How can I process Unacknowledged messages in Activemq

How can I process Unacknowledged messages in Activemq

I wrote a class in Java name as ExternalFileProcess.java for processing
externalFile(.exe). I have written one more class name as MyListener for
processing messages in Activemq.
Whenever JMS Provider(Activemq) gets the messages, subscriber needs to run
External(.EXE) file.For this I called ExternalFileProcess.java in
MyListener.It's working fine.
Now I am trying to implement error handling. Assume If external(.EXE) file
process is crashed because of some reasons, I want to resend processing
message to JMS Provider(Activemq).
If I resend processing message from MyListener to Activemq, I will process
that message again and never lose any message.
if (Whether process is successfully done or not) {
//process completed sucessfully
msg.acknowledge();
}else {
//Process Not completely done
Here I want to resend the message to Activemq. If I resend
messages,Activemq deliver that message to
Subscriber(MessageListener) so I can process that message one more
time.
}
Thanks.