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，这种类型意味着它可以像非关系型数据库那样存储数据，数据扩展性非常好，这也是项目使用它的主要原因之一。 在Hibernate和PostgreSQL的结合过程中遇到了针对Json数据类型，Hibernate并没有Json这种数据映射字段，想了下是不是版本的问题，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，而且现在MySQL最新版本也已经支持Json数据格式啦，身为主流ORM框架怎么到现在还没有提供这种数据类型，不管啦，也许人家提供了其他想法，大公司的想法是我们这些吊私无法猜测的，作为我们就是寻找解决方案就可以啦，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，这种常见问题肯定有了很好的解决方案，果然在stackoverflow找到了一些解决方案，这里只做整理说明，能够解决当前项目中的问题，不做深入探究。

现在有俩种方案：

1、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

2、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，这样就可以定义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形成映射

例如

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

这样session.save(entity);的时候就可以将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，本身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，而且不符合Hibernate存在的原因，二第二种方法很好的实现了所谓的映射关系，数据库端不需要做任何多余的事情。所以我选择了第二种方案。