为什么FixedThreadPool无法正常工作

Question

Here's my following code:

    ExecutorService executor = Executors.newFixedThreadPool(5);

    executor.submit(new Runnable() {
        public void run() {
            for (int i = 0; i < 5; i++) {
              System.out.println("Start"+"  "+Thread.currentThread().getName());
              try {
                Thread.sleep(100);
              } catch (InterruptedException e) {
                e.printStackTrace();
              }
              System.out.println("End!"+"  "+Thread.currentThread().getName());
            }
        }
    });

    executor.shutdown();
    executor.awaitTermination(1, TimeUnit.SECONDS);

Here's my output:

    Start  pool-1-thread-1
    End!  pool-1-thread-1
    Start  pool-1-thread-1
    End!  pool-1-thread-1
    Start  pool-1-thread-1
    End!  pool-1-thread-1
    Start  pool-1-thread-1
    End!  pool-1-thread-1
    Start  pool-1-thread-1
    End!  pool-1-thread-1

My understanding is that in my code there're 5 threads in FixedThreadPool . So when I run my code, it's supposed to output all 5 threads, right? Or Am I misunderstanding something?

In my output thread-1 starts and end everytime but isn't it supposed to output all 5 threads when it loops in for loop? Cause if only 1 thread is doing the task then why do we even need 5 threads ? Is there any way all 5 threads can output on the console?(I'm a newbie)

Answer 1

You are posting only one Runnable so your ExecutorService runs one task. It will use only one thread. To use multiple threads you must call submit multiple times or you can call invokeAll with a Collection of runnables.

EDIT

Something like this:

public void test() {
    int numberOfThreads = 5;
    ExecutorService executorService = Executors.newFixedThreadPool(numberOfThreads);
    for(int i=0; i<numberOfThreads; i++){
        executorService.submit(createRunnable());
    }
}

private Runnable createRunnable() {
    return new Runnable() {
        @Override
        public void run() {
            for (int i = 0; i < 5; i++) {
                System.out.println("Start" + "  " + Thread.currentThread().getName());
                try {
                    Thread.sleep(100);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println("End!" + "  " + Thread.currentThread().getName());
            }
        }
    };
}