How to compile other java files and run them from my own programs?

I'm developing a unit testing tool for AspectJ, which can originally generate the stubs for aspect unit to be tested. All these stubs are java files. After generation of stubs, how can I program the code to compile this unit together with these stubs, i.e. how can I put the functionality of compilation and run, e.g. command "javac *.java" and "java *", into my own code.
Hope you can understand what I mean.
Thank you in advance.

When I use Runtime.getRuntime().exec(command) for executing the AspectJ compile command "ajc", "IOException createProcess error=2" always occurs, while it is ok for Java comiple command "javac".
Why this happens when I use Runtime.getRuntime().exec("ajc Test.aj")? Is it the reason that Runtime.exec(String) may not support any command in DOS?
(ajc is the compile command for the java file and aj file.)
Thank you in advance.