如何编写一个了解线程已完成并开始新线程的循环？

Question

I want to write a thread which understand a thread has finished and starts new thread. I mean I wrote this code :

 new Thread(new Runnable(){ 
            @Override public void run(){
    //code here
                } 
           }).start();

But I want to do it in for loop. I want to create just 5 thread.But when a thread has finished I want to create a new one.

for(int i=0;i<300;i++)
{
 //I want to create 5 thread here and stop code  and then when a thread has finished I want //to create  new thread.
}

Answer 1

The thread class has these methods, which could be used to do what you want:

Thread.join()
Thread.isAlive()

But, you probably really want to use a thread pool, like this:

    ExecutorService executor = Executors.newFixedThreadPool(5);
    for(int i=0;i<N;i++) {
        executor.submit(new Runnable() {
            @Override
            public void run() {
            }
        });
    }

Answer 2

If you want a more universal method, but more low level you can use a semaphore :

final Semaphore s = new Semaphore(5);
for (int i = 0; i < 20; ++i)
{
    final int j = i;

    s.acquire();

    new Thread(new Runnable()
    {
        @Override
        public void run()
        {
            try
            {
                System.out.println("Thread " + j + " starts.");
                Thread.sleep(1000);
                System.out.println("Thread " + j + " ends.");
            }
            catch (InterruptedException e)
            {
                e.printStackTrace();
            }
            finally
            {
                s.release();
            }
        }

    }).start();
}

Answer 3

You sounded like you want to create task base on the currently running task. Here I have an example which you could create new task in another task. Perhaps, you may also want to look at java.util.concurrent.ForkJoinPool

final ExecutorService executorService = Executors.newFixedThreadPool(5);

executorService.submit(new Runnable(){
    @Override
    public void run() {
        //code here which run by 5 threads, thread can be reused when the task is finished

        //new task can be created at the end of another task
        executorService.submit(...)
    }
});