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From James House <ja...@interobjective.com>
Subject RE: [POOL] Bug
Date Wed, 03 Apr 2002 16:36:27 GMT

I think this is reasonable... of course I am then going to desire and good 
implementation of equals() on PoolableConnection ;-)

Which I suppose could be as simple as this:

     public boolean equals(Object obj) {
       if(this.hashCode() == obj.hashCode())
         return true;

       return false;
     }

or even this:

     public boolean equals(Object obj) {
       if(this == obj)
         return true;

       return false;
     }

-- Since it IS the same object that we're worried about.

James


At 4/2/2002 04:52 PM -0600, you wrote:
>To clarify, I think it's pretty much as simple as this:
>
>public class CheckingObjectPool extends GenericObjectPool {
>   public void returnObject(Object obj) throws Exception {
>     if(_pool.contains(obj)) {
>        throw new Exception(obj + " is already in the pool.");
>     }
>   }
>}
>
>to extend GenericObjectPool with this behavior.
>
>-----Original Message-----
>From: Waldhoff, Rodney [mailto:rwaldhof@us.britannica.com]
>Sent: Monday, April 01, 2002 12:25 PM
>To: 'Jakarta Commons Developers List'
>Subject: RE: [POOL] Bug
>
>
> > This could easily be done by doing a "contains()"
> > check on the collection that backs the pool before
> > adding the object to it.
>
>If and only if the equality method identifies unique instances of objects
>within the pool.
>
>You could implement a [Keyed]ObjectPool to do this, or perhaps better still,
>implement it via the passivateObject method of [Keyed]PoolableObjectFactory.
>Then it would be tied to the top of object being pooled, when you could
>enforce a meaningful contains() method.
>
>I'm not sure it makes sense to do this check in the general case, since, as
>you note:
>
> > if there is code that returns the object to the pool more
> > than once, then that code itself is buggy,
>
>What do the rest of you think?
>
>-----Original Message-----
>From: James House [mailto:jhouse@part.net]
>Sent: Monday, April 01, 2002 10:26 AM
>To: commons-dev@jakarta.apache.org
>Subject: [POOL] Bug
>
>
>
>I've ran into another bug with Pool / DBCP.
>
>The problem lies in GenericObjectPool's returnObject() method.
>
>There is no check to see if the object has already been returned, it simply
>adds the object back into the pool.
>
>If the object has already been returned, then the object now appears two or
>more times in the pool.  This leads to serious problems.  For one, it leads
>to multiple threads "borrowing" the same object at the same time.  It also
>makes the pool "fill up" which causes very serious problems -- once the
>maxIdle pool count is reached (which happens quickly in this scenario), the
>pool starts "tossing" out objects that are being returned.  It does so by
>calling "destroyObject".  The problem with this is that the object (in this
>case, a DB connection) gets destroyed, but there are multiple pointers to
>it still in the pool!  - the next time the object is borrowed from the pool
>it is completely invalid! (the connection has been closed).
>
>I understand that if there is code that returns the object to the pool more
>than once, then that code itself is buggy, however I think there should
>also be real safeguards in the pool (returnObject()) that disallow the same
>object to be in the pool multiple times.  This could easily be done by
>doing a "contains()" check on the collection that backs the pool before
>adding the object to it.  If the object is found to already be in the pool,
>an exception should be thrown so that the code utilizing the pool can be
>corrected.  Perhaps "contains()" is not the most efficient way to do the
>check (if the pool is large) -- but it's probably the most quick and easy
>way to make the fix, as it simply adds 2 lines of code:  a line to test the
>condition, and another line to throw an exception.
>
>James
>
>
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