spring jdbc包提供了JdbcTemplate和它的兩個兄弟SimpleJdbcTemplate和NamedParameterJdbcTemplate,我們先從JdbcTemplate入手,
引入問題,領略一下類NamedParameterJdbcTemplate在處理where中包含in時的便利和優雅。
首先創建實體類Employee:
1 public class Employee { 2 private Long id; 3 private String name; 4 private String dept; 5 // omit toString, getters and setters 6 }
使用JdbcTemplate訪問一批數據
比較熟悉的使用方法如下:
public List<Employee> queryByFundid(int fundId) { String sql = "select * from employee where id = ?"; Map<String, Object> args = new HashMap<>(); args.put("id", 32); return jdbcTemplate.queryForList(sql, args , Employee.class ); }
但是,當查詢多個部門,也就是使用in的時候,這種方法就不好用了,只支持Integer.class String.class 這種單數據類型的入參。如果用List匹配問號,你會發現出現這種的SQL:
select * from employee where id in ([1,32])
執行時一定會報錯。解決方案——直接在Java拼湊入參,如:
String ids = "3,32";
String sql = "select * from employee where id in (" + ids +")";
如果入參是字符串,要用兩個''號引起來,這樣跟數據庫查詢方式相同。示例中的入參是int類型,就沒有使用單引號。但是,推薦使用NamedParameterJdbcTemplate類,然后通過: ids方式進行參數的匹配。
使用NamedParameterJdbcTemplate訪問一批數據
public List<Employee> queryByFundid(int fundId) { String sql = "select * from employee where id in (:ids) and dept = :dept"; Map<String, Object> args = new HashMap<>();
args.put("dept", "Tech"); List<Integer> ids = new ArrayList<>(); ids.add(3); ids.add(32); args.put("ids", ids); NamedParameterJdbcTemplate givenParamJdbcTemp = new NamedParameterJdbcTemplate(jdbcTemplate); List<Employee> data = givenParamJdbcTemp.queryForList(sql, args, Employee.class); return data; }
如果運行以上程序,會采坑,拋出異常:org.springframework.jdbc.IncorrectResultSetColumnCountException: Incorrect column count: expected 1, actual 3。
拋出此異常的原因是方法queryForList 不支持多列,但是,API(spring-jdbc-4.3.17.RELEASE.jar)並未說明。請看queryForList 在JdbcTemplate的實現:
public <T> List<T> queryForList(String sql, SqlParameterSource paramSource, Class<T> elementType) throws DataAccessException { return query(sql, paramSource, new SingleColumnRowMapper<T>(elementType)); } /** * Create a new {@code SingleColumnRowMapper}. * <p>Consider using the {@link #newInstance} factory method instead, * which allows for specifying the required type once only. * @param requiredType the type that each result object is expected to match */ public SingleColumnRowMapper(Class<T> requiredType) { setRequiredType(requiredType); }
查詢API發現,需要換一種思路,代碼如下:
public List<Employee> queryByFundid(int fundId) { String sql = select * from employee where id in (:ids) and dept = :dept Map<String, Object> args = new HashMap<>(); args.put("dept", "Tech"); List<Integer> ids = new ArrayList<>(); ids.add(3); ids.add(32); args.put("ids", ids); NamedParameterJdbcTemplate givenParamJdbcTemp = new NamedParameterJdbcTemplate(jdbcTemplate); List<Employee> data = givenParamJdbcTemp.jdbc.query(sql, args, new RowMapper<Employee>() { @Override public Employee mapRow(ResultSet rs, int index) throws SQLException { Employee emp = new Employee(); emp.setId(rs.getLong("id")); emp.setName(rs.getString("name")); emp.setDept(rs.getString("dept")); return emp; } }); return data; }
歡迎拍磚。