What are the advantages of using an ExecutorService?

Question

What is the advantage of using ExecutorService over running threads passing a Runnable into the Thread constructor?

Answer 1

Is it really that expensive to create a new thread?

As a benchmark, I just created 60,000 threads with Runnables with empty run() methods. After creating each thread, I called its start(..) method immediately. This took about 30 seconds of intense CPU activity. Similar experiments have been done in response to this question. The summary of those is that if the threads do not finish immediately, and a large number of active threads accumulate (a few thousand), then there will be problems: (1) each thread has a stack, so you will run out of memory, (2) there might be a limit on the number of threads per process imposed by the OS, but not necessarily, it seems.

So, as far as I can see, if we're talking about launching say 10 threads per second, and they all finish faster than new ones start, and we can guarantee that this rate won't be exceeded too much, then the ExecutorService doesn't offer any concrete advantage in visible performance or stability. (Though it may still make it more convenient or readable to express certain concurrency ideas in code.) On the other hand, if you might be scheduling hundreds or thousands of tasks per second, which take time to run, you could run into big problems straight away. This might happen unexpectedly, e.g. if you create threads in response to requests to a server, and there is a spike in the intensity of requests that your server receives. But e.g. one thread in response to every user input event (key press, mouse motion) seems to be perfectly fine, as long as the tasks are brief.