什么是DDD中的模型？

在 DDD 中，模型是由领域专家和开发人员协作设计的领域模型。它是包含与解决特定领域问题有关的实体、值对象、聚合根、仓储和服务等概念的一个抽象层。模型是语言无关的，因为它们反映了解决特定领域问题所需的方式，而不是与编程语言相关的技术实现细节。

下面是一个简单的代码示例，它展示如何使用 Java 实现一个领域模型：

public class Order {
    private long id;
    private Customer customer;
    private List<OrderItem> items;

    public Order(long id, Customer customer, List<OrderItem> items) {
        this.id = id;
        this.customer = customer;
        this.items = items;
    }

    public long getId() {
        return id;
    }

    public Customer getCustomer() {
        return customer;
    }

    public List<OrderItem> getItems() {
        return items;
    }
}

public class Customer {
    private long id;
    private String name;

    public Customer(long id, String name) {
        this.id = id;
        this.name = name;
    }

    public long getId() {
        return id;
    }

    public String getName() {
        return name;
    }
}

public class OrderItem {
    private long id;
    private Product product;
    private int quantity;

    public OrderItem(long id, Product product, int quantity) {
        this.id = id;
        this.product = product;
        this.quantity = quantity;
    }

    public long getId() {
        return id;
    }

    public Product getProduct() {
        return product;
    }

    public int getQuantity() {
        return quantity;
    }
}

public class Product {
    private long id;
    private String name;
    private double price;

    public Product(long id, String name, double price) {
        this.id = id;
        this.name = name;
        this.price = price;
    }

    public long getId() {