Would an inner static runnable in a singleton block if used as wrapper for callables, if it modifies a singleton's static param?

Question

1) Is there a chance that an inner static Runnable class can block if it modifies a static ConcurrentHashMap in the outer singleton, after it calls a Callable? I'm thinking about a scenario where the modification is made to the same item in the map by multiple runnables at the same time. Multiple runnables are ran in a static ThreadPoolExecutor in the outer singleton.

2) Would the callable be called in the same Thread as the runnable, if i do a val = myCallable.call()? It modifies the concurrenthashmap depending on the result of a callable.

Answer 1

1. ConcurrentHashMap is designed to be non-blocking, although some contention is possible if multiple updates to the same hash segment happened simultaneously.
2. Yes.

Answer 2

1. Except when running static initializers, Java will never implicitly block.

2. call() is a normal method call and will run synchronously in the calling thread, like any other method call.