Mybatis custom TypeHandler implements database Json array to List<T> object

Realize function

In the development process, it is often encountered that the entire JSON data is stored in the database as a field, but we do not want to use a String to receive it in the corresponding entity table. As shown in the figure below, there is an address field in the database t_user table. A JSON array is stored, and List<Address> objects are used in the TUserEntity entity to receive it. At this time, a custom TypeHanlder is needed to help us define the JSON serialization and deserialization when entering and exiting the library.

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@Data
@TableName("t_user")
public class TUserEntity  implements Serializable {
    private static final long serialVersionUID = 1L;
    private Long id;
    @TableField(typeHandler = AddressListTypeHandler.class)
    private List<Address> address;
}
    ```
@Data
public class Address implements Serializable {
    private static final long serialVersionUID = 1L;
    private Long id;
    private String name;
}
```

Define an abstract List to handle TypeHandler, and other types of data may be stored in the List

import com.fasterxml.jackson.core.type.TypeReference;
import org.apache.ibatis.type.BaseTypeHandler;
import org.apache.ibatis.type.JdbcType;

import java.sql.CallableStatement;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.util.List;


public abstract class ListTypeHandler<T> extends BaseTypeHandler<List<T>> {
    @Override
    public void setNonNullParameter(PreparedStatement ps, int i, List<T> parameter, JdbcType jdbcType) throws SQLException {
        ps.setString(i, JsonUtils.toJson(parameter));
    }

    @Override
    public List<T> getNullableResult(ResultSet rs, String columnName) throws SQLException {
        return rs.wasNull() ? null : JsonUtils.readList(rs.getString(columnName), specificType());
    }

    @Override
    public List<T> getNullableResult(ResultSet rs, int columnIndex) throws SQLException {
        return rs.wasNull() ? null : JsonUtils.readList(rs.getString(columnIndex), specificType());
    }

    @Override
    public List<T> getNullableResult(CallableStatement cs, int columnIndex) throws SQLException {
        return cs.wasNull() ? null : JsonUtils.readList(cs.getString(columnIndex), specificType());
    }

    protected abstract TypeReference<List<T>> specificType();

Define the AddressListTypeHandler class, if there are other classes inheriting ListTypeHandler

@MappedTypes({List.class})
public class AddressListTypeHandler extends  ListTypeHandler<Address> {
    @Override
    protected TypeReference<List<Address>> specificType() {
        return new TypeReference<List<Address>>() {
        };
    }
}

The JsonUtils.readList() method in jaskson can also use fastjson JSON.parseObject(content, this.specificType())


public static <T> List<T> readList(String json, TypeReference<List<T>> tTypeReference) {
    if (json == null || "".equals(json)) {
        return null;
    }
    try {
        return mapper.readValue(json,tTypeReference);
    } catch (IOException e) {
        throw new RuntimeException(e);
    }
}

Test result query

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