jpa查询blob数据抛出异常:No Dialect mapping for JDBC type -4

用entityManager.createNativeQuery(sql).list()查询blob数据时，抛出了下面的异常：

Caused by: org.hibernate.MappingException: No Dialect mapping for JDBC type: -4

at org.hibernate.dialect.TypeNames.get(TypeNames.java:56)

at org.hibernate.dialect.TypeNames.get(TypeNames.java:81)

at org.hibernate.dialect.Dialect.getHibernateTypeName(Dialect.java:370)

其中entityManager为javax.persistence.EntityManager类型，此时，需要注册BLOB类型，编写一个继承MySQL5Dialect的类：

package com.kin.mail.dialect;
import java.sql.Types;
import org.hibernate.Hibernate;
import org.hibernate.dialect.MySQL5Dialect;
/**
* 注册BLOB类型
* @author kin
*
*/
public class BlobMySQLDialect extends MySQL5Dialect {
public BlobMySQLDialect () {
super();
registerHibernateType(Types.LONGVARBINARY, Hibernate.BLOB.getName());//注册blob，Types.LONGVARBINARY的值就是-4
}
}

然后在persistence.xml配置文件里修改数据库方言声明：

<property name="hibernate.dialect" value="com.kin.mail.dialect.BlobMySQLDialect" />