External Connection Pool - No Available Connections

Question

I am using Toplink 9.0.4 in an application deployed on WebLogic 7. Toplink is configured to use an external connection pool.
Under heavy load we sometimes exhaust the connection pool. When this occurs we start to get failures when trying to acquire connections.
It appears that when Toplink tries to obtain a connection from the pool and none are available the attempt immediately fails with an exception. What we would prefer to happen is for the thread to block and wait for a connection to become available.
Do you know of any way to produce this behavior? Is there a way to configure Toplink so this occurs?
Alternatively is there a practical way to produce this behavior by implementing logic in a ServerSession exception handler? We have implemented an exception handler that handles dropped and stale database connections, but I'm not sure if this is the appropriate place to handle a failure retrieving a connection from the external pool.
Any suggestions are appreciated.

Answer 1

Well I have tried but I am unable to write an effective exception handler that can recover from an exception retrieving a connection from an external pool. I wrote the following handler and attached it to the ServerSession.
     public Object handleException(RuntimeException exception) throws DatabaseException{
          if(exception instanceof DatabaseException){
               DatabaseException dbex = (DatabaseException)exception;
               String exceptionMessage = dbex.getMessage();
               if(exceptionMessage != null && exceptionMessage.indexOf("No available connections in pool") >= 0){
                    try{
                         Thread.sleep(5000);                    
                    }catch(InterruptedException e){}
                    oracle.toplink.publicinterface.Session session = null;
                    session = serverSession.acquireClientSession();
                    dbex.setSession(session);
                    if(session != null){
                         DatabaseQuery query = dbex.getQuery();
                         if(query != null){
                              return session.executeQuery(query);
                         }else{
                              return null;
                    throw exception;
               }else{
                    throw exception;
          }else{
               throw exception;
It works sometimes but other times I get strange errors when executing the query. I know the mappings are good, the queries succeed under normal circumstances. The following error is commmon:
Exception Description: The parameter name [UNIQUE_ID] in the query's selection criteria does not match any parameter name defined in the query.
The other problem I have is the recursive nature of the exception handling. If calling ServerSession.acquireClientSession() in the exception handler causes an exception, I end up in another exception handler. It's not clear which instance should execute and return the query in this case.
Any suggestions or examples of exception handlers for this situation are appreciated.