How to restart the threads using ExecutorService which were terminated previously in a Swing application

Question

I am creating a Swing application. It contains a JFrame, inside JFrame, I added a JButton to start and stop some tasks. I am using the same JButton to start and stop the tasks which are threads and executed by ExecutorService.

While clicking on the Start button, threads will be executed and the button label will be changed to Stop, while clicking on the Stop button will stop all threads (I have done this using ExecutorService shutdownNow() method) and the button label will be again changed to Start, but the applicaiton will not be closed. Now, if I click Start button again, applicaion gets hanged, the threads are not restarted from the beginning.

import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class MultipleThreads {
    public static ExecutorService executor = Executors.newFixedThreadPool(4);

    public static void main(String[] args) {
        JFrame frame = new JFrame("Stop Thread");
        frame.setSize(200,200);
        frame.setLocationRelativeTo(null);
        frame.setLayout(null);
        JPanel panel = new JPanel();
        panel.setBounds(5,5,150,150);
        panel.setLayout(null);
        JButton btn = new JButton("Start");
        btn.setBounds(10,10,80,25);

        btn.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent ae) {
                if (btn.getText().equals("Start")) {
                    btn.setText("Stop");
                    MultipleThreads2 runThreads = new MultipleThreads2();
                    runThreads.runThreadMethod();
                } else if (btn.getText().equals("Stop")) {
                    try {
                        if (!executor.awaitTermination(800, TimeUnit.MILLISECONDS)) {
                            executor.shutdownNow();
                        } 
                    } catch (InterruptedException e) {
                        executor.shutdownNow();
                    }

                    btn.setText("Start");
                }
            }
        });
        panel.add(btn);
        frame.add(panel);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);
    }
}

class MultipleThreads2 {
    public volatile boolean flag = true;

    public void stopRunning() {
        flag = false;
    }

    public MultipleThreads2() {
        while (flag) {
            try {
                MultipleThreads.executor.submit(t1);
                MultipleThreads.executor.submit(t2);
                flag = false;
                System.out.println(t1.isAlive());
            } catch (Exception e) {

            }
        }
    }

    public void runThreadMethod() {
        flag = true;
        while (flag) {
            try {
                MultipleThreads.executor.submit(t3);
                MultipleThreads.executor.submit(t4);
                    flag = false;
            } catch (Exception e) {

            }
        }
    }

    Thread t1 = new Thread(new Runnable() {
        @Override
        public void run() {
            try {
                for (int i = 0; i < 10; i++) {
                    System.out.println("From t1 Thread");
                    Thread.sleep(1000);
                }
            } catch (Exception e) {

            }
        }
    });

    Thread t2 = new Thread(new Runnable() {
        @Override
        public void run() {
            try {
                for (int i = 0; i < 10; i++) {
                    System.out.println("From t2 Thread");
                    Thread.sleep(500);
                }
            } catch (Exception e) {

            }
        }
    });

    Thread t3 = new Thread(new Runnable() {
        @Override
        public void run() {
            try {
                for (int i = 0; i < 10; i++) {
                    System.out.println("From t3 Thread");
                    Thread.sleep(500);
                }
            } catch (Exception e) {

            }
        }
    });

    Thread t4 = new Thread(new Runnable() {
        @Override
        public void run() {
            try {
                for (int i = 0; i < 10; i++) {
                    System.out.println("From t4 Thread");
                    Thread.sleep(500);
                }
            } catch (Exception e) {

            }
        }
    });
}

Expected: While clicking on Start button for the second time should restart all the threads from the beginning.

Answer 1

Here:

public static ExecutorService executor = Executors.newFixedThreadPool(4);

You are creating that thread pool once. Later on, you are calling

executor.shutdownNow();

In other words: you are starting your car, and at some point, you stop the car, you get out, and you set it on fire. Then you ask yourself: "ok, how can I use that car to drive home?". Well, you can't. You just set it on fire.

Same thing here: when you shutdown the service, it is gone.

Long story short, a simple (not necessarily ideal) solution would be to do:

public static ExecutorService executor = null;

and later on:

if (executor == null) 
  executor = Executors.newFixedThreadPool(4);

and

executor.shutdownNow();
executor = null;

In other words: you drive with your car, you set it on fire, and then you buy a new one, to make another drive, before setting that on fire.

Of course, that approach of setting to null and checking for that could lead to various problems. It would be a bit better to do:

  executor.shutdownNow();
  executor = Executors.newFixedThreadPool(4);

Meaning: instead of leaving the executor around as null, you simply create a new instance, as soon as the "last one" was told to shutdown. So, theoretically, whenever the executor gets used to submit tasks OR it gets shutdown, you are talking to a currently "valid" instance.