The Strange behavior of CompletableFuture [duplicate]

Question

Why the below code does not print I just woke up

    import java.util.concurrent.CompletableFuture;
    public class CompletableFutureTest {
        public static void main(String[] args) {
            CompletableFuture completableFuture = CompletableFuture.runAsync(()->{
                try {
                    Thread.sleep(4000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println("I just woke up.");
            });
            System.out.println("I am main");
        }
    }

And the code given below prints I just woke up

import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
public class CompletableFutureTest {
    public static void main(String[] args) {
        ExecutorService executorService = Executors.newSingleThreadExecutor();
        CompletableFuture completableFuture = CompletableFuture.runAsync(()->{
            try {
                Thread.sleep(4000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("I just woke up.");
        }, executorService);
        executorService.shutdown();
        System.out.println("I am main");
    }
}

The possible noticeable difference between both the codes is that the second sample of code takes Single Thread pool into runAsync method, while the first one does not take any argument that is It uses ForkJoin pool. Please help me get the reason why is I just woke up not printed in first program