【发布时间】：2009-08-11 14:00:44
【问题描述】：
我编写了这个小测试，唯一的目的是为了更好地理解 jdbc 中的事务。尽管我按照文档进行了所有操作，但测试并不希望正常工作。
这是表结构：
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();
}
}
}
我编写了脚本，每次运行都会用 10 万条记录（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：
驱动是否支持事务隔离？是的
未找到默认条目。失败。
这段代码有什么问题？
【问题讨论】：
-
我看到一些答案开始谈论声明性事务和使用 Spring（我确实喜欢 Spring），但他们只会混淆情况。这个问题是可以解决的，并且您将更好地理解使用原始 JDBC，如果您开始将其包装在更高级别的框架（例如 JEE 或 Spring）中，这将变得模糊不清。从长远来看，我建议不要使用原始 JDBC，除非您处于真正受限的环境中。它很容易出错，而且是 PITA。
-
你为什么认为 DefaultReader 应该成功？仅仅因为线程在删除器之前启动，并不意味着它会在删除发生之前实际运行？请记住，MyISAM 表不支持事务，InnoDB 表支持。 SERIALIZABLE 隔离级别可能会失败，如果有 2 个并发 SERIALIZABLE 事务正在进行，dbs 通常会失败事务。
标签： java jdbc transactions