To allow proper sandboxing and let users run their own script, it is necessary to control the time a script is allowed to run and avoid runaway executions (for instance while(true); ).
Support for asynchronuous execution is usually based on Runnable/Callable/Future and Thread interruption. The Callable interface seems the most versatile so a method to transform a Script into a Callable seems in order. Thread interruption is in most cases an indication that must be checked (besides locking methods) somehow.
Creating a Callable from a Script, Context (and arguments) is straightforward. Checking thread interruption can be performed in the Interpreter loop at "key" points namely getting/setting properties, calling methods/functions, resolving identifiers and of course, within loop constructs (for, while).