JdbcTemplate ResultSetExtractor 执行报异常：No current row in the ResultSet

同一个查询，使用：

List<T> query(String sql, RowMapper<T> rowMapper, Object... args)

void query(String sql, Object[] args, RowCallbackHandler rch)

这两个方法可以执行成功，但是

T query(String sql, ResultSetExtractor<T> rse, Object... args)

执行时会报如下异常信息：

Caused by: java.sql.SQLException: No current row in the ResultSet.

代码为：

String sql = "select id, name from user where id = ?";

// 1. RowMapper
List<User> users = jdbcTemplate.query(sql, (rs, rowMap) -> {
    User user = new User();
    user.setId(rs.getInt("id"));
    user.setName(rs.getString("name"));
    return user;
}, userId);

// 2. RowCallbackHandler
User user = new User();
jdbcTemplate.query(sql, new Object[]{damId}, rs -> {
    user.setId(rs.getInt("id"));
    user.setName(rs.getString("name"));
}, userId);

// 3. ResultSetExtractor
return jdbcTemplate.query(sql, rs -> {
    User user = new User();
    user.setId(rs.getInt("id"));
    user.setName(rs.getString("name"));
    return user;
}, userId);

第一种和第二种写法中的 RowMapper 和 RowCallbackHandler 对象分别构造了 RowMapperResultSetExtractor 和
RowCallbackHandlerResultSetExtractor 对象，它们都是 ResultSetExtractor 的子类。

ResultSetExtractor 类中的 extractData 方法定义为：

T extractData(ResultSet rs) throws SQLException, DataAccessException;

RowMapperResultSetExtractor 类定义为：

public class RowMapperResultSetExtractor<T> implements ResultSetExtractor<List<T>> {

	private final RowMapper<T> rowMapper;

	private final int rowsExpected;


	/**
	 * Create a new RowMapperResultSetExtractor.
	 * @param rowMapper the RowMapper which creates an object for each row
	 */
	public RowMapperResultSetExtractor(RowMapper<T> rowMapper) {
		this(rowMapper, 0);
	}

	/**
	 * Create a new RowMapperResultSetExtractor.
	 * @param rowMapper the RowMapper which creates an object for each row
	 * @param rowsExpected the number of expected rows
	 * (just used for optimized collection handling)
	 */
	public RowMapperResultSetExtractor(RowMapper<T> rowMapper, int rowsExpected) {
		Assert.notNull(rowMapper, "RowMapper is required");
		this.rowMapper = rowMapper;
		this.rowsExpected = rowsExpected;
	}


	@Override
	public List<T> extractData(ResultSet rs) throws SQLException {
		List<T> results = (this.rowsExpected > 0 ? new ArrayList<T>(this.rowsExpected) : new ArrayList<T>());
		int rowNum = 0;
		while (rs.next()) {
			results.add(this.rowMapper.mapRow(rs, rowNum++));
		}
		return results;
	}

}

第一种写法中形参表里的 lambda 表达式就是 rowMapper 的匿名构造类。

RowCallbackHandlerResultSetExtractor 类定义为：

private static class RowCallbackHandlerResultSetExtractor implements ResultSetExtractor<Object> {

    private final RowCallbackHandler rch;

    public RowCallbackHandlerResultSetExtractor(RowCallbackHandler rch) {
        this.rch = rch;
    }

    @Override
    public Object extractData(ResultSet rs) throws SQLException {
        while (rs.next()) {
            this.rch.processRow(rs);
        }
        return null;
    }
}

第二种写法中形参表里的 lambda 表达式就是 rch 的匿名构造类。

所以前两种写法，已经默认执行了循环解析 ResultSet 结果集这个操作。而第三种写法需要自己重写
ResultSetExtractor 类的 extractData() 方法。所以将第三种写法改成以下形式即可：

// ResultSetExtractor
return jdbcTemplate.query(sql, rs -> {
    if (rs.next()) {
        User user = new User();
        user.setId(rs.getInt("id"));
        user.setName(rs.getString("name"));
        return user;
    }
    return null;
}, userId);