Enum *seems* to be initialized more than once, the constructor is called more than once. If I’m right, then why?

I’m reading thru this tutorial:

JAX-RS Delete Example

It first runs: ClientAllOrders(), which creates 5 orders. Then it runs: ClientDeleteById(), which deletes orders 2 and 4. Then it runs ClientAllOrders(), and gets all orders except for orders 2, 4. It creates orders here:

public enum OrderService {
    Instance;
    private Map<Integer, Order> orders = new HashMap<>();

    OrderService() {
        Instant instant = OffsetDateTime.now().toInstant();
        for (int i = 1; i <= 5; i++) {
            Order order = new Order();
            order.setId(i);
            order.setItem("item " + i);
            order.setQty((int) (1 + Math.random() * 100));
            long millis = instant.minus(Period.ofDays(i))
                                 .toEpochMilli();
            order.setOrderDate(new Date(millis));
            orders.put(i, order);
        }
    }
//---
}

Note that OrderResource is not a Singleton, so by default a new instance of the resource class is created for each new request, however, the OrderService.constructor is called only ones and Enum in initialized only once as expected.

The above scenario results in this output:

// ClientAllOrders:

Order{id=1, item='item 1', qty=62, orderDate=Fri Jul 24 18:27:51 EDT 2020}
Order{id=2, item='item 2', qty=100, orderDate=Thu Jul 23 18:27:51 EDT 2020}
Order{id=3, item='item 3', qty=29, orderDate=Wed Jul 22 18:27:51 EDT 2020}
Order{id=4, item='item 4', qty=28, orderDate=Tue Jul 21 18:27:51 EDT 2020}
Order{id=5, item='item 5', qty=28, orderDate=Mon Jul 20 18:27:51 EDT 2020}

// ClientDeleteById – deletes orders 2 and 4:

true
true

// Run ClientAllOrders again:

Order{id=1, item='item 1', qty=62, orderDate=Fri Jul 24 18:27:51 EDT 2020}
Order{id=3, item='item 3', qty=29, orderDate=Wed Jul 22 18:27:51 EDT 2020}
Order{id=5, item='item 5', qty=28, orderDate=Mon Jul 20 18:27:51 EDT 2020}

However, if I add this line in ClientAllOrders:

Collection<Order> c = OrderService.Instance.getAllOrders();
c.forEach(System.out::println);

Then OrderService.constructor is called again, and the map now has 5 new orders in it. Why unlike the REST requests that all share the same Enum object, here, Enum was initialized again and the constructor() was called again creating 5 new orders?

    public class ClientAllOrders {
    	
        public static void main(String[] args) {
            Client client = ClientBuilder.newClient();
            
            //get all  orders
            WebTarget allOrderTarget = client.target("http://localhost:8080/jaxrs-delete-example/orders");
    
            Response response = allOrderTarget.request().get();
            List<Order> orders = response.readEntity(new GenericType<List<Order>>() {});
            System.out.println("Orders by REST call:");
            orders.forEach(System.out::println);
            
// Added this line - Enum is initialized again and constructor called again, creating new orders:
            Collection<Order> c = OrderService.Instance.getAllOrders();
            System.out.println("Orders created again:");
            c.forEach(System.out::println);
        }
    }

This is the output if I run ClientAllOrders, note the orders created are different:

Orders by REST call:
Order{id=1, item='item 1', qty=59, orderDate=Fri Jul 24 18:35:24 EDT 2020}
Order{id=2, item='item 2', qty=14, orderDate=Thu Jul 23 18:35:24 EDT 2020}
Order{id=3, item='item 3', qty=78, orderDate=Wed Jul 22 18:35:24 EDT 2020}
Order{id=4, item='item 4', qty=3, orderDate=Tue Jul 21 18:35:24 EDT 2020}
Order{id=5, item='item 5', qty=2, orderDate=Mon Jul 20 18:35:24 EDT 2020}

Orders created again:
Order{id=1, item='item 1', qty=1, orderDate=Fri Jul 24 18:35:24 EDT 2020}
Order{id=2, item='item 2', qty=53, orderDate=Thu Jul 23 18:35:24 EDT 2020}
Order{id=3, item='item 3', qty=76, orderDate=Wed Jul 22 18:35:24 EDT 2020}
Order{id=4, item='item 4', qty=31, orderDate=Tue Jul 21 18:35:24 EDT 2020}
Order{id=5, item='item 5', qty=25, orderDate=Mon Jul 20 18:35:24 EDT 2020}

The client and the server run in separate processes, in separate JVMs, independently. The server has one version of enum OrderService, and the client has another. Nothing links these two instances of the enum to each other.

In your case, the "server" is an application server, where you deploy the web application "war" archive. The "client" is a separate Java program with a main method. that you run from your IDE with the "run as Java application" action.