API throwing StackOverflow Error with a @OneToMany Relationship

Question

I have made some changes to my MYSQL database so that a Product can have multiple Images associated with it (One to Many), however since making this change I am seeing some weird behaviour and the program throws a StackOverflow Exception. It seems as if the program is stuck in a continuous loop before crashing and throwing the error.

My model classes are structured as follows:

@Entity
@Table(name="products")
public class Products {
    
    public Products(String name, String price, String added_on, String category_id, String image_name, String description, List<ImageModel> imageModel) {
        super();
        this.name = name;
        this.price = price;
        this.added_on = added_on;
        this.category_id = category_id;
        this.image_name = image_name;
        this.description = description;
        this.imageModel = imageModel;
    }
    
    public Products(String name, String price, String added_on, String category_id, String description) {
        super();
        this.name = name;
        this.price = price;
        this.added_on = added_on;
        this.category_id = category_id;
        this.description = description;
    }
    
    public Products() {}
    
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private long id;
    private String name;
    private String price;
    private String added_on;
    private String category_id;
    private String image_name;
    private String description;
    private String image_id;
    
    @ManyToOne(optional=false)
    @JoinColumn(name = "category_id", insertable=false, updatable=false)
    private Category category;
    
    @OneToMany(mappedBy = "product")
    private List<ImageModel> imageModel;

    // Getters & Setters

This class is then linked to ModelImage as follows:

@Entity
@Table(name = "image_table")
public class ImageModel {

public ImageModel() {
    super();
}

public ImageModel(String name, String type, byte[] picByte, Products product) {
    this.name = name;
    this.type = type;
    this.picByte = picByte;
    this.product = product;
}

public ImageModel(String name, String type, byte[] picByte) {
    this.name = name;
    this.type = type;
    this.picByte = picByte;
}


@Id
@Column(name = "id")
@GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;

@Column(name = "name")
private String name;

@Column(name = "type")
private String type;

// image bytes can have large lengths so we specify a value
// which is more than the default length for picByte column
@Column(name = "picByte", length = 10000)
private byte[] picByte;

@ManyToOne
@JoinColumn(name="product_id")
private Products product;

// Getters Setters

When I add a product, the following code executes and seems to add everything as expected:

@PostMapping(value = "imageUploadMultiple")
public ResponseEntity<ImageResponse> addProductAndImages(@RequestParam("imageFiles") MultipartFile[] files, @RequestParam("productName") String productName, @RequestParam("productDescription") String productDescription, @RequestParam("productPrice") String productPrice, @RequestParam("categoryId") String categoryId) throws IOException {      
    // Need to save product first then get the id and save all images with the productId
    
    Products products = productService.addProduct(productName,  productDescription,  productPrice,  categoryId);

    
    Arrays.asList(files).stream().forEach(file -> {
        ImageModel img = null;
        try {
            img = new ImageModel(file.getOriginalFilename(), file.getContentType(), compressBytes(file.getBytes()), products);
            imageRepository.save(img);
        } catch (IOException e) {
