Configuration.buildEntityManagerFactory(Ejb3Configuration.java:677)
	at org.hibernate.ejb.HibernatePersistence.createEntityManagerFactory(HibernatePersistence.java:126)
	at javax.persistence.Persistence.createEntityManagerFactory(Persistence.java:52)
	at com.tibco.hibernate.jpa.JPAUtil.createEMF(JPAUtil.java:20)
	at com.tibco.hibernate.jpa.JPAClient.main(JPAClient.java:26)
Caused by: org.hibernate.loader.MultipleBagFetchException: cannot simultaneously fetch multiple bags
	at org.hibernate.loader.BasicLoader.postInstantiate(BasicLoader.java:94)
	at org.hibernate.loader.entity.EntityLoader.<init>(EntityLoader.java:98)
	at org.hibernate.loader.entity.EntityLoader.<init>(EntityLoader.java:66)
	at org.hibernate.loader.entity.EntityLoader.<init>(EntityLoader.java:56)
	at org.hibernate.loader.entity.BatchingEntityLoader.createBatchingEntityLoader(BatchingEntityLoader.java:126)
	at org.hibernate.persister.entity.AbstractEntityPersister.createEntityLoader(AbstractEntityPersister.java:1765)
	at org.hibernate.persister.entity.AbstractEntityPersister.createEntityLoader(AbstractEntityPersister.java:1769)
	at org.hibernate.persister.entity.AbstractEntityPersister.createLoaders(AbstractEntityPersister.java:3002)
	at org.hibernate.persister.entity.AbstractEntityPersister.postInstantiate(AbstractEntityPersister.java:2995)
	at org.hibernate.persister.entity.SingleTableEntityPersister.postInstantiate(SingleTableEntityPersister.java:712)
	at org.hibernate.impl.SessionFactoryImpl.<init>(SessionFactoryImpl.java:329)
	at org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:1341)
	at org.hibernate.cfg.AnnotationConfiguration.buildSessionFactory(AnnotationConfiguration.java:867)
	at org.hibernate.ejb.Ejb3Configuration.buildEntityManagerFactory(Ejb3Configuration.java:669)
	... 4 more

错误的原因是：如果一个实体里要映射多个集合实体时，我们不能把两个集合的的FetchType设置为EAGER,此时只能设置为LAZY,否则会报：cannot simultaneously fetch multiple bags。

解决办法：将一个FetchType设置为Lazy

4 Lazy和Eager的主要区别在于效率，当效率要求高时必须用Lazy，但有些时候既要要求高效率（Lazy），但相应Lazy标记的属性不要是相关的代理，这时就需要把相关属性代理转换成原属性，下面是一个实现这个功能的类，主要思想是Java反射：

import java.beans.Introspector;
import java.beans.PropertyDescriptor;
import java.lang.reflect.Method;
import java.util.ArrayList;
import java.util.List;

/**
 * 
 * @author Kylin Soong
 * 
 */
public class POPropertyUtil {

	public static Object setBeanProperty(Object bean) throws Throwable  {
		PropertyDescriptor[] propertyDescriptors = Introspector.getBeanInfo(bean.getClass()).getPropertyDescriptors();
		for(PropertyDescriptor propertyDescriptor : propertyDescriptors) {
			if(propertyDescriptor.getReadMethod() == null) {
				continue;
			}
			Object obj = propertyDescriptor.getReadMethod().invoke(bean, new Object[]{});
			if(obj == null) {
				continue;
			}
			if(obj.getClass().getPackage().getName().compareTo("com.staffware.frameworks.po") == 0) {
				setBeanProperty(obj);
			}
			if(obj instanceof List) {
				List lists = (List) obj; 
				for(Object o : lists) {
					setBeanProperty(o);
				}
			}
			if(isProxyProperty(obj)) {
				if(propertyDescriptor.getWriteMethod() != null) {
					Object invokeObj = clone(obj, propertyDescriptor);
					propertyDescriptor.getWriteMethod().invoke(bean, new Object[]{invokeObj});
				} else {
					throw new RuntimeException(propertyDescriptor.getDisplayName() + "has a null getWriteMethod");
				}
			}
		}
		return bean;
	}

	private static Object clone(Object obj, PropertyDescriptor propertyDescriptor) throws Throwable {
		if(obj instanceof List) {
			List lists = (List) obj;
			List newList = new ArrayList();
			for(Object o : lists) {
				newList.add(o);
			}
			return newList;
		} else {
			Object newObj = propertyDescriptor.getPropertyType().newInstance();
			return replication(newObj, obj);
			
