MySQL ResultSet可滚动/可更新无法按预期工作

• Z时代
• 2024-01-10
• 分类：问答

我有一个测试JDBC程序，该程序试图更改ResultSet的可滚动性和可更新性功能。不幸的是，所有组合TYPE_，并CONCUR_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），也可以在ResultSet中滚动。谁能解释为什么？

我正在使用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();
    }
}

回答：

正如马克Rotteveel到问题的评论中提到，MySQL的高速缓存默认的ResultSet数据（也由Ben J.

Christensen的一篇博客文章中讨论这里）。这种缓存的明显副作用是MySQL Connector / J将“升级”

TYPE_FORWARD_ONLY ResultSet使其实际上是可滚动的：

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

Current row number: 3
Current row number: 2

根据上面引用的博客文章，防止缓存和“流式处理” 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

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