what is RMI TCP connection

The threads are used to feed a remote JMX client (in your case Java VisualVM) with data from your JVM.

Once you disconnect the threads should not allocate so much data anymore.

To verify this you can go to the Threads tab and look at the thread dump of a RMI TCP Connection thread. You should see that the RMI operations trigger JMX beans.

RMI is a Java API, which allows you to divide the implementation of parts of the same application on multiple computers. Do you use java.rmi library in your project?