		}
	}

	private static void invokeCollectionEntity(Object o) throws Throwable {
		PropertyDescriptor[] propertyDescriptors = Introspector.getBeanInfo(o.getClass()).getPropertyDescriptors();
		for(PropertyDescriptor propertyDescriptor : propertyDescriptors) {
			if(propertyDescriptor.getReadMethod() != null) {
				propertyDescriptor.getReadMethod().invoke(o, new Object[]{});
			}
		}
	}

	private static Object replication(Object newObj, Object oldObj) throws Throwable {
		PropertyDescriptor[] newPropertyDescriptors = Introspector.getBeanInfo(newObj.getClass()).getPropertyDescriptors();
		PropertyDescriptor[] oldPropertyDescriptors = Introspector.getBeanInfo(oldObj.getClass()).getPropertyDescriptors();
		for(PropertyDescriptor propertyDescriptor : newPropertyDescriptors) {
			if(propertyDescriptor.getWriteMethod() != null) {
				Method readMethod = getReadMethod(propertyDescriptor.getDisplayName(), oldPropertyDescriptors);
				propertyDescriptor.getWriteMethod().invoke(newObj, new Object[]{readMethod.invoke(oldObj, new Object[]{})});
			}
		}
		return newObj;
	}

	private static Method getReadMethod(String displayName,
			PropertyDescriptor[] oldPropertyDescriptors) {
		for(PropertyDescriptor propertyDescriptor : oldPropertyDescriptors) {
			if(propertyDescriptor.getDisplayName().compareTo(displayName) == 0) {
				if(propertyDescriptor.getReadMethod() == null){
					throw new RuntimeException("Get Read method return a null value: " + propertyDescriptor.getPropertyType() + " " + propertyDescriptor.getDisplayName());
				}
				return propertyDescriptor.getReadMethod();
			}
		}
		throw new RuntimeException("can not find read method in:" + oldPropertyDescriptors);
	}

	private static boolean isProxyProperty(Object obj) throws Throwable {
		String name = obj.getClass().getName();
		return name.contains("_$$_javassist_") || name.contains("org.hibernate.collection.PersistentBag");
	}
}

对上述类进行测试，如下代码：

public static void main(String[] args) throws Throwable {
		EntityManagerFactory emf = JPAUtil.createEMF("oracle");
		EntityManager em = emf.createEntityManager();
		EntityTransaction t = em.getTransaction();
		t.begin();

		User user = em.find(User.class, new Long(1));
		System.out.println(user.getEvents().getClass());
		System.out.println(user.getUserCard().getClass());
		System.out.println(user.getWife().getClass());
		POPropertyUtil.setBeanProperty(user);
		System.out.println(user.getEvents().getClass());
		System.out.println(user.getUserCard().getClass());
		System.out.println(user.getWife().getClass());
		
//		t.commit();
		em.close();
		emf.close();
	}

运行结果：

class org.hibernate.collection.PersistentBag
class com.tibco.hibernate.po.UserCard_$$_javassist_0
class com.tibco.hibernate.po.Wife_$$_javassist_3
class java.util.ArrayList
class com.tibco.hibernate.po.UserCard
class com.tibco.hibernate.po.Wife

既将相应的代理还原了；

5 说明一个常见的错误：“detached entity passed to persist”，先还原错误：

如上在获取User对象后给其设定ID值，然后保存如下代码

public static void main(String[] args) throws Throwable {
		EntityManagerFactory emf = JPAUtil.createEMF("oracle");
		EntityManager em = emf.createEntityManager();
		EntityTransaction t = em.getTransaction();
		t.begin();

		User user = JPAClient.getUser();
		user.setId(new Long(1212));
		em.persist(user);
		
		t.commit();
		em.close();
		emf.close();
	}

运行代码抛出异常，异常堆栈信息如下：

Exception in thread "main" javax.persistence.PersistenceException: org.hibernate.PersistentObjectException: detached entity passed to persist: com.tibco.hibernate.po.User
	at org.hibernate.ejb.AbstractEntityManagerImpl.throwPersistenceException(AbstractEntityManagerImpl.java:614)
	at org.hibernate.ejb.AbstractEntityManagerImpl.persist(AbstractEntityManagerImpl.java:226)
	at com.tibco.hibernate.jpa.JPAClient.main(JPAClient.java:33)
Caused by: org.hibernate.PersistentObjectException: detached entity passed to persist: com.tibco.hibernate.po.User
	at org.hibernate.event.def.DefaultPersistEventListener.onPersist(DefaultPersistEventListener.java:102)
	at org.hibernate.event.def.DefaultPersistEventListener.onPersist(DefaultPersistEventListener.java:61)
	at org.hibernate.impl.SessionImpl.firePersist(SessionImpl.java:646)
	at org.hibernate.impl.SessionImpl.persist(SessionImpl.java:620)
	at org.hibernate.impl.SessionImpl.persist(SessionImpl.java:624)
	at org.hibernate.ejb.AbstractEntityManagerImpl.persist(AbstractEntityManagerImpl.java:220)

错误原因是：实体的状态变化EntityManager是根据实体的ID来判断，一般一个实体Persist的过程就是给他分配一个ID的过程，当一个实体有唯一ID时，EntityManager就认为它已经处于游离（detached ）状态，而游离态实例不能够持久化

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EJB6: EntityBean例子 | JPA dev: 几个问题总结

2010-11-25 18:02
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