最近看到hibernate的自定义类型，这个以前没接触过，在这里记录一下，当是对自己知识的巩固，也让没有接触过的朋友一起学习研究一番。
1）自定义类型，顾名思义，当然就是由于内部的类型不满足需求，而自己来进行实现的类型。这种情况不多，但我们还是有必要学习一下，技多不压身嘛。也学习一下，别人在做框架的时候是怎么去考虑的，怎么去思考扩展性的。
自定义类型有两个方法来实现，一种是实现UserType，另外一种实现CompositeUserType，另外可能还有一些方法，但我暂时没用到，先不讲了。
我暂时只用到UserType，我们就先看一下UserType接口的定义：
public interface UserType { /** * Return the SQL type codes for the columns mapped by this type. The * codes are defined on <tt>java.sql.Types</tt>. */ public int[] sqlTypes(); /** * The class returned by <tt>nullSafeGet()</tt>. */ public Class returnedClass(); /** * Compare two instances of the class mapped by this type for persistence "equality". * Equality of the persistent state. */ public boolean equals(Object x, Object y) throws HibernateException; /** * Get a hashcode for the instance, consistent with persistence "equality" */ public int hashCode(Object x) throws HibernateException; /** * Retrieve an instance of the mapped class from a JDBC resultset. Implementors * should handle possibility of null values. */ public Object nullSafeGet(ResultSet rs, String[] names, Object owner) throws HibernateException, SQLException; /** * Write an instance of the mapped class to a prepared statement. Implementors * should handle possibility of null values. A multi-column type should be written * to parameters starting from <tt>index</tt>. */ public void nullSafeSet(PreparedStatement st, Object value, int index) throws HibernateException, SQLException; /** * Return a deep copy of the persistent state, stopping at entities and at * collections. It is not necessary to copy immutable objects, or null * values, in which case it is safe to simply return the argument. */ public Object deepCopy(Object value) throws HibernateException; /** * Are objects of this type mutable? * * @return boolean */ public boolean isMutable(); /** * Transform the object into its cacheable representation. At the very least this * method should perform a deep copy if the type is mutable. That may not be enough * for some implementations, however; for example, associations must be cached as * identifier values. (optional operation) * * @param value the object to be cached * @return a cachable representation of the object * @throws HibernateException */ public Serializable disassemble(Object value) throws HibernateException; /** * Reconstruct an object from the cacheable representation. At the very least this * method should perform a deep copy if the type is mutable. (optional operation) */ public Object assemble(Serializable cached, Object owner) throws HibernateException; /** * During merge, replace the existing (target) value in the entity we are merging to * with a new (original) value from the detached entity we are merging. For immutable * objects, or null values, it is safe to simply return the first parameter. For * mutable objects, it is safe to return a copy of the first parameter. For objects * with component values, it might make sense to recursively replace component values. */ public Object replace(Object original, Object target, Object owner) throws HibernateException; } 其实大家看英文一般情况下都能理解，不再多做解释了，这里我们最主要的就是实现nullSafeSet() 方法，这个方法主要用到把此类型的值保存到数据库，这一次我们先学怎么用，以后我们再慢慢研究内部是怎么来实现的。
2）我学习时写的例子是参照夏昕的例子，所以肯定和网上的大部分都一样，我们只是大概分析一下：
下面是User类
package org.hibernate.tutorial.domain; import java.io.Serializable; import java.util.List; public class User implements Serializable{ public Long id; private String name; private List emails; 省略Get/Set方法 } 下来是自定义的EmailList类：
