摘要：關于的一些總結一個小問題的思考起因當前項目中一直使用的都是，即這種用法考慮到確實有一定的局限性，在部分查詢中使用到了進行復雜查詢操作由于本人年也曾使用過，古語溫故而知新，所以做此總結梳理。

當前項目中一直使用的都是 SpringData JPA ，即 public interface UserRepository extends JpaRepository 這種用法；

考慮到 SpringData JPA 確實有一定的局限性，在部分查詢中使用到了 JdbcTemplate 進行復雜查詢操作；
由于本人16年也曾使用過 JdbcTemplate，古語溫故而知新，所以做此總結梳理。

首先列出同事的做法：

public class xxx{

    xxx method(){
        ...
        List list = jdbcTemplate.query(sql, new WishDTO());
        ...
    }
}

@Data
public class WishDTO implements RowMapper, Serializable {
    String xxx;
    Long xxx;
    Date xxx;
    BigDecimal xxx;

    @Override
    public WishDTO mapRow(ResultSet rs, int rowNum) {
        WishDTO dto = new WishDTO();
        Field[] fields = dto.getClass().getDeclaredFields();
        for (Field field : fields) {
            try {
                field.setAccessible(true);
                field.set(dto, rs.getObject(field.getName()));
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
        return dto;
    }

}

個人感覺讓 WishDTO 再實現實現一遍 RowMapper 有點麻煩，畢竟 WishDTO 實體類的所有字段都是需要賦值的，并沒有定制化需求。

所以想著有沒有更好地寫法，然后就翻了一下 jdbcTemplate 的方法，找到了一個自認為滿足自己這個需求的方法:

public  List queryForList(String sql, Class elementType)
即 將代碼改為：

public class xxx{

    xxx method(){
        ...
        List list = jdbcTemplate.queryForList(sql, WishDTO.class);
        ...
    }
}

@Data
public class WishDTO implements Serializable {
    String xxx;
    Long xxx;
    Date xxx;
    BigDecimal xxx;
}

一切看起來都很完美，但執行卻報錯了：Incorrect column count: expected 1, actual 13

經過一番對源碼進行debug，結合網上的一些資料，大概知道了是什么原因了，分析如下；

    public  List queryForList(String sql, Class elementType) throws DataAccessException {
        return query(sql, getSingleColumnRowMapper(elementType));
    }

其本質是還是調用public List query(String sql, RowMapper rowMapper) ，只是將 Class elementType 封裝成一個 RowMapper 實現實例;

    protected  RowMapper getSingleColumnRowMapper(Class requiredType) {
        return new SingleColumnRowMapper<>(requiredType);
    }

現在我們可以看一下 SingleColumnRowMapper 類的描述：

/**
 * {@link RowMapper} implementation that converts a single column into a single
 * result value per row. Expects to operate on a {@code java.sql.ResultSet}
 * that just contains a single column.
 *
 * 
The type of the result value for each row can be specified. The value
 * for the single column will be extracted from the {@code ResultSet}
 * and converted into the specified target type.
 */

其實從類名也可以看出，這是一個 RowMapper 的 簡單實現，且僅能接收一個字段的數據，如 String.class 和 Integer.class 等基礎類型；

網上的參考資料：https://blog.csdn.net/qq_4014...

使用 BeanPropertyRowMapper 進行封裝 ；
即 將代碼改為：

public class xxx{

    xxx method(){
        ...
        List list = jdbcTemplate.query(sql, new BeanPropertyRowMapper(WishDTO.class));
        ...
    }
}

@Data
public class WishDTO implements Serializable {
    String xxx;
    Long xxx;
    Date xxx;
    BigDecimal xxx;
}

接下來看一下 BeanPropertyRowMapper 的類描述：

/**
 * {@link RowMapper} implementation that converts a row into a new instance
 * of the specified mapped target class. The mapped target class must be a
 * top-level class and it must have a default or no-arg constructor.
 *
 * 
Column values are mapped based on matching the column name as obtained from result set
 * meta-data to public setters for the corresponding properties. The names are matched either
 * directly or by transforming a name separating the parts with underscores to the same name
 * using "camel" case.
 *
 * 
Mapping is provided for fields in the target class for many common types, e.g.:
 * String, boolean, Boolean, byte, Byte, short, Short, int, Integer, long, Long,
 * float, Float, double, Double, BigDecimal, {@code java.util.Date}, etc.
 *
 * 
To facilitate mapping between columns and fields that don"t have matching names,
 * try using column aliases in the SQL statement like "select fname as first_name from customer".
 *
 * 
For "null" values read from the database, we will attempt to call the setter, but in the case of
 * Java primitives, this causes a TypeMismatchException. This class can be configured (using the
 * primitivesDefaultedForNullValue property) to trap this exception and use the primitives default value.
 * Be aware that if you use the values from the generated bean to update the database the primitive value
 * will have been set to the primitive"s default value instead of null.
 *
 * 
Please note that this class is designed to provide convenience rather than high performance.
 * For best performance, consider using a custom {@link RowMapper} implementation.
 */

其作用就是講一個Bean class 轉化成相對應的 Bean RowMapper 實現類。

https://docs.spring.io/spring...

int rowCount = this.jdbcTemplate.queryForObject("select count(*) from t_actor", Integer.class);

int countOfActorsNamedJoe = this.jdbcTemplate.queryForObject("select count(*) from t_actor where first_name = ?", Integer.class, "Joe");

String lastName = this.jdbcTemplate.queryForObject("select last_name from t_actor where id = ?", new Object[]{1212L}, String.class);

Actor actor = this.jdbcTemplate.queryForObject(
        "select first_name, last_name from t_actor where id = ?",
        new Object[]{1212L},
        new RowMapper() {
            public Actor mapRow(ResultSet rs, int rowNum) throws SQLException {
                Actor actor = new Actor();
                actor.setFirstName(rs.getString("first_name"));
                actor.setLastName(rs.getString("last_name"));
                return actor;
            }
        });

List actors = this.jdbcTemplate.query(
        "select first_name, last_name from t_actor",
        new RowMapper() {
            public Actor mapRow(ResultSet rs, int rowNum) throws SQLException {
                Actor actor = new Actor();
                actor.setFirstName(rs.getString("first_name"));
                actor.setLastName(rs.getString("last_name"));
                return actor;
            }
        });

---

public List findAllActors() {
    return this.jdbcTemplate.query( "select first_name, last_name from t_actor", new ActorMapper());
}
private static final class ActorMapper implements RowMapper {
    public Actor mapRow(ResultSet rs, int rowNum) throws SQLException {
        Actor actor = new Actor();
        actor.setFirstName(rs.getString("first_name"));
        actor.setLastName(rs.getString("last_name"));
        return actor;
    }
}

this.jdbcTemplate.update(
        "insert into t_actor (first_name, last_name) values (?, ?)",
        "Leonor", "Watling");

this.jdbcTemplate.update(
        "update t_actor set last_name = ? where id = ?",
        "Banjo", 5276L);

this.jdbcTemplate.update(
        "delete from actor where id = ?",
        Long.valueOf(actorId));

this.jdbcTemplate.execute("create table mytable (id integer, name varchar(100))");

https://docs.spring.io/spring...