java.lang.OutOfMemoryError: unable to create new native thread

OS：Red Hat Enterprise Linux Server release 6.4 (Santiago)
Jboss：jboss-eap-6.2

I maintain a program.
It's copy folder A file to folder B.
I check the code. It use Thread to do.
But if the folder A files too over(Maybe 500 files or 1000 files)

system give me a error message:

Message: java.lang.reflect.InvocationTargetException
java.lang.OutOfMemoryError: unable to create new native thread

I don't know what happened.