This method is called to ask the resource manager to prepare for a transaction commit of the transaction specified in xid.
If XA_OK is returned then any call other than xa_commit() or xa_abort() will throw exceptions. If XA_RDONLY is returned then any call other than destroy() will throw exceptions.
@return A value indicating the resource manager's vote on thethe outcome of the transaction. The possible values are: XA_RDONLY or XA_OK. If the resource manager wants to roll back the transaction, it should do so by throwing an appropriate XAException in the prepare method.
@exception StandardException Standard exception policy.