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。