java - MySQL 结果集可滚动/可更新未按预期工作

Question

我有一个测试 JDBC 程序，它尝试更改 ResultSet 的可滚动性和可更新性功能。不幸的是，TYPE_ 和 CONCUR_ 的所有组合似乎都会产生相同的结果(TYPE_SCROLL_INSENSITIVE 和 CONCUR_READ_ONLY)。

即使使用默认值(TYPE_FORWARD_ONLY)，也可以滚动结果集。谁能解释一下这是为什么？

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();
    }
}

Answer 1

正如 Mark Rotteveel 在对该问题的评论中提到的那样，MySQL 默认情况下会缓存 ResultSet 数据(也在博客文章中进行了讨论) 本·克里斯滕森 here )。这种缓存的一个明显的副作用是 MySQL Connector/J 会将 TYPE_FORWARD_ONLY 结果集“升级”为实际上可滚动的:

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

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());
}

结果

Data from first row: Gord
now let's try rs.last() ...
... Exception: Operation not supported for streaming result sets