JdbcTemplate中queryForObject的EmptyResultDataAccessException问题

Posted 2020-08-19 Mr. Ma

在使用Spring提供的JdbcTemplate中名为queryForObject API进行数据库查询时有时会抛出如下异常提示息，org.springframework.dao.EmptyResultDataAccessException: Incorrect result size: expected 1, actual 0 或者 org.springframework.dao.IncorrectResultSizeDataAccessException: Incorrect result size: expected 1, actual 2 

在解决这些异常之前，我们首先来看看queryForObject API的源代码，假设我们调用的是queryForObjec(String sql, Class requiredType)。 

JdbcTemplate.class 

public <T> T queryForObject(String sql, Class<T> requiredType) 
		throws DataAccessException 
{return queryForObject(sql, getSingleColumnRowMapper(requiredType));}

public <T> T queryForObject(String sql, RowMapper<T> rowMapper) 
		throws DataAccessException {
    List<T> results = query(sql, rowMapper);
    return DataAccessUtils.requiredSingleResult(results);
}

public <T> List<T> query(String sql, RowMapper<T> rowMapper) 
		throws DataAccessException {
    return query(sql, new RowMapperResultSetExtractor<T>(rowMapper));
}

public <T> T query(final String sql, final ResultSetExtractor<T> rse) 
		throws DataAccessException {
	Assert.notNull(sql, "SQL must not be null");
	Assert.notNull(rse, "ResultSetExtractor must not be null");
	if (logger.isDebugEnabled()) {
		logger.debug("Executing SQL query [" + sql + "]");
	}
	class QueryStatementCallback implements StatementCallback<T>, SqlProvider {
		public T doInStatement(Statement stmt) throws SQLException {
			ResultSet rs = null;
			try {
				rs = stmt.executeQuery(sql);
				ResultSet rsToUse = rs;
				if (nativeJdbcExtractor != null) {
					rsToUse = nativeJdbcExtractor.getNativeResultSet(rs);
				}
				return rse.extractData(rsToUse);
			}
			finally {
				JdbcUtils.closeResultSet(rs);
			}
		}
		public String getSql() {
			return sql;
		}
	}
	return execute(new QueryStatementCallback());
}

　　DataAccessUtils.class 

public static <T> T requiredSingleResult(Collection<T> results) 
        throws IncorrectResultSizeDataAccessException {
    int size = (results != null ? results.size() : 0);
    if (size == 0) {
        throw new EmptyResultDataAccessException(1);
    }
    if (results.size() > 1) {
        throw new IncorrectResultSizeDataAccessException(1, size);
    }
    return results.iterator().next();
}

通过阅读源代码，可以很清楚看到在DataAccessUtils.class中requiredSingleResult方法中，当结果集合的size为0或者大于1时，就会抛出以上两个异常。 

为了避免出现以上的异常，最好还是使用queryForList 或者query来进行查询，这样返回的结果为List，即使返回的结果集合的size为0，即Zero Row，也不会抛出异常。如果size不为0，即可以使用get(0)取得第一个查询对象。