JdbcTemplate中queryForObject的空结果或不正确结果数量问题
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());}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();}