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简要分析Java的Hibernate框架中的自定义类型

时间:2020-03-20 13:29     来源/作者:cxshun

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:

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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

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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:

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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:

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<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:

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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:

简要分析Java的Hibernate框架中的自定义类型

而当我们只保存一个时,异常如下:

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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,

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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:
 

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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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