JDBC使用SELECT FOR UPDATE锁定一行,不起作用

Question

我遇到MySQL的SELECT问题.. FOR UPDATE,这是我试图运行的查询:

SELECT * FROM tableName WHERE HostName='UnknownHost' 
        ORDER BY UpdateTimestamp asc limit 1 FOR UPDATE

在此之后,相关的线程将执行UPDATE并更改HostName,然后它应该解锁该行.

我正在运行一个多线程java应用程序,所以3个线程正在运行这个SQL语句,但是当线程1运行它时,它不会从线程2和3锁定其结果.因此线程2和3得到相同的结果,他们可以更新同一行.

每个线程也都有自己的mysql连接.

我正在使用Innodb,事务隔离= READ-COMMITTED,并且在执行select for update之前自动提交已关闭

我可能会错过什么吗？或许还有更好的解决方案？非常感谢.

代码:

public BasicJDBCDemo()
{
    Le_Thread newThread1=new Le_Thread();
    Le_Thread newThread2=new Le_Thread();
    newThread1.start();
    newThread2.start();         
}

线程:

class Le_Thread extends Thread  
{

    public void run() 
    {
    tring name = Thread.currentThread().getName();
        System.out.println( name+": Debut.");
    long oid=Util.doSelectLockTest(name);
    Util.doUpdateTest(oid,name);        
    }

}

选择 :

public  static long doSelectLockTest(String threadName)
  {
    System.out.println("[OUTPUT FROM SELECT Lock ]...threadName="+threadName);
    PreparedStatement pst = null;
    ResultSet rs=null;
    Connection conn=null;
    long oid=0;
    try
    {
     String query = "SELECT * FROM table WHERE Host=? 
                               ORDER BY Timestamp asc limit 1 FOR UPDATE";


      conn=getNewConnection();
      pst = conn.prepareStatement(query);
      pst.setString(1, DbProperties.UnknownHost);
      System.out.println("pst="+threadName+"__"+pst);
      rs = pst.executeQuery();

      if (rs.first())
      {
        String s = rs.getString("HostName");
        oid = rs.getLong("OID");
        System.out.println("oid_oldest/host/threadName=="+oid+"/"+s+"/"+threadName);

      }   

    }
    catch (SQLException ex)
    {
      ex.printStackTrace();
    }
    finally
    {
        DBUtil.close(pst);
        DBUtil.close(rs);
        DBUtil.close(conn);
    }
    return oid;
  }

请帮忙.... :

结果:

Thread-1: Debut.
Thread-2: Debut.
[OUTPUT FROM SELECT Lock ]...threadName=Thread-1
New connection..
[OUTPUT FROM SELECT Lock ]...threadName=Thread-2
New connection..
pst=Thread-2: SELECT * FROM b2biCheckPoint  WHERE HostName='UnknownHost' ORDER BY UpdateTimestamp asc limit 1 FOR UPDATE
pst=Thread-1: SELECT * FROM b2biCheckPoint  WHERE HostName='UnknownHost' ORDER BY UpdateTimestamp asc limit 1 FOR UPDATE
oid_oldest/host/threadName==1/UnknownHost/Thread-2
oid_oldest/host/threadName==1/UnknownHost/Thread-1
[Performing UPDATE] ... oid = 1, thread=Thread-2
New connection..
[Performing UPDATE] ... oid = 1, thread=Thread-1
pst_threadname=Thread-2: UPDATE b2bicheckpoint SET HostName='1_host_Thread-2',UpdateTimestamp=1294940161838 where OID = 1
New connection..
pst_threadname=Thread-1: UPDATE b2bicheckpoint SET HostName='1_host_Thread-1',UpdateTimestamp=1294940161853 where OID = 1

Answer 1

你非常困惑,但至少在编辑后事情看起来更好.有多种方法可以做到这一点,但我发现的最好方法是实际使用JDBC的ResultSet.update*方法:

首先,您需要SELECT ... FOR UPDATE使用ResultSet.CONCUR_UPDATABLE参数准备语句,如下所示:

ps = conn.prepareStatement(query,
                           ResultSet.TYPE_FORWARD_ONLY,
                           ResultSet.CONCUR_UPDATABLE);

然后,您必须使用ResultSet实际更新表:

if(rs.next())
{
    rs.updateString(columnIndex, "new_hostname");
    rs.updateRow();
}

第三,您可能需要使用我可以在更新中看到的事务.希望你的DbUtil.close方法不会抛出任何异常,检查null等等.另外,如果你的方法变得更复杂,你也应该有回滚逻辑.

您不应该my.ini因任何原因进行修改.

Answer 2

您创建的选择更新的连接必须与用于执行更新的连接相同。否则，它不是同一事务的一部分，并且它会释放锁，因此其他线程也开始执行它。所以在你的代码中你需要这样做：

if (rs.first())
  {
    String s = rs.getString("HostName");
    oid = rs.getLong("OID");
    System.out.println("oid_oldest/host/threadName=="+oid+"/"+s+"/"+threadName);

  }   
Util.doUpdateTest(oid,name,conn);
conn.commit();