This method is called by the Queued Transaction Scheduler to determine whether the queue has a transaction that should be executed. If so, the method returns the transaction object. Otherwise, it returns null. The method should only return a transaction if the Arbortext PE サブプロセス pool is willing to execute transactions of the same type. The method can call the testRequest method of com.arbortext.e3.E3SubprocessPool to make the determination.