Tukukkk

I'm trying to create a REST service with Spring.
Everything works until I try to add a List of object (CartItem) to my main object (Cart).

This is my main object

@Entity

@Table(name="cart")

public class Cart implements Serializable{

   ...

   @GeneratedValue(strategy=GenerationType.IDENTITY)

   @Id

   @Column(name="id")

   private Integer id;  



   /*when I add this I get the error. If I remove this, the 

   REST service works*/

   @OneToMany(mappedBy="cart", fetch = FetchType.EAGER)

   private List<CartItem> cartItems; 



   //getter, setter, constructors, other fields ecc.

}

This is the object inside the List:

@Entity

@Table(name="cart_item")

public class CartItem implements Serializable{

   ...

   @GeneratedValue(strategy=GenerationType.IDENTITY)

   @Id

   @Column(name="id")

   private Integer id; 



   @OneToOne(targetEntity = Product.class, cascade = CascadeType.ALL)

   @JoinColumn(referencedColumnName="productId", name="product_id" )    

   private Product product;     



   @ManyToOne 

   @JoinColumn(name="cart_id", nullable=false)     

   private Cart cart;



  //getter, setter, constructors, other fields ecc.

}

This is my controller

@RestController

@RequestMapping(value="rest/cart")

public class CartRestController {

  ...

    @RequestMapping(value = "/", method = RequestMethod.GET) 

    public List<Cart> readAll() { 

        return cartService.read(); 

    }

 ...

}

I get this error:

SEVERE: Servlet.service() for servlet [dispatcher] in context with path 

[/webstore] threw exception [Request processing failed; nested exception

 is org.springframework.http.converter.HttpMessageNotWritableException: 

Could not write JSON: Infinite recursion (StackOverflowError); nested

 exception is com.fasterxml.jackson.databind.JsonMappingException: 

Infinite recursion (StackOverflowError) (through reference chain:...

I suppose that I had to manage the List inside the Cart object in a particular manner, maybe because i'm using JPA, but I still didn't find a solution on the internet.
Can anyone help me?

This is a serialization recursion problem, it happens because CartItem has a bidirectional mapping back to Cart. So what happens is that

• a Cart gets serialized to JSON
  
  
  
  
  • all the CartItems inside it get serialized to JSON
    
    
    
    
    • the Cart property inside CartItem get serialized to JSON
      
      
      
      
      • the CartItems inside the cart get serialized to json, etc. etc.
      
      
      
      
    
    
    
    
  
  
  
  

You will probably want to exclude the CartItem.cart field from serialization by marking it with the @JsonIgnore annotation.

It is only too easy to expose far too much information to the outside world if you use JPA entities directly inside your webservices. Jackson actually has a useful feature called a JsonView which allows you to define which properties get exposed, you can even tailor it per webservice call if you want.

Never ending list? Did you mean a stackOverFlow exception?

If the situation is just like I said,then you should check something like fetch type and the entities' toString() or equal() method or something like that.

For example,there are to entities named A and B and their relationship is one to many(A is the one).If you config both of their fetchType as Eager,then when jpa query A,it will query B too.But B also contains A,so jpa will query A again.This kind of circle loop will cause a stackOverFlow.

By the way, how about providing more info about your problem like the Exception name?It's too hard for me to give you a specific solution,All I can do is to tell you some experiences I have met before.

Well,I created a small project with SpringBoot 2.1.0 and MySql.

It's my cartItem

public class CartItem {

    @GeneratedValue(strategy=GenerationType.IDENTITY)

    @Id

    @Column(name="id")

    private Integer id;



    @JsonIgnore

    @ManyToOne

    @JoinColumn(name="cart_id", nullable=false)

    private Cart cart;

}

and my cart:

public class Cart {

    @GeneratedValue(strategy= GenerationType.IDENTITY)

    @Id

    @Column(name="id")

    private Integer id;



    @OneToMany(mappedBy="cart", fetch = FetchType.EAGER)

    private List<CartItem> cartItems;

}

Controller is as same as you wrote.After adding a @JsonIgnore to cart filed of CartItem,circle loop is over(before i do that,the program did had a circle loop problem).

Every time you use jpa with @oneToMany,@ManyToOne or @ManyToMany,you should be careful about this problem.This circular reference case could happen when instantiating a object, printing a object or something like this.And of course there is a lot of way to solve it like changing fetch type to LAZY,adding @JsonIgnore,overriding toString() and equal() method.