问题
As we create a Thread pool using Java's Executor service and submit threads to this thread pool, what is the order in which those threads get executed?
I want to ensure that threads submitted first, execute first. For example, in the code below, I want first 5 threads to get executed first, followed by the next 5 threads and so on...
// Create a thread pool of 5 threads.
ScheduledExecutorService exService = Executors.newScheduledThreadPool(5, new ModifiedThreadFactory("ReadThreadPool"));
// Create 100 threads.
MyThread[] threads = createMyThreads(100);
// Submit these 100 threads to thread pool for execution.
for(MyThread thread : threads) {
exService.submit(thread);
}
Does Java's Thread Pool provide any API for this purpose, or do we need to implement a FIFO queue at our end to achieve this. If Java's thread pool does not provide any such functionality, I am really interested to understand the reason behind the non-existence of this functionality as it appears like a very common use-case to me. Is it technically not possible (which I think is quite unlikely), or is it just a miss?
回答1:
That's the default behavior. ScheduledThreadExecutor (that you're using although you're not scheduling anything) extends from ThreadPoolExecutor. Tasks submitted to a ThreadPoolExecutor are stored in a BlockingQueue until one thread is available to take them and execute them. And queues are FIFO.
This is decscribed in details in the javadoc.
回答2:
Threads do not get executed. Threads are the entities running taska like Runnable and Callable . Submiting such a task to a executor service will put it in it's inner BlockingQueue until it gets picked up by a thread from it's thread pool. This will still tell you nothing about the order of execution as different classes can do different things while implementing Runnable
来源：https://stackoverflow.com/questions/24477316/java-thread-pool-executorservice-threads-execution-order