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。