@Convert jpa中用于进行数据库存储类型与程序中类型的转换

我就是我 2022-05-13 12:24 163阅读 0赞

用于数据库属性类型与java存储的类型做转换，例如枚举类型，在存储到数据库时或者在数据库取出来时，不用手动转换。

必须实现接口AttributeConverter<X,Y>，源码如下：

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);
    }

convertToDatabaseColumn（X attribute）用于把输入的类型转成数据库存储的类型

convertToEntityAttribute (Y dbData) 用于把数据库搜出来的类型转成实体中想要的类型

下面看实例：

我自己定义了一个枚举

package com.xhx.springboot.enums;
    
    public enum GenderEnum{
        MAN("1","男"),WOMAN("2","女");
        GenderEnum(String code,String value){
            this.code = code;
            this.value = value;
        }
        private String code;
        private String value;
        private String description;
    
        public String getCode() {
            return this.code;
        }
    
        public String getValue() {
            return this.value;
        }
    
        public String getDescription() {
            return this.description;
        }
    
    }

定义一个转换器类，实现AttributeConverter<X,Y>接口

package com.xhx.springboot.converters;
    
    import com.xhx.springboot.enums.GenderEnum;
    
    import javax.persistence.AttributeConverter;
    import java.util.Arrays;
    
    public class GenderEnumConverter implements AttributeConverter<GenderEnum,String> {
        @Override
        public String convertToDatabaseColumn(GenderEnum genderEnum) {
            return genderEnum.getCode();
        }
    
        @Override
        public GenderEnum convertToEntityAttribute(String s) {
            return Arrays.stream(GenderEnum.values()).filter(en -> en.getCode().equals(s)).findFirst().orElse(null);
        }
    }

在实体类属性上，加入@Convert注解，把转换器设置成上面实现的转换器。存储值时就会调用convertToDatabaseColumn方法，把code存入数据库，查询值时就会调用convertToEntityAttribute方法，转成枚举值，赋值到实体类中。

package com.xhx.springboot.entity;
    
    import com.xhx.springboot.converters.GenderEnumConverter;
    import com.xhx.springboot.enums.GenderEnum;
    
    import javax.persistence.*;
    
    /**
     * @author xuhaixing
     * @date 2018/4/28 10:29
     */
    @Entity
    @Table(name = "USER")
    public class User {
        @Id
        @GeneratedValue(strategy = GenerationType.SEQUENCE)
        private int id;
        private String name;
        private int age;
        @Convert(
                converter = GenderEnumConverter.class
        )
        private GenderEnum gender;
        @Version
        private int version;
    
        public int getId() {
            return id;
        }
    
        public void setId(int id) {
            this.id = id;
        }
    
        public String getName() {
            return name;
        }
    
        public void setName(String name) {
            this.name = name;
        }
    
        public int getAge() {
            return age;
        }
    
        public void setAge(int age) {
            this.age = age;
        }
    
        public GenderEnum getGender() {
            return gender;
        }
    
        public void setGender(GenderEnum gender) {
            this.gender = gender;
        }
    
        public int getVersion() {
            return version;
        }
    
        public void setVersion(int version) {
            this.version = version;
        }
    
        @Override
        public String toString() {
            return "User{" +
                    "id=" + id +
                    ", name='" + name + '\'' +
                    ", age=" + age +
                    ", gender=" + gender +
                    ", version=" + version +
                    '}';
        }
    }

写了一个测试用例：调用controller接口，一个是添加数据，一个是查询数据

package com.xhx.springboot;
    
    import com.xhx.springboot.controller.UserController;
    import com.xhx.springboot.entity.User;
    import com.xhx.springboot.enums.GenderEnum;
    import org.junit.Test;
    import org.junit.runner.RunWith;
    import org.springframework.beans.factory.annotation.Autowired;
    import org.springframework.boot.test.context.SpringBootTest;
    import org.springframework.test.context.junit4.SpringRunner;
    
    @RunWith(SpringRunner.class)
    @SpringBootTest
    public class Springboot5ApplicationTests {
    
        @Autowired
        private UserController userController;
    
        @Test
        public void testConvertToDB() {
            User user = new User();
            user.setName("小徐");
            user.setAge(25);
            user.setGender(GenderEnum.MAN);
            userController.add(user); //GenderEnum类型已经转换
        }
    
        @Test
        public void testDBToEntity() {
            User user = userController.findById(2);//GenderEnum类型已经转换
            System.out.println(user);
            //User{id=2, name='小徐', age=25, gender=MAN, version=0}
        }
    
    }

执行第一个后，数据库中已经有值，如下图

![70][]

执行第二个返回结果如上面注释。

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[70]: /images/20220513/9bfe4b9aefe74c72be52fed77096247c.png
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