Wsrtjtyk

I want to create a new thead and assign it to a thread pool executor so that it can be processed concurrently. The core size of such a thread pool is 5 and maximum of it is 20. However it cannot create more threads than core size in the code below. If I uncommented the code if (socket != null) { socket.close(); } (Actually, I can't close it here as the socket should be processed in the TestHandler), more threads can be created upto maximum of the thread pool which is 20.
So why this can happened? Does anyone can help to explain it?
The workQueue size can be adjusted accordingly and I was using apache ab to do test. e.g. ab -n 1000 -c 10 http://localhost:8080/

import com.google.common.util.concurrent.ThreadFactoryBuilder;



import java.io.IOException;

import java.net.ServerSocket;

import java.net.Socket;

import java.util.Random;

import java.util.concurrent.*;



public class MultiThreadServer {

    public static void main(String args) throws IOException {

        MultiThreadServer server = new MultiThreadServer();

        server.start();

    }



    private static ThreadFactory builder = new ThreadFactoryBuilder().setNameFormat("Demo Task Executor #%d").build();



    public void start() throws IOException {

        ServerSocket serverSocket = new ServerSocket(8080);

        Socket socket = null;

        ExecutorService executor = new ThreadPoolExecutor(5,

                20,

                10,

                TimeUnit.MILLISECONDS,

                new ArrayBlockingQueue<>(10),

                builder);

        for (; ; ) {

            socket = serverSocket.accept();



            executor.execute(new TestHandler(socket));

//            if (socket != null) {

//                socket.close();

//            }

        }

    }



    class TestHandler implements Runnable {

        private Socket socket;



        public TestHandler(Socket socket) {

            this.socket = socket;

        }



        @Override

        public void run() {

            try {

                Thread.sleep(new Random().nextInt(100));

                socket.close();

            } catch (Exception e) {

                e.printStackTrace();

            }

            System.out.println(Thread.currentThread());

        }

    }



}

If you check the implementation of execute method in ExecutorService class you can see the following code:

//...................



        if (isRunning(c) && workQueue.offer(command)) {

//...................

        }

        else if (!addWorker(command, false))

            reject(command);



//...................

So a new thread will be created only if the workQueue is full. You have initialized your pool with new ArrayBlockingQueue<>(10). That means no new threads will be created for the first 15 concurrent requests (corePoolSize + queue capacity).

To see for example 6 threads just try to run ab -n 1000 -c 16 http://localhost:8080/ or reduce the queue initial capacity. But take into account that when the queue is full and the number of thread equals maximumPullSize any new request will be rejected.