I resolved the issue by my self, for those who are facing the same problem , i am writing solution for them,
Actually In a decision handler when you try to initialize the beanshell engine , you have ProcessContextClassLoader but actually you suppose to have webContextClassloader so it is not able to load the BeanShellEngine because it is not able to find the relavent classes that are available in a jar for beanshell. You actually have WebContextClass Loader , following code in decision handler will work
ClassLoader oldClassLoader = Thread.currentThread().getContextClassLoader(); // This code will give you ProcessClassLoader
Thread.currentThread().setContextClassLoader(Thread.currentThread().getContextClassLoader().getParent());
//Above line will set WebContextClassLoader in current thread's classloader now you can write you Beanshell related stuff
and finally in finally block you can set your class loader to
Thread.currentThread().setContextClassLoader(oldClassloader); // that is your processClass Loader
Thanks
Sunay