如何正确设置 Map 为 javaType in<arg> 在 MyBatis 中？</arg>

Question

Problem description:

I have java.lang.NoSuchMethodException: bar.Foo.<init>(java.lang.Long, java.lang.String, java.util.Map) when I try to get immutable Foo objects from DB with MyBatis and can't figure out what's wrong:(

I have an unsuccessful case by annotating result object with @Value and successful case annotating with @Data (they are below under corresponding headers). So, I'd like to use @Value but don't get what I'm doing wrong..

Unsuccessful case:

So, I have the result class which is immutable:

@Value
public class Foo {
    long entryId;
    String description;
    Map<String, String> params;
}

It is stored in postgres in a table foos, params field value is stored in JSON format in a corresponding column of the same table (type is VARCHAR, entries are "{key=value}" in case of params = Map.of("key", "value") . I'd like to instantiate my object of this class from DB and so I use the following MyBatis code:

<resultMap id="foo" type="bar.Foo">
     <constructor>
         <arg column="entry_id" javaType="java.lang.Long"/>
         <arg column="description" javaType="java.lang.String"/>
         <arg column="params" javaType="java.util.Map" jdbcType="VARCHAR" typeHandler="bar.MapToJsonStringHandler"/>
     </constructor>
</resultMap>

Here is my type handler (it's already been successfully used in another case with @Data object instead of @Value, so I guess it's correct by itself):

public class MapToJsonStringHandler implements TypeHandler<Map<String, String>> {

    private final ObjectMapper objectMapper = new ObjectMapper();

    @SneakyThrows(IOException.class)
    @Override
    public void setParameter(PreparedStatement ps, int i, Map<String, String> parameter, JdbcType jdbcType) throws SQLException {
        if (parameter == null) {
        ps.setNull(i, Types.VARCHAR);
    } else {
        ps.setString(i, objectMapper.writeValueAsString(parameter));
    }
}

@Override
public Map<String, String> getResult(ResultSet rs, String columnName) throws SQLException {
    String result = rs.getString(columnName);
    return convertToMap(result);
}


@Override
public Map<String, String> getResult(ResultSet rs, int columnIndex) throws SQLException {
    String result = rs.getString(columnIndex);
    return convertToMap(result);
}

@Override
public Map<String, String> getResult(CallableStatement cs, int columnIndex) throws SQLException {
    String result = cs.getString(columnIndex);
    return convertToMap(result);
}

@SneakyThrows(IOException.class)
private Map<String, String> convertToMap(String result) {
    return result == null ? Collections.emptyMap() : objectMapper.readValue(result, Map.class);
}

}

Here is SQL:

<select id="getFoos" resultMap="foo">
     SELECT *
     FROM foos
     <![CDATA[WHERE needed_tm < now()]]>
</select>

And when I get my Foos I have java.lang.NoSuchMethodException: bar.Foo.<init>(java.lang.Long, java.lang.String, java.util.Map) .

Funny thing is that it works like a charm when I use @Data instead of @Value:

Successful case:

@Data
@NoArgsConstructor
@AllArgsConstructor
public class Foo {
    long entryId;
    String description;
    Map<String, String> params;
}

<resultMap id="foo" type="bar.Foo">
     <result column="entry_id" property="entryId"/>
     <result column="description" property="description"/>
     <result column="params" property="params" typeHandler="bar.MapToJsonStringHandler"/>
</resultMap>

SQL and type handler are the same that in unsuccessful case.

May you be so kind to give me a hint what I am possibly missing? Thanks in advance!

Update 1:

@Value
@AllArgsConstructor
public class Foo {
     long entryId;
     String description;
     Map<String, String> params;
}

gives the same exception. Anyway as I understand, @Value already has @AllArgsConstructor by design ( https://projectlombok.org/features/Value )

Answer 1

java.lang.NoSuchMethodException: bar.Foo.(java.lang.Long, java.lang.String, java.util.Map) <--- Means there is no constructor defined.

When you use @AllArgsConstructor, you are telling lombok to generate a constructor with all the arguments, (Long, String, Map), that is why that works for you.

According to documentation @Data:

@Data All together now: A shortcut for @ToString, @EqualsAndHashCode, @Getter on all fields, @Setter on all non-final fields, and @RequiredArgsConstructor

https://projectlombok.org/features/Data

That is why at that point it finds a constructor and does not give you an error (NoSuchMethod) when you use @Data.

---

<resultMap id="foo" type="bar.Foo">
     <constructor>
         <arg column="entry_id" javaType="java.lang.Long"/>
         <arg column="description" javaType="java.lang.String"/>
         <arg column="params" javaType="java.util.Map" jdbcType="VARCHAR" typeHandler="bar.MapToJsonStringHandler"/>
     </constructor>
</resultMap>

Since you are specifying the datatype to be java.lang.Long, it expects the constructor to be defined to have that parameter type.

I believe with @Data, it is not the constructor that helped you, it was the Setters (boxing) long To Long.

Try to use @Value, and change long to Long in your class. See if that works.

Lombok will generate a constructor with (long, String, Map), not (Long, String, Map)

Curious to know what java version you are using.