内容

• 需求场景
• 实现步骤

需求场景

比如,在编写用户类时,用户类型(UserType)是一个枚举类,这时候我把将user模型保存到数据库的时候会有问题,我们可以在使用jpa将user保存到数据库中的时候,将该枚举类转换为String类型保存在数据库中,从数据库中拿出来的时候,将String类型转化为枚举类型

实现步骤

核心步骤是编写一个类实现接口AttributeConverter<X,Y>,通过该接口实现"数据库存储类型-自定义类型"之间的自动转换

package javax.persistence;
 
/**
 * A class that implements this interface can be used to convert
 * entity attribute state into database column representation
 * and back again.
 * Note that the X and Y types may be the same Java type.
 *
 * @param X the type of the entity attribute
 * @param Y the type of the database column
 *
 * @since Java Persistence 2.1
 */
public interface AttributeConverter<X,Y> {
    /**
     * Converts the value stored in the entity attribute into the
     * data representation to be stored in the database.
     *
     * @param attribute the entity attribute value to be converted
     * @return the converted data to be stored in the database column
     */
    public Y convertToDatabaseColumn (X attribute);
 
    /**
     * Converts the data stored in the database column into the
     * value to be stored in the entity attribute.
     * Note that it is the responsibility of the converter writer to
     * specify the correct dbData type for the corresponding column
     * for use by the JDBC driver: i.e., persistence providers are
     * not expected to do such type conversion.
     *
     * @param dbData the data from the database column to be converted
     * @return the converted value to be stored in the entity attribute
     */
    public X convertToEntityAttribute (Y dbData);
}

上面的代码是AttributeConverter<X,Y>接口,在实现该接口的时候需要实现

public Y convertToDatabaseColumn (X attribute);

public Y convertToDatabaseColumn (X attribute);

定义一个自定义的类型,这本文的demo中使用的是enum类型

public enum MyEnum{
    MAN("1","男"),WOMAN("2","女");
}

实现AttributeConverter<X,Y>接口,定义一个转换类

下面的例子只是demo,没有严格的逻辑可言

import javax.persistence.AttributeConverter;

public class MyConverter implements AttributeConverter<MyEnum,String> {

    @Override
    public String convertToDatabaseColumn(MyEnum myEnum) {
        return myEnum.toString();
    }

    @Override
    public MyEnum convertToEntityAttribute(String s) {
        return MyEnum.MAN;
    }

}

在实体属性上加入@Convert注解,把转换器设置成上面的实现类

@Entity
@Table(name = "USER")
public class User {
    @Convert(
            converter = MyConverter.class
    )
    private MyEnum myEnum;
}

如此,在使用jpa进行数据库的增删改查,jpa会通过实现类将自定义类型和数据库类型自动转换