MySQL ResultSet可滚动/可更新不按预期工作
我有一个测试JDBC程序，试图改变ResultSet的可滚动性和可更新性function。 不幸的是， TYPE_和CONCUR_所有组合似乎产生相同的结果（ TYPE_SCROLL_INSENSITIVE和CONCUR_READ_ONLY ）。
即使使用默认值（ TYPE_FORWARD_ONLY ），也可以滚动ResultSet。 谁能解释为什么会这样？
我使用的是MySQL 5.6和JDK7。 这是代码：
public class ResultSetTest3 { public static void main(String[] args) { Connection conn; try { conn = DriverManager.getConnection("jdbc:mysql://localhost/bd", "user", "password"); Statement sta = conn.createStatement(); sta.execute("DELETE FROM test"); sta.close(); PreparedStatement ps = conn.prepareStatement("INSERT INTO test VALUES(?, ?)"); for(int i=1; i<=100; i++) { ps.setInt(1, i); ps.setString(2, "Teste " + i); ps.addBatch(); } ps.executeBatch(); ps.close(); System.out.println("TYPE_FORWARD_ONLY CONCUR_READ_ONLY"); result(conn, ResultSet.TYPE_FORWARD_ONLY, ResultSet.CONCUR_READ_ONLY); System.out.println("==================================="); System.out.println("TYPE_SCROLL_INSENSITIVE CONCUR_READ_ONLY"); result(conn, ResultSet.TYPE_SCROLL_INSENSITIVE, ResultSet.CONCUR_READ_ONLY); System.out.println("==================================="); System.out.println("TYPE_SCROLL_SENSITIVE CONCUR_READ_ONLY"); result(conn, ResultSet.TYPE_SCROLL_SENSITIVE, ResultSet.CONCUR_READ_ONLY); System.out.println("==================================="); System.out.println("TYPE_FORWARD_ONLY CONCUR_UPDATABLE"); result(conn, ResultSet.TYPE_FORWARD_ONLY, ResultSet.CONCUR_UPDATABLE); System.out.println("==================================="); System.out.println("TYPE_SCROLL_INSENSITIVE CONCUR_UPDATABLE"); result(conn, ResultSet.TYPE_SCROLL_INSENSITIVE, ResultSet.CONCUR_UPDATABLE); System.out.println("==================================="); System.out.println("TYPE_SCROLL_SENSITIVE CONCUR_UPDATABLE"); result(conn, ResultSet.TYPE_SCROLL_SENSITIVE, ResultSet.CONCUR_UPDATABLE); System.out.println("==================================="); conn.close(); } catch (SQLException e) { e.printStackTrace(); } } private static void result(Connection conn, int type, int update) throws SQLException { Statement sta = conn.createStatement(ResultSet.TYPE_FORWARD_ONLY, ResultSet.CONCUR_READ_ONLY); ResultSet rs = sta.executeQuery("SELECT * FROM test"); System.out.println(rs.getConcurrency() + " " + update); System.out.println(rs.getType() + " " + type); try { rs.absolute(10); System.out.println(rs.getInt(1) + " - " + rs.getString(2)); rs.relative(20); System.out.println(rs.getInt(1) + " - " + rs.getString(2)); rs.previous(); System.out.println(rs.getInt(1) + " - " + rs.getString(2)); rs.first(); System.out.println(rs.getInt(1) + " - " + rs.getString(2)); try { System.out.println("AGORA!!!"); Thread.sleep(20000); } catch (Exception e) { System.out.println(e); } rs.absolute(3); System.out.println(rs.getInt(1) + " - " + rs.getString(2)); } catch(SQLException e) { System.out.println("Not Scrollable"); } try { rs.next(); rs.next(); rs.next(); rs.next(); rs.deleteRow(); rs.next(); rs.updateString(2, "TesteUpdate"); rs.insertRow(); } catch(SQLException e) { System.out.println("Not Updatable"); } rs.close(); sta.close(); } }
正如Mark Rotteveel在对该问题的评论中提到的那样，MySQL默认缓存ResultSet数据（在Ben J. Christensen的博客文章中也有讨论）。 这种缓存的一个明显的副作用是MySQL Connector / J将“升级”TYPE_FORWARD_ONLY ResultSet实际上是可滚动的：
Statement s = dbConnection.createStatement( ResultSet.TYPE_FORWARD_ONLY, ResultSet.CONCUR_READ_ONLY); ResultSet rs = s.executeQuery("SELECT * FROM testdata"); rs.last(); System.out.println(String.format("Current row number: %d", rs.getRow())); rs.previous(); System.out.println(String.format("Current row number: %d", rs.getRow()));
显示器
Current row number: 3 Current row number: 2
根据上面引用的博客文章，防止缓存和“流”ResultSet数据的方法是使用Statement.setFetchSize ：
Statement s = dbConnection.createStatement( ResultSet.TYPE_FORWARD_ONLY, ResultSet.CONCUR_READ_ONLY); s.setFetchSize(Integer.MIN_VALUE); ResultSet rs = s.executeQuery("SELECT * FROM testdata"); rs.next(); System.out.println("Data from first row: " + rs.getString(2)); System.out.println("now let's try rs.last() ..."); try { rs.last(); System.out.println("... Okay, done."); } catch (Exception e) { System.out.println("... Exception: " + e.getMessage()); }
导致
Data from first row: Gord now let's try rs.last() ... ... Exception: Operation not supported for streaming result sets
尝试以这种方式创建Scroll-Insensitive ， 只读 ResultSet对象语句对象。
Statement sta = conn.createStatement( ResultSet.TYPE_SCROLL_INSENSITIVE, ResultSet.CONCUR_READ_ONLY);
代替
Statement sta = conn.createStatement();
对PreparedStatement也这样做。
通过此
ResultSet类型，光标可以向任何方向移动。 它不敏感，这意味着结果集不能反映在打开时所做的更改。 它是MySql的默认结果集类型。
必须阅读 从结果集中检索和修改值