Hibernate 映射数据库中Json字段的解决方案

最近的项目ORM框架使用的是Hibernate，数据库使用PostgreSQL，俩个都是开源的高大上产品，Hibernate就不用说啦，主流的ORM框架，PostgreSQL第一次接触，其中包含Json数据类型的字段，这种类型意味着它能够像非关系型数据库那样存储数据，数据扩展性很是好，这也是项目使用它的主要缘由之一。 在Hibernate和PostgreSQL的结合过程当中遇到了针对Json数据类型，Hibernate并无Json这种数据映射字段，想了下是否是版本的问题，也许高版本Hibernate已经支持这种类型啦，下载最新版本Hibernate看下啦，仍然不支持，这点不是很明白，PostgreSQL已经支持Json这么长时间啦，并且如今MySQL最新版本也已经支持Json数据格式啦，身为主流ORM框架怎么到如今尚未提供这种数据类型，无论啦，也许人家提供了其余想法，大公司的想法是咱们这些吊私没法猜想的，做为咱们就是寻找解决方案就能够啦，毕竟人家已经开源了嘛。
毕竟这俩框架已经诞生这么长时间啦，这种常见问题确定有了很好的解决方案，果真在stackoverflow找到了一些解决方案，这里只作整理说明，可以解决当前项目中的问题，不作深刻探究。

如今有俩种方案：

一、在数据库端处理

CREATE OR REPLACE FUNCTION json_intext(text) RETURNS json AS $$ SELECT json_in($1::cstring); 
$$ LANGUAGE SQL IMMUTABLE;

上面的函数是将指定text类型数据转换成json数据个数

二、在Hibernate端处理

import org.hibernate.HibernateException;
import org.hibernate.engine.spi.SessionImplementor;
import org.hibernate.usertype.UserType;

import java.io.Serializable;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Types;

/** * @author timfulmer */
public class StringJsonUserType implements UserType {

    /** * Return the SQL type codes for the columns mapped by this type. The * codes are defined on <tt>java.sql.Types</tt>. * * @return int[] the typecodes * @see java.sql.Types */
    @Override
    public int[] sqlTypes() {
        return new int[] { Types.JAVA_OBJECT};
    }

    /** * The class returned by <tt>nullSafeGet()</tt>. * * @return Class */
    @Override
    public Class returnedClass() {
        return String.class;
    }

    /** * Compare two instances of the class mapped by this type for persistence "equality". * Equality of the persistent state. * * @param x * @param y * @return boolean */
    @Override
    public boolean equals(Object x, Object y) throws HibernateException {

        if( x== null){

            return y== null;
        }

        return x.equals( y);
    }

    /** * Get a hashcode for the instance, consistent with persistence "equality" */
    @Override
    public int hashCode(Object x) throws HibernateException {

        return x.hashCode();
    }

    /** * Retrieve an instance of the mapped class from a JDBC resultset. Implementors * should handle possibility of null values. * * @param rs a JDBC result set * @param names the column names * @param session * @param owner the containing entity  @return Object * @throws org.hibernate.HibernateException * * @throws java.sql.SQLException */
    @Override
    public Object nullSafeGet(ResultSet rs, String[] names, SessionImplementor session, Object owner) throws HibernateException, SQLException {
        if(rs.getString(names[0]) == null){
            return null;
        }
        return rs.getString(names[0]);
    }

    /** * 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>. * * @param st a JDBC prepared statement * @param value the object to write * @param index statement parameter index * @param session * @throws org.hibernate.HibernateException * * @throws java.sql.SQLException */
    @Override
    public void nullSafeSet(PreparedStatement st, Object value, int index, SessionImplementor session) throws HibernateException, SQLException {
        if (value == null) {
            st.setNull(index, Types.OTHER);
            return;
        }

        st.setObject(index, value, Types.OTHER);
    }

    /** * 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. * * @param value the object to be cloned, which may be null * @return Object a copy */
    @Override
    public Object deepCopy(Object value) throws HibernateException {

        return value;
    }

    /** * Are objects of this type mutable? * * @return boolean */
    @Override
    public boolean isMutable() {
        return true;
    }

    /** * 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 org.hibernate.HibernateException * */
    @Override
    public Serializable disassemble(Object value) throws HibernateException {
        return (String)this.deepCopy( value);
    }

    /** * 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) * * @param cached the object to be cached * @param owner the owner of the cached object * @return a reconstructed object from the cachable representation * @throws org.hibernate.HibernateException * */
    @Override
    public Object assemble(Serializable cached, Object owner) throws HibernateException {
        return this.deepCopy( cached);
    }

    /** * 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. * * @param original the value from the detached entity being merged * @param target the value in the managed entity * @return the value to be merged */
    @Override
    public Object replace(Object original, Object target, Object owner) throws HibernateException {
        return original;
    }
}

上面的类实现Hibernate的UserType接口，这样就能够定义Hibernate端的数据类型，这样将对应数据库中Json字段的JavaBean中的数据字段定义成String类型，这样javaBean中指定字段就能和数据库中json造成映射

例如

<property name="atts" type="com.quangao.hibernate.StringJsonUserType">
     <column name="atts"  />
</property>

这样session.save(entity);的时候就能够将json字段成功保存

上述俩种方法，第一种虽能实现，但需在数据中频繁转换，自己简单的保存修改变得很麻烦，明显不切符合解耦概念，这是将前端的事情强加给数据库来处理的，并且不符合Hibernate存在的缘由，二第二种方法很好的实现了所谓的映射关系，数据库端不须要作任何多余的事情。因此我选择了第二种方案。

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