Using same instance of ExecutorService in different threads is thread safe?

Question

For example, I execute runnable block via ExecutorService instance and that runnable block execute some asynchronous code via the same ExecutorService instance. So my code looks like:

final ExecutorService eService = Executors.newFixedThreadPool(nThreads);
    eService.execute(new Runnable() {
        public void run() {
            eService.execute(new Runnable() {
                public void run() {
                    System.out.println("Successfully created");
                }
            });
        }
    });

1. Is this code thread safe, because ExecutorService doesn't have a state?

In my real app I have some threads that created new threads inside them and I want to use one ExecutorService instance(maybe a bad practice?) with configurable thread pool size.

2. Is it a bad practice to do have one ExecutorService instance for different threads?
3. And if yes, maybe there are some alternatives?

Answer 1

It look fine. It is a good practice to reuse threads instead of spawning new ones (which is expensive) and one of possible implementations of such thread sharing can be usage of ExecutorService

Just keep in mind, that it will work for the code you have mentioned, It might break if you will try to await for the result of scheduled operation. This can lead to obvious deadlock when all the threads (from the pool) will be waiting for the result of the operation that awaits a free thread (in that pool) to execute it.