how to start a .jar from another Java program?

as above. currently, i have a program with multiple unrelated functions... i wrote it like that because i wanted integration at first, but as the program became more complicated and the number of records in each database grew, the different parts made the main program become much slower (it's already very heavily Threaded and i've already set all Threads to Thread.MAX_PRIORITY already)... and if it screws up for some reason, i'll have to restart the WHOLE thing instead of the problematic part.

now, i'd like to split the whole program into smaller programs, but would still like the main GUI to *feel* integrated.

so, can anyone pls tell me the method to use to start a .jar (as well as pass in arguments if it's not just appending the arguments to the end of the file path) from another program?

thanks.