CODEWITHSUNDEEP
javaexecutorservicethreadpoolexecutor
user1730789
user1730789

Reputation: 5177

java Executor Service

When I create ExecutorService with below code in JAVA,can someone explain how the ExecutorService works ? 

ExecutorService executor = Executors.newFixedThreadPool(400);

    for (int i = 0; i < 500; i++) {
        Runnable worker = new MyRunnable(10000000L + i);
        executor.execute(worker);
    }

I believe that there will be a single Queue of work and my for loop will add 500 Runnable tasks to this queue. Now the ExecutorService has been created with a Thread Pool of 400 threads.

So of those 500 tasks in the queue, the 400 threads in the ExecutorService will execute this 400 tasks at a time, and the remaining as slots are freed up?

Am I correct in my understanding ?

Upvotes: 0

Views: 2545

Answers (2)

Ravindra babu
Ravindra babu

Reputation: 38950

If tasks are more than number of processing threads, the tasks which haven't been picked up by threads will wait. Once the thread completes one task, it will pick-up one more waiting task.

But these thread pools (other than ForkJoinPool) are not efficient in stealing worker thread tasks.

Assume that one thread is backlog of 10 tasks to be executed and it's running first task. At that same time, some other thread in pool is idle. In this scenario, once the task is allocated a Thread, only that thread will execute the task even though other threads are idle.

ForkJoinPool differs from other kinds of ExecutorService mainly by virtue of employing work-stealing: all threads in the pool attempt to find and execute tasks submitted to the pool and/or created by other active tasks (eventually blocking waiting for work if none exist)

One more new API has been added in Java 8.

public static ExecutorService newWorkStealingPool()

Creates a work-stealing thread pool using all available processors as its target parallelism level.

Relate SE question : ThreadPoolExecutor vs ForkJoinPool: stealing subtasks

Upvotes: 0

moodywoody
moodywoody

Reputation: 2159

JavaDoc newFixedThreadPool

Creates a thread pool that reuses a fixed number of threads operating off a shared unbounded queue. At any point, at most nThreads threads will be active processing tasks. If additional tasks are submitted when all threads are active, they will wait in the queue until a thread is available. If any thread terminates due to a failure during execution prior to shutdown, a new one will take its place if needed to execute subsequent tasks. The threads in the pool will exist until it is explicitly shutdown.

Upvotes: 2

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