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("[e-mail]"); 
      list.add("[e-mail]"); 
 
      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:[[[e-mail], [e-mail]]]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:[[[e-mail], [e-mail]]]y:[[e-mail], [e-mail]] 

 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。