如何解决如何解决此错误：无法编写JSON：无法延迟初始化角色集合：com.cashregister.demo.model
尝试创建一个Transaction对象然后返回一个事务列表时，我陷入了困境。呼叫http：// localhost：8080 / transactions时出现的错误是：
{
"timestamp": "2020-08-13T14:24:11.113+0000","status": 500,"error": "Internal Server Error","message": "Could not write JSON: failed to lazily initialize a collection of role: com.cashregister.demo.model.Transaction.items,could not initialize proxy - no Session; nested exception is com.fasterxml.jackson.databind.JsonMappingException: failed to lazily initialize a collection of role: com.cashregister.demo.model.Transaction.items,could not initialize proxy - no Session (through reference chain: java.util.HashMap[\"transactions\"]->java.util.ArrayList[0]->com.cashregister.demo.model.Transaction[\"items\"])","path": "/transactions"
}
我一直在寻找解决方案，但似乎没有一个摆脱错误并返回。有人可以帮助解决此问题，以便在获取所有交易时以以下格式返回响应：
{
"transaction": {
"id": 1,"total": 46.44,"customer": {
"id": 1,"phoneNumber": "9416970394","lastName": "Weber","loyaltyNumber": "2484801419"
},"items": [
// list of each item in the transaction
]
这是我的代码：
Item.java模型
@Entity
public class Item {
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;
@ManyToOne(fetch = FetchType.LAZY)
@JoinColumn(name = "transaction_id",nullable = false)
private Transaction transaction;
@NotNull
private double total;
@NotNull
@OneToOne
private Product product;
@NotNull
private int quantity;
// Getters and setters
}
Transaction.java模型
@Entity
public class Transaction {
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;
private double total;
@ManyToOne
private Customer customer;
@OneToMany(mappedBy = "transaction",fetch = FetchType.LAZY,cascade = CascadeType.ALL)
private List<Item> items = new ArrayList<>();
// Getters and setters
}
用于数据库访问的TransactionDaoImpl.java
@Repository
public class TransactionDaoImpl implements TransactionDao {
@Autowired
private SessionFactory sessionFactory;
@Override
public List<Transaction> findAll() {
Session session = sessionFactory.openSession();
List<Transaction> transactions = session.createCriteria(Transaction.class).list();
session.close();
return transactions;
}
@Override
public Transaction findById(Long id) {
Session session = sessionFactory.openSession();
Transaction transaction = session.get(Transaction.class,id);
session.close();
return transaction;
}
@Override
public Long save(Transaction transaction) {
Session session = sessionFactory.openSession();
session.beginTransaction();
session.saveOrUpdate(transaction);
session.getTransaction().commit();
session.close();
return transaction.getId();
}
}
TransctionController.java
@RestController
@RequestMapping("/transactions")
public class TransactionController {
@Autowired
private TransactionService transactionService;
@Autowired
private CustomerService customerService;
@Autowired
private ProductService productService;
@Autowired
private ItemService itemService;
@RequestMapping(value = "")
public Map<String,List<Transaction>> listTransactions() {
Map<String,List<Transaction>> response = new HashMap<>();
List<Transaction> transactions = transactionService.findAll();
response.put("transactions",transactions);
return response;
}
@RequestMapping(value = "{transaction_id}")
public Map<String,Transaction> findTransactionById(@PathVariable Long transaction_id) {
Map<String,Transaction> response = new HashMap<>();
Transaction transaction = transactionService.findById(transaction_id);
response.put("transaction",transaction);
return response;
}
@RequestMapping(value = "",method = RequestMethod.POST)
public Map<String,Transaction> createTransaction(@RequestBody List<Product> products,@RequestParam("user_id") Long customerId) {
Map<String,Transaction> response = new HashMap<>();
Customer customer = customerService.findById(customerId);
// Get unique skus to get the product objects from the database (source of truth).
Set<String> skus = new HashSet<>();
for (final Product product : products) {
skus.add(product.getSku());
}
List<String> skusStrings = new ArrayList<String>();
for(String sku : skus) {
skusStrings.add(sku);
}
List<Product> productsFromDb = productService.findBySkus(skusStrings);
// Loop through products in payload to calculate quantity.
List<Item> items = new ArrayList<>();
Map<String,Integer> productQuantity = new HashMap<>();
for(Product product : products) {
if(productQuantity.containsKey(product.getSku())) {
productQuantity.put(product.getSku(),productQuantity.get(product.getSku()) + 1);
} else {
productQuantity.put(product.getSku(),1);
}
}
// Calculate total cost
double total = 0;
for(Product product : productsFromDb) {
if(!customer.getLoyaltyNumber().isEmpty()) {
total += product.getDiscountPrice() * productQuantity.get(product.getSku());
} else {
total += product.getDefaultPrice() * productQuantity.get(product.getSku());
}
}
// Calculate item total and append to Items list
Map<String,Double> productTotal = new HashMap<>();
for(Product product : productsFromDb) {
if(!customer.getLoyaltyNumber().isEmpty()) {
productTotal.put(product.getSku(),productQuantity.get(product.getSku()) * product.getDiscountPrice());
} else {
productTotal.put(product.getSku(),productQuantity.get(product.getSku()) * product.getDefaultPrice());
}
}
Transaction t = new Transaction();
t.setTotal(total);
t.setCustomer(customer);
t.setItems(items);
Long transactionId = transactionService.save(t);
for(Product product : productsFromDb) {
Item item = new Item(t,productTotal.get(product.getSku()),product,productQuantity.get(product.getSku()));
itemService.save(item);
}
Transaction transaction = transactionService.findById(transactionId);
response.put("transaction",transaction);
return response;
}
}
解决方法
您的交易实体与项目实体有关系。因此，在您的控制器中，您没有任何用于获取项目的数据库会话。您必须将所有代码放入Service层，然后在其上放入@Transactional。简短响应和不良响应会将spring.jpa.open-in-view属性设置为true。
这里的问题是Lazy Loading by Hibernate。这实际上是JPA规范，由Hibernate实现。主要概念是不加载所有关联，从而提高性能。
为什么要遇到此问题：
- 您已经将FETCH策略定义为双方都是LAZY（在这里，从事务到项目的关系很重要，因为您获取了事务）
- TransactionDaoImpl＃findAll（）方法打开一个Session并获取记录。根据“延迟加载”概念，Hibernate不会从数据库中获取关联的项目，而是创建代理。
- 由于这些是代理对象，因此HttpMessageConverter无法将这些对象转换为JSON表示形式。
如何解决此问题：
- 您必须在项目关系上调用获取访问器。
@Transactional // This is required since you are not using Spring data Jpa Repository abstractions. Or else you can explicitly define the transaction boundary inside the method by opening the transaction and closing it.
@Override
public List<Transaction> findAll() {
Session session = sessionFactory.openSession();
List<Transaction> transactions = session.createCriteria(Transaction.class).list();
transactions.foreach(Transaction::getItems);
session.close();
return transactions;
}
但这会产生N+1 issue。
主要。对于每笔交易，将为每笔交易触发N个查询（N为一次交易的商品数）-因此将触发K * N个查询。
-
要解决N + 1问题，您可以使用“本地连接”，使用“联接”通过一个查询来获取所有数据。由于加入了JOIN，因此性能仍然会很慢，但比第一个选项要好得多。或者，您可以使用Hibernate Annotation或JPQL / HQL JOIN FETCH，它们实际上为您创建了JOIN。
-
另一个无需N + 1查询即可加载数据的选项是
的信息@EntityGraphy。我没有用过。但是您可以了解更多有关here
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