一个简单的关于Swing 与并发知识点的程序，为什么按下b2按钮后，再次添加任务就会出现异常？

与 executors. newSingleThreadpool ( ) 有关吗？

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.util.concurrent.*;
import static net.mindview.util.SwingConsole.*;

class Task implements Runnable {
  private static int counter = 0;
  private final int id = counter++;
  public void run() {
    System.out.println(this + " started");
    try {
      TimeUnit.SECONDS.sleep(3);
    } catch(InterruptedException e) {
      System.out.println(this + " interrupted");
      return;
    }
    System.out.println(this + " completed");
  }
  public String toString() { return "Task " + id; }
  public long id() { return id; }
};

public class InterruptableLongRunningTask extends JFrame {
  private JButton
    b1 = new JButton("Start Long Running Task"),
    b2 = new JButton("End Long Running Task");
  ExecutorService executor =
    Executors.newSingleThreadExecutor();
  public InterruptableLongRunningTask() {
    b1.addActionListener(new ActionListener() {
      public void actionPerformed(ActionEvent e) {
        Task task = new Task();
        executor.execute(task);
        System.out.println(task + " added to the queue");
      }
    });
    b2.addActionListener(new ActionListener() {
      public void actionPerformed(ActionEvent e) {
        executor.shutdownNow(); // Heavy-handed
      }
    });
    setLayout(new FlowLayout());
    add(b1);
    add(b2);
  }
  public static void main(String[] args) {
    run(new InterruptableLongRunningTask(), 200, 150);
  }
}