Using Process Runtime.getRuntime().exec

Question

http://im1.shutterfly.com/procserv/47b4d633b3127cceb46774c84ee80000001610
Here is a URL to a part of the current process I need to run. Actually there are 4 steps that preceed these but they are ran in order and must be done before these (that part is easy). Sorry it kinda chopped off the left side a little bit.
My question is, can I do this entirely using Runtime.getRuntime().exec() commands or do I need to involve multi threading. The reason I am asking is because P13 cannot run until P8 and P9 completes, but that shouldn't stop the rest of the program from running.
If I have something like
Process P4 = Runtime.getRuntime().exec(P4);
Process P5 = Runtime.getRuntime().exec(P2);
// Some handling for the streams of these processes goes here
P4.waitFor();
P2.waitFor();
if(P2.exitValue() != 0){
Process P5 = Runtime.getRuntime().exec(P5);
Process P6 = Runtime.getRuntime().exec(P6);
// Some handling for the streams of these processes goes here
Does that mean the whole program will stop and wait until P4 is done before even checking for P2? If P2 is finished would the program continue and run P5 and P6 even if P4 is still running or will it wait for P4 to complete also. P4 has nothing to do with P2?
Also any advice ???

Answer 1

P4 and P2 will both be running in parallel (or as much as your OS and hardware allows them to be). As soon as both are done, and regardless of which finishes first, whatever follows P2.waitFor() will execute.
If you have multiple groups of processes, where the processes within a group must execute sequentially, but the groups themselves can run independently of each other, then you'll need to either use Java's threads, or wrap each group of sequential processes in a shell script or batch file the executes that group sequentially.
If there are complex dependencies among the groups--that is, multiple groups must wait on the same process, or one group must wait for multiple other groups, then it might be easier to control the concurrency in Java.