org.hibernate.PersistentObjectException：传递给persist的分离实体

我用hibernate成功编写了我的第一个master child示例 . 几天后，我再次拿起它并升级了一些图书馆 . 不知道我做了什么，但我再也不能让它再次运行 . 有人会帮我弄清楚返回错误信息的代码有什么问题：

org.hibernate.PersistentObjectException: detached entity passed to persist: example.forms.InvoiceItem
    at org.hibernate.event.def.DefaultPersistEventListener.onPersist(DefaultPersistEventListener.java:127)
    at org.hibernate.impl.SessionImpl.firePersist(SessionImpl.java:799)
    at org.hibernate.impl.SessionImpl.persist(SessionImpl.java:791)
    .... (truncated)

hibernate映射：

<hibernate-mapping package="example.forms">
    <class name="Invoice" table="Invoices">
        <id name="id" type="long">
            <generator class="native" />
        </id>
        <property name="invDate" type="timestamp" />
        <property name="customerId" type="int" />
        <set cascade="all" inverse="true" lazy="true" name="items" order-by="id">
            <key column="invoiceId" />
            <one-to-many class="InvoiceItem" />
        </set>
    </class>
    <class name="InvoiceItem" table="InvoiceItems">
        <id column="id" name="itemId" type="long">
            <generator class="native" />
        </id>
        <property name="productId" type="long" />
        <property name="packname" type="string" />
        <property name="quantity" type="int" />
        <property name="price" type="double" />
        <many-to-one class="example.forms.Invoice" column="invoiceId" name="invoice" not-null="true" />
    </class>
</hibernate-mapping>

EDIT: InvoiceManager.java

class InvoiceManager {

    public Long save(Invoice theInvoice) throws RemoteException {
        Session session = HbmUtils.getSessionFactory().getCurrentSession();
        Transaction tx = null;
        Long id = null;
        try {
            tx = session.beginTransaction();
            session.persist(theInvoice);
            tx.commit();
            id = theInvoice.getId();
        } catch (RuntimeException e) {
            if (tx != null)
                tx.rollback();
            e.printStackTrace();
            throw new RemoteException("Invoice could not be saved");
        } finally {
            if (session.isOpen())
                session.close();
        }
        return id;
    }

    public Invoice getInvoice(Long cid) throws RemoteException {
        Session session = HbmUtils.getSessionFactory().getCurrentSession();
        Transaction tx = null;
        Invoice theInvoice = null;
        try {
            tx = session.beginTransaction();
            Query q = session
                    .createQuery(
                            "from Invoice as invoice " +
                            "left join fetch invoice.items as invoiceItems " +
                            "where invoice.id = :id ")
                    .setReadOnly(true);
            q.setParameter("id", cid);
            theInvoice = (Invoice) q.uniqueResult();
            tx.commit();
        } catch (RuntimeException e) {
            tx.rollback();
        } finally {
            if (session.isOpen())
                session.close();
        }
        return theInvoice;
    }
}

Invoice.java

public class Invoice implements java.io.Serializable {

    private Long id;
    private Date invDate;
    private int customerId;
    private Set<InvoiceItem> items;

    public Long getId() {
        return id;
    }

    public Date getInvDate() {
        return invDate;
    }

    public int getCustomerId() {
        return customerId;
    }

    public Set<InvoiceItem> getItems() {
        return items;
    }

    void setId(Long id) {
        this.id = id;
    }

    void setInvDate(Date invDate) {
        this.invDate = invDate;
    }

    void setCustomerId(int customerId) {
        this.customerId = customerId;
    }

    void setItems(Set<InvoiceItem> items) {
        this.items = items;
    }
}

InvoiceItem.java

public class InvoiceItem implements java.io.Serializable {

    private Long itemId;
    private long productId;
    private String packname;
    private int quantity;
    private double price;
    private Invoice invoice;

    public Long getItemId() {
        return itemId;
    }

    public long getProductId() {
        return productId;
    }

    public String getPackname() {
        return packname;
    }

    public int getQuantity() {
        return quantity;
    }

    public double getPrice() {
        return price;
    }

    public Invoice getInvoice() {
        return invoice;
    }

    void setItemId(Long itemId) {
        this.itemId = itemId;
    }

    void setProductId(long productId) {
        this.productId = productId;
    }

    void setPackname(String packname) {
        this.packname = packname;
    }

    void setQuantity(int quantity) {
        this.quantity = quantity;
    }

    void setPrice(double price) {
        this.price = price;
    }

    void setInvoice(Invoice invoice) {
        this.invoice = invoice;
    }
}

EDIT: 从客户端发送的JSON对象：

{"id":null,"customerId":3,"invDate":"2005-06-07T04:00:00.000Z","items":[
{"itemId":1,"productId":1,"quantity":10,"price":100},
{"itemId":2,"productId":2,"quantity":20,"price":200},
{"itemId":3,"productId":3,"quantity":30,"price":300}]}

EDIT: 一些细节：
我试图通过以下两种方式保存发票：

• 手动编写上面提到的json对象并将其传递给服务器的新会话 . 在这种情况下，在调用save方法之前绝对没有进行任何活动，因此除了在save方法中打开的会话之外，不应该有任何打开的会话

• 使用getInvoice方法加载现有数据，并在删除键值后传递相同的数据 . 这也是我认为应该在保存之前关闭会话，因为事务是在getInvoice方法中提交的 .

在这两种情况下，我都得到相同的错误消息，迫使我认为hibernate配置文件或实体类或save方法有问题 .

Please let me know if I should provide more details