[Hibernate-issues] Re: [Hibernate] Hibernate Bug? [ConcurrentModificationException]

[William R. L. <wr...@ex...>]

On Sat, 27 Sep 2003, Juozas Baliuka wrote:

> It must be bug in "SessionHolder".

Hi, Juozas.  That code below was actuallly incorrect in my last email, and
I have corrected it in-line below: I am actually storing this.ftpSession
instead of this.session (line 71 in the original quoted code below).

As for the SessionHolder object, I'm using that so that I don't have
hundreds of connections to the back-end PostgreSQL database.  I've include
the code beow -- it should connect to the database if needed and just
return a connected net.sf.hibernate.Session object from its method.

01:public class SessionHolder
02:{
03:  private static final boolean DEBUG = true;
04:  private static Session session;
05:
06:  public static Session getSession()
07:  {
08:    SessionHolder.connect();
09:
10:    return(SessionHolder.session);
11:  }
12:
13:  public static void connect()
14:  {
15:    if (SessionHolder.session == null)
16:    {
17:      try
18:      {
19:        SessionFactory sessionFactory = new Configuration()
20:          .configure().buildSessionFactory();
21:        SessionHolder.session = sessionFactory.openSession();
22:      } catch (HibernateException e)
23:      {
24:        if (SessionHolder.DEBUG) e.printStackTrace();
25:      }
26:    } else
27:    {
28:      try
29:      {
30:        SessionHolder.session.reconnect();
31:      } catch (Exception e) {}
32:    }
33:  }
34:
35:  public static void disconnect()
36:  {
37:    if (SessionHolder.session != null)
38:    {
39:      try
40:      {
41:        SessionHolder.session.close();
42:        SessionHolder.session.disconnect();
43:      } catch (HibernateException e) {}
44:    }
45:  }
46:}

QHat do you think?  Is there a better way that I should be doing this 
without rewriting a heck of a lot of code?  I like the idea of having the 
session establishment self-contained within the SessionHolder object?

Does this SessionHolder seem to be the culprit behind the 
ConcurrentModificationException issues?

> > We seem to be having some problems like this in more than one section
> > of code. I really don't understand it at all; we suspect some kind of
> > JVM bug. Unfortunately, no-one has been able to deliver a testcase
> > that will help me reproduce this on my machine.

> > I can't really do much, until I can reproduce it.

> > P.S. You might also get this if you share a session between two
> > threads - but at least some occurrences of this problem do not seem to
> > be caused by this....

> > >I'm receiving an Exception in a Hibernate application I've built, and
> > >I'm a bit confused as to why this might be happening.  The code at
> > >where the Exception is thrown is quite simple, and I think this might
> > >be a bug in Hibernate itself.  I've searched Google for this to no
> > >avail, and I'm using the latest recommended Hibernate release as
> > >available on the site.

> > >The exception I receive has a stacktrace as follows:

> > > java.util.ConcurrentModificationException
> > >  at java.util.AbstractList$Itr.checkForComodification(Unknown Source)
> > >  at java.util.AbstractList$Itr.remove(Unknown Source)
> > >  at net.sf.hibernate.impl.SessionImpl.executeAll(SessionImpl.java:2101)
> > >  at net.sf.hibernate.impl.SessionImpl.execute(SessionImpl.java:2061)
> > >  at net.sf.hibernate.impl.SessionImpl.flush(SessionImpl.java:2005)
> > >  at net.sf.hibernate.transaction.JDBCTransaction.commit(
> > >       JDBCTransaction.java:57
> > >     )
> > >  at
> org.express.ftpsrv.ConnectionHandler.<init>(ConnectionHandler.java:72)
> > >  at
> org.express.ftpsrv.ConnectionListener.accept(ConnectionListener.java:56)
> > >  at
> org.express.ftpsrv.ConnectionListener.main(ConnectionListener.java:19)
> > >
> > >And the code at ConnectionHandler.java:72 that causes this looks like:
> > >
> > >  52:this.ftpSession = new FtpSession();
> > >  53:
> > >  54:/* ftpSession.setSessionId() does not operate on the unique
> > >  55: * 'id' field that is also part of the ftpSession object.
> > >  56: */
> > >  57:ftpSession.setSessionId(UniqueGenerator.makeUniqueNumber());
> > >  58:
> > >  59:Date currentDate = new Date();
> > >  60:currentDate.setTime(System.currentTimeMillis());
> > >  61:this.ftpSession.setBegTime(currentDate);
> > >  62:
> > >  63:net.sf.hibernate.Session hSession = null;
> > >  64:Transaction transaction = null;
> > >  65:
> > >  66:try
> > >  67:{
> > >  68:  hSession = SessionHolder.getSession();
> > >  69:
> > >  70:  transaction = hSession.beginTransaction();
> > >  71:  hSession.save(this.ftpSession);
> > >  72:  transaction.commit();
> > >  73:} catch (HibernateException e)
> > >  74:{
> > >  75:  transaction.rollback();
> > >  76:}

> > >I'd like to point out that this seems to be an intermittent thing,
> > >occurring more often when I instantiate many objects quickly (the
> > >quoted lines 52-72, above, are within a cronstructor in the Object).

> > >Does anyone have any ideas as to what might be happening or where I
> > >might look for additional information in order to track down this
> > >bug? Thanks, in advance, for any response on this.

--          _ 
__ __ ___ _| | William R. Lorenz <wr...@ex...> 
\ V  V / '_| | http://www.clevelandlug.net/ ; "Every revolution was 
 \./\./|_| |_| first a thought in one man's mind." - Ralph Waldo Emerson