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From "Asif H. Shahid (JIRA)" <j...@apache.org>
Subject [jira] Created: (DERBY-2142) NullPointerException while using XAConnection/PooledConnection in a heavily contended multithreaded scenario
Date Mon, 04 Dec 2006 10:56:21 GMT
NullPointerException while using XAConnection/PooledConnection in a heavily contended multithreaded
scenario
------------------------------------------------------------------------------------------------------------

                 Key: DERBY-2142
                 URL: http://issues.apache.org/jira/browse/DERBY-2142
             Project: Derby
          Issue Type: Bug
          Components: JDBC
    Affects Versions: 10.2.1.6, 10.1.3.1, 10.1.2.1, 10.1.1.0, 10.0.2.1, 10.0.2.0
         Environment: The issue can appear in any environment as the bug is in the source
code . The bug has been verified in Suse Linux env.
            Reporter: Asif H. Shahid
            Priority: Critical


We are using the Derby's Transactional DataSource  class ( org.apache.derby.jdbc.EmbeddedXADataSource
) to create a pool of  XAConnections in our application.
Whenever a thread in a JTA transaction requests for a SQLConnection , we retrieve an XAConnection
from the pool. From the XAConnection , we  register the XAResource with the TransactionManager
& return a java.sql.Connection to the application. 
A class implementing the ConnectionEventListener is registered with the XAConnection to get
callback  connectionClosed ( ) when the thead closes the java.sql.Connection. In this callback,
 we invoke XAResource.end & return the XAConnection to our pool  so that other threads
can use it.

We have encountered NullPointerException  , when performing operation on java.sql.Connection.
The stacktrace is as follows
at
org.apache.derby.jdbc.XAStatementControl.<init>(XAStatementControl.java:71)
   at
org.apache.derby.jdbc.EmbedXAConnection.wrapStatement(EmbedXAConnection.java:162)
   at
org.apache.derby.iapi.jdbc.BrokeredConnection.createStatement(Unknown
Source)
   at
com.gemstone.gemfire.internal.datasource.ConnectionPoolingTest$1.run(ConnectionPoolingTest.java:174)
   at java.lang.Thread.run(Thread.java:595)


I have done some debugging on source code of  db-derby-10.2.1.6-src & have following explanation
of the bug & a suggested fix. However, I want to confirm that it is genuinely a bug &
not a problem in our understanding of the Datasource spec behaviour.

Reason for the bug:-

The class EmbedPooledConnection.java   stores in the field currentConnectionHandle ( of  class
BrokeredConnection)  a reference of the java.sql.Connection object , being returned to the
application, 
Now ,whenever the client closes the java.sql.Connection ,  the code flow is 

EmbedPooledConnection.close() --> EmbedPooledConnection.notifyClose().
In the function EmbedPooledConnection.notifyClose(), it notifies  my listener ( javax.sql.ConnectionEventListener)
) where I return the XAConnection to the pool ( after deregistering the XAResource). 
The last line of EmbedPooledConnection.close()  makes the currentConnectionHandle  field as
null.

The issue here is that  javax.sql.ConnectionEventListener.connectionClosed is invoked before
making the currentConenctionHandle field as null.  Thus XAConnection is returned to the pool
, ready for pickup by a new thread. This new thread obtains a java.sql.Connection whose reference
gets assigned to the currentConnectionHandle field, meanwhile the previous thread completes
the EmbedPooledConnection.close  making the newly assigned currentConnectionHandle as null.

Thus a previous thread's close makes a field null of an XAConnection, which has been assigned
to a new thread.
The bug is easily reproducible  in a multi threaded scenario ( 20 threads or so) with a pool
size of around 4 XAConnections so that there is heavy  contention on XAConnection. 

The fix is to rearrange the code of EmbedPooledConenction.java 's closingConnection () as

bug :
public  boolean closingConnection() {
     notifyClose();
     currentConnectionHandle = null;
    return false;

}

bug fix :
public  boolean closingConnection() {
     currentConnectionHandle = null;
     notifyClose();
     return false;
}










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