Signal Handlers: In some operating systems, signal handlers are executed on a thread stack; they “overlay” the current thread execution (which may be used, for example, in combination with setjmp/longjmp to create user-domain lightweight threads). The thread model supported by modeling languages often cannot fully model signal handlers. This is certainly true in the case of a language like Java. Real-Time Specification for Java tries to address this to some extent, but it is not a complete solution.