简要分析Java的Hibernate框架中的自定义类型

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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