Let's assume that we have the following Order class:

public class Order {

  private final int customerId;

  public Order(int customerId) {
    this.customerId = customerId;
  }

  // getter and toString() omitted for brevity
}

And, we write CustomerOrder as follows:

public class CustomerOrder implements Runnable {

  private static final Logger logger
    = Logger.getLogger(CustomerOrder.class.getName());
  private static final Random rnd = new Random();

  private static final ThreadLocal<Order> 
    customerOrder = new ThreadLocal<>();

  private final int customerId;

  public CustomerOrder(int customerId) {
    this.customerId = customerId;
  }

  @Override
  public void run() {
    logger.info(() -> "Given customer id: " + customerId 
      + " | " + customerOrder.get() 
      + " | " + Thread.currentThread().getName());

    customerOrder.set(new Order(customerId));

    try {
      Thread.sleep(rnd.nextInt(2000));
    } catch (InterruptedException ex) {
      Thread.currentThread().interrupt();
      logger.severe(() -> "Exception: " + ex);
    }

    logger.info(() -> "Given customer id: " + customerId 
      + " | " + customerOrder.get() 
      + " | " + Thread.currentThread().getName());

    customerOrder.remove();
  }
}

For each customerId, we have a dedicated thread that we control:

CustomerOrder co1 = new CustomerOrder(1);
CustomerOrder co2 = new CustomerOrder(2);
CustomerOrder co3 = new CustomerOrder(3);

new Thread(co1).start();
new Thread(co2).start();
new Thread(co3).start();

So, each thread modifies a certain instance of CustomerOrder (there is a particular thread for each instance).

The run() method fetches the order for the given customerId and stores it in the ThreadLocal variable, using the set() method.

A possible output will be as follows:

[14:48:20] [INFO] 
  Given customer id: 3 | null | Thread-2
[14:48:20] [INFO] 
  Given customer id: 2 | null | Thread-1
[14:48:20] [INFO] 
  Given customer id: 1 | null | Thread-0

[14:48:20] [INFO] 
  Given customer id: 2 | Order{customerId=2} | Thread-1
[14:48:21] [INFO] 
  Given customer id: 3 | Order{customerId=3} | Thread-2
[14:48:21] [INFO] 
  Given customer id: 1 | Order{customerId=1} | Thread-0

In scenarios like the preceding one, avoid using ExecutorService. There is no guarantee that each Runnable (of a given customerId) will be handled by the same thread at every execution. This may lead to weird results.