使用@Version与LockModeType.OPTIMISTIC时Hibernate抛异常原因

使用Hibernate时遇到如下报错：

Caused by: org.hibernate.TransientPropertyValueException: object references an unsaved transient instance - save the transient instance before flushing : api.models.SaleItem.product -> api.models.Product

但移除@Version字段和LockModeType.OPTIMISTIC后，功能可正常运行。

相关实体类、仓库层及控制器层代码如下：

实体类代码 #

Product类 #

@Entity
@Table(name = "products")
public class Product {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Integer id;
    @Column(unique = true, nullable = false)
    @NotNull(message = "name is required")
    @Size(min = 3, max = 50, message = "name must be between 3 and 50 characters")
    private String name;
    @Min(value = 1, message = "price not be less than 1")
    @Max(value = 1000000, message = "price should not be greater than 1000000")
    private float price;
    @Min(value = 0, message = "stock not be less than 0")
    @Max(value = 10000, message = "stock should not be greater than 10000")
    private int stock;
    @Min(value = 1, message = "minimum stock not be less than 1")
    @Max(value = 1000, message = "minimum stock should not be greater than 1000")
    @Column(name = "minimum_stock")
    private int minimumStock;
    private Status status;
    @ImageSize
    private String image;
    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "category_id", nullable = false)
    @NotNull(message = "category cannot be null")
    private Category category;
    @Version
    private Integer version;

    public Product() {
        this.status = Status.ACTIVE;    
    }
    
    public Product(Integer id) {
        this.id=id;
        this.status=Status.ACTIVE;
    }

    @PrePersist
    void onCreate() {
        for (SaleItem item : items)
            item.setSale(this);
        for (Payment payment : payments)
            payment.setSale(this);
        if(this.discount!=null)
            this.discount.setSale(this);
        if(this.surcharge!=null)
            this.surcharge.setSale(this);
    }
    // 省略其他getter/setter
}

SaleItem类 #

@Entity
@Table(name = "sales_items")
public class SaleItem {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Integer id;
    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name="product_id")
    @NotNull(message="product cannot be null")
    private Product product;
    @Min(value = 1, message = "quantity not be less than 1")
    @Max(value = 10000, message = "quantity should not be greater than 10000")
    private int quantity;
    @Min(value = 0, message = "price not be less than 1")
    @Max(value = 1000000, message = "price should not be greater than 1000000")
    private float price;
    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "sale_id")
    @JsonIgnore
    private Sale sale;
    // 省略其他getter/setter
}

仓库层代码 #

public Sale save(Sale entity) {
    EntityManager em = emf.createEntityManager();
    try {
        em = emf.createEntityManager(); // 重复创建EntityManager，需修复
        em.getTransaction().begin();
        em.persist(entity);
        updateStock(entity.getItems(), em);
        em.getTransaction().commit();
        em.close();
        return entity;
    } catch (PersistenceException e) {
        em.getTransaction().rollback();
        e.printStackTrace();
    } finally {
        em.close();
    }
}

private void updateStock(List<SaleItem> items, EntityManager em) {
    for (SaleItem item : items) {
        Product product = em.find(Product.class, item.getProduct().getId(),LockModeType.OPTIMISTIC);
        product.setStock(product.getStock() - item.getQuantity());
    }
}

控制器层代码 #

public void doPost(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {
    Sale sale = mapper.readValue(Util.getPayload(request), Sale.class);
    List<String> errors = validate(sale);
    if (!errors.isEmpty())
        Util.sendAsJson(response, errors, mapper);
    else {
        sale.setUser(new User(Double.valueOf(request.getAttribute("id").toString()).intValue()));
        sale.setDate(new Date());
        Util.sendAsJson(response, saleRepo.save(sale), mapper);
    }
}

已尝试更换不同版本的Hibernate，问题仍存在，请问该如何解决？

问题根源 #

报错的核心原因是：调用em.persist(entity)保存Sale时，Sale关联的SaleItem中的Product是仅包含ID的临时对象（transient instance），并未被Hibernate的持久化上下文托管。

未使用@Version和乐观锁时，Hibernate对关联对象的状态检查相对宽松，仅通过ID关联即可完成操作；但启用乐观锁后，Hibernate需要验证关联对象的版本状态，而临时对象无有效版本信息，因此触发TransientPropertyValueException。

修复步骤 #

1. 在保存Sale前，将SaleItem中的Product替换为持久化上下文托管的实例
   修改仓库层的save方法，在em.persist(entity)前，先通过EntityManager查询获取已托管的Product实例，并设置回SaleItem：

   public Sale save(Sale entity) {
       EntityManager em = emf.createEntityManager();
       try {
           em.getTransaction().begin();
           // 先将每个SaleItem的Product替换为托管实例
           for (SaleItem item : entity.getItems()) {
               Product managedProduct = em.find(Product.class, item.getProduct().getId());
               item.setProduct(managedProduct);
           }
           em.persist(entity);
           updateStock(entity.getItems(), em);
           em.getTransaction().commit();
           return entity;
       } catch (PersistenceException e) {
           em.getTransaction().rollback();
           e.printStackTrace();
       } finally {
           em.close();
       }
   }
   

2. 修复重复创建EntityManager的问题
   原代码中em = emf.createEntityManager();重复执行，会导致初始创建的EntityManager未被关闭，存在资源泄漏风险，需移除该行冗余代码。

3. 可选：为SaleItem的Product关联添加nullable=false约束
   在SaleItem的@JoinColumn中显式设置nullable=false，与@NotNull注解保持一致，增强数据完整性约束：

   @ManyToOne(fetch = FetchType.LAZY)
   @JoinColumn(name="product_id", nullable = false)
   @NotNull(message="product cannot be null")
   private Product product;
   

原理说明 #

通过em.find()获取的Product实例会被Hibernate持久化上下文托管，此时SaleItem关联的是已存在的、受托管的持久化对象，Hibernate可以正常验证其版本信息，乐观锁机制也能正常工作，同时避免了临时对象关联导致的报错。

内容的提问来源于stack exchange，提问作者maxc5