package org.hibernate.tutorial.domain; import java.io.Serializable; import java.sql.PreparedStatement; import java.sql.ResultSet; import java.sql.SQLException; import java.sql.Types; import java.util.ArrayList; import java.util.List; import org.hibernate.Hibernate; import org.hibernate.HibernateException; import org.hibernate.usertype.UserType; public class EmailList implements UserType { private static final char SPLITTER = ';'; private static final int[] TYPES = new int[] {Types.VARCHAR}; private String assemble(List emailList) { StringBuilder strBuf = new StringBuilder(); for (int i = 0; i < emailList.size() - 1; i++){ strBuf.append(emailList.get(i)).append(SPLITTER); } strBuf.append(emailList.get(emailList.size()-1)); return strBuf.toString(); } private List parse(String value) { String[] strs = org.hibernate.util.StringHelper.split(value,String.valueOf(SPLITTER)); List emailList = new ArrayList(); for (int i = 0;i < strs.length; i++) { emailList.add(strs[i]); } return emailList; } public Object deepCopy(Object value) throws HibernateException { List sourceList = (List)value; List targetList = new ArrayList(); targetList.add(sourceList); return targetList; } public Serializable disassemble(Object value) throws HibernateException { return null; } public boolean equals(Object x, Object y) throws HibernateException { if (x == y) return true; System.out.println("X:"+x+"Y:"+y); if (x != null && y != null) { List xList = (List)x; List yList = (List)y; if(xList.size() != yList.size()) return false; for (int i = 0; i < xList.size(); i++) { String str1 = (String)xList.get(i); String str2 = (String)yList.get(i); if (!str1.equals(str2)) return false; } return true; } return false; } public boolean isMutable() { return false; } public Object nullSafeGet(ResultSet rs, String[] names, Object owner) throws HibernateException, SQLException { String value = (String)Hibernate.STRING.nullSafeGet(rs, names[0]); if (value != null) { return parse(value);//把List通过;分割 } else{ return null; } } public void nullSafeSet(PreparedStatement st, Object value, int index) throws HibernateException, SQLException { System.out.println("Set Method Executed!"); System.out.println("value:" + value); if (value != null){ String str = assemble((List)value);//把字符串用;拼接 Hibernate.STRING.nullSafeSet(st, str, index); } else { Hibernate.STRING.nullSafeSet(st, value, index); } } public Class returnedClass() { return List.class; } public int[] sqlTypes() { return TYPES; } //省略其他不需要修改的方法 } 类中实现的方法是需要修改的方法，其他不需要修改暂时不用的方法则没有写出来，但还是需要实现的。
3）接下来就是User类的映射文件：
<class name="User" table="USER"> <id name="id" column="USER_ID" type="java.lang.Long"> <generator class="native" /> </id> <property name="name" type="string" column="USER_NAME"/> <property name="emails" type="org.hibernate.tutorial.domain.EmailList" column="emails"/> </class>
相信大家都知道怎么进行修改，这里也不进行讲解了，主要是修改emails的type，修改为我们刚才定义的EmailList类。
4）最后我们来写一个测试类：
import java.util.HashMap; import java.util.List; import java.util.Map; import java.util.ArrayList; import junit.framework.TestCase; import org.hibernate.EntityMode; import org.hibernate.Session; import org.hibernate.SessionFactory; import org.hibernate.Transaction; import org.hibernate.cfg.Configuration; import org.hibernate.tutorial.domain.User; public class HibernateTest extends TestCase{ private Session session = null; protected void setUp() throws Exception { Configuration cfg = new Configuration().configure(); SessionFactory sessionFactory = cfg.buildSessionFactory(); session = sessionFactory.openSession(); } public void testInsert(){ Transaction tran = null; try{ tran = session.beginTransaction(); User user = new User(); user.setName("shun"); List list = new ArrayList(); list.add("12312@sfsdf.com"); list.add("123@123.com"); user.setEmails(list); session.save(user); tran.commit(); } catch (Exception ex) { ex.printStackTrace(); if (tran != null){ tran.rollback(); } } } protected void tearDown() throws Exception { session.close(); } } 这里可能会出现问题，当我们只保存一个email时，它会出现异常，在数据库里面是email字段是空的，而当我们如上面代码一样，有两个时，并不会出现问题，数据库中结果如图：
而当我们只保存一个时，异常如下：
java.lang.ClassCastException: java.util.ArrayList cannot be cast to java.lang.String
它发生在EmailList的equals方法中的String str1 = (String)xList.get(i);这句代码中，经检查是在插入数据传到EmailList的nullSafeSet方法时变成了List的List，即
value:[[12312@sfsdf.com, 123@123.com]]这样的形式，这样在比较的时候就会出问题，它永远都只有一个值，而在比较的时候却是不同的，
if(xList.size() != yList.size()) return false;
所以在强制转换时会出问题。
而经过检查，equals方法里：
X:[[12312@sfsdf.com, 123@123.com]]Y:[12312@sfsdf.com, 123@123.com]
这样的结果却是很奇怪的。网上并没有讲到为什么会出现这种情况。这里提出一下：我用的hibernate版本是Hibernate 3.3.2.GA。不知道是版本问题还是其他问题，我们明天再研究一下。如果有哪位兄弟知道为什么的，希望也不吝告诉我一下。
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