Java sql 事务。我究竟做错了什么？

Question

我编写这个小测试的唯一目的是更好地理解 jdbc 中的事务。虽然我按照文档做了所有的事情，但测试并不希望正常工作。

表结构如下:

CREATE TABLE `default_values` (
   `id` INT UNSIGNED NOT auto_increment,
   `is_default` BOOL DEFAULT false,
   PRIMARY KEY(`id`)
);

测试包含 3 个类:

public class DefaultDeleter implements Runnable
{

    public synchronized void deleteDefault() throws SQLException
    {
        Connection conn = null;
        Statement deleteStmt = null;
        Statement selectStmt = null;
        PreparedStatement updateStmt = null;
        ResultSet selectSet = null;

        try
        {
            conn = DriverManager.getConnection("jdbc:mysql://localhost/xtest", "root", "");
            conn.setAutoCommit(false);
            conn.setTransactionIsolation(Connection.TRANSACTION_SERIALIZABLE);

            // Deleting current default entry
            deleteStmt = conn.createStatement();
            deleteStmt.executeUpdate("DELETE FROM `default_values` WHERE `is_default` = true");

            // Selecting first non default entry
            selectStmt = conn.createStatement();
            selectSet = selectStmt.executeQuery("SELECT `id` FROM `default_values` ORDER BY `id` LIMIT 1");

            if (selectSet.next())
            {
                int id = selectSet.getInt("id");

                // Updating found entry to set it default
                updateStmt = conn.prepareStatement("UPDATE `default_values` SET `is_default` = true WHERE `id` = ?");
                updateStmt.setInt(1, id);
                if (updateStmt.executeUpdate() == 0)
                {
                    System.err.println("Failed to set new default value.");
                    System.exit(-1);
                }
            }
            else
            {
                System.err.println("Ooops! I've deleted them all.");
                System.exit(-1);
            }

            conn.commit();
            conn.setAutoCommit(true);
        }
        catch (SQLException e)
        {
            try { conn.rollback(); } catch (SQLException ex)
            {
                ex.printStackTrace();
            }

            throw e;
        }
        finally
        {
            try { selectSet.close(); } catch (Exception e) {}
            try { deleteStmt.close(); } catch (Exception e) {}
            try { selectStmt.close(); } catch (Exception e) {}
            try { updateStmt.close(); } catch (Exception e) {}
            try { conn.close(); } catch (Exception e) {}
        }
    }

    public void run()
    {
        while (true)
        {
            try
            {
                deleteDefault();
            }
            catch (SQLException e)
            {
                e.printStackTrace();
                System.exit(-1);
            }

            try
            {
                Thread.sleep(20);
            }
            catch (InterruptedException e) {}
        }
    }

}

public class DefaultReader implements Runnable
{

    public synchronized void readDefault() throws SQLException
    {
        Connection conn = null;
        Statement stmt = null;
        ResultSet rset = null;

        try
        {
            conn = DriverManager.getConnection("jdbc:mysql://localhost/xtest", "root", "");

            conn.setAutoCommit(false);
            conn.setTransactionIsolation(Connection.TRANSACTION_SERIALIZABLE);

            stmt = conn.createStatement();
            rset = stmt.executeQuery("SELECT * FROM `default_values` WHERE `is_default` = true");

            int count = 0;
            while (rset.next()) { count++; }

            if (count == 0)
            {
                System.err.println("Default entry not found. Fail.");
                System.exit(-1);
            }
            else if (count > 1)
            {
                System.err.println("Count is " + count + "! Wtf?!");
            }

            conn.commit();
            conn.setAutoCommit(true);
        }
        catch (SQLException e)
        {
            try { conn.rollback(); } catch (Exception ex)
            {
                ex.printStackTrace();
            }

            throw e;
        }
        finally
        {
            try { rset.close(); } catch (Exception e) {}
            try { stmt.close(); } catch (Exception e) {}
            try { conn.close(); } catch (Exception e) {}
        }
    }

    public void run()
    {
        while (true)
        {
            try
            {
                readDefault();
            }
            catch (SQLException e)
            {
                e.printStackTrace();
                System.exit(-1);
            }

            try
            {
                Thread.sleep(20);
            }
            catch (InterruptedException e) {}
        }
    }

}

public class Main
{

    public static void main(String[] args)
    {
        try
        {
            Driver driver = (Driver) Class.forName("com.mysql.jdbc.Driver")
                    .newInstance();
            DriverManager.registerDriver(driver);

            Connection conn = null;
            try
            {
                conn = DriverManager.getConnection("jdbc:mysql://localhost/xtest", "root", "");
                System.out.println("Is transaction isolation supported by driver? " +
                        (conn.getMetaData()
                        .supportsTransactionIsolationLevel(
                        Connection.TRANSACTION_SERIALIZABLE) ? "yes" : "no"));
            }
            finally
            {
                try { conn.close(); } catch (Exception e) {}
            }

            (new Thread(new DefaultReader())).start();
            (new Thread(new DefaultDeleter())).start();

            System.in.read();
            System.exit(0);
        }
        catch (Exception e)
        {
            e.printStackTrace();
        }
    }

}

我已经编写了脚本，每次运行都会用 100k 条记录(其中一条是默认记录)填充表格。但是每次我运行这个测试时，输出都是:

Is transaction isolation supported by driver? yes

Default entry not found. Fail.

这段代码有什么问题？

Answer 1

请确保您正在创建 InnoDB 表，MyISAM(默认)不支持事务。您可以将您的数据库创建更改为:

CREATE TABLE `default_values` (
   `id` INT UNSIGNED NOT auto_increment,
   `is_default` BOOL DEFAULT false,
   PRIMARY KEY(`id`)
) Engine=InnoDB;

另一个例子:MySQL transaction with accounting application