9.9. 拦截器(Interceptors)
Interceptor接口提供从session到你的应用程序的回调方法，让你的程序可以观察和在持久化对象保存/更改/删除或者装载的时候操作它的属性。一种可能的用途是用来监视统计信息。比如，下面的Interceptor会自动在一个Auditable创建的时候设置其createTimestamp,并且当它被更改的时候，设置其lastUpdateTimestamp属性。

package net.sf.hibernate.test;

import java.io.Serializable;
import java.util.Date;
import java.util.Iterator;

import net.sf.hibernate.Interceptor;
import net.sf.hibernate.type.Type;

public class AuditInterceptor implements Interceptor, Serializable {

    private int updates;
    private int creates;

    public void onDelete(Object entity,
                         Serializable id,
                         Object[] state,
                         String[] propertyNames,
                         Type[] types) {
        // do nothing
    }

    public boolean onFlushDirty(Object entity,
                                Serializable id,
                                Object[] currentState,
                                Object[] previousState,
                                String[] propertyNames,
                                Type[] types) {

        if ( entity instanceof Auditable ) {
            updates++;
            for ( int i=0; i < propertyNames.length; i++ ) {
                if ( "lastUpdateTimestamp".equals( propertyNames[i] ) ) {
                    currentState[i] = new Date();
                    return true;
                }
            }
        }
        return false;
    }

    public boolean onLoad(Object entity,
                          Serializable id,
                          Object[] state,
                          String[] propertyNames,
                          Type[] types) {
        return false;
    }

    public boolean onSave(Object entity,
                          Serializable id,
                          Object[] state,
                          String[] propertyNames,
                          Type[] types) {
       
        if ( entity instanceof Auditable ) {
            creates++;
            for ( int i=0; i<propertyNames.length; i++ ) {
                if ( "createTimestamp".equals( propertyNames[i] ) ) {
                    state[i] = new Date();
                    return true;
                }
            }
        }
        return false;
    }

    public void postFlush(Iterator entities) {
        System.out.println("Creations: " + creates + ", Updates: " + updates);
    }

    public void preFlush(Iterator entities) {
        updates=0;
        creates=0;
    }
   
    ......
    ......
   
}
当session被创建的时候，就应该指定拦截器。

Session session = sf.openSession( new AuditInterceptor() );
9.10. 元数据(Metadata) API
Hibernate对所有的实体和值类型都需要一个非常丰富的元级别(meta-level)模型。有时候，这个模型对应用程序本身也会非常有用。比如说，应用程序可能使用Hibernate的元数据来实现一种“智能”的深度拷贝算法，来理解哪些对象应该被拷贝（比如，可变的值类型），那些不应该（不可变的值类型和可能的被关联的实体）。

Hibernate通过ClassMetadata接口，CollectionMetadata接口和Type对象树，暴露出元数据。可以通过SessionFactory获取metadata接口的实例。

Cat fritz = ......;
Long id = (Long) catMeta.getIdentifier(fritz);
ClassMetadata catMeta = sessionfactory.getClassMetadata(Cat.class);
Object[] propertyValues = catMeta.getPropertyValues(fritz);
String[] propertyNames = catMeta.getPropertyNames();
Type[] propertyTypes = catMeta.getPropertyTypes();
// get a Map of all properties which are not collections or associations
// TODO: what about components?
Map namedValues = new HashMap();
for ( int i=0; i<propertyNames.length; i++ ) {
    if ( !propertyTypes[i].isEntityType() && !propertyTypes[i].isCollectionType() ) {
        namedValues.put( propertyNames[i], propertyValues[i] );
    }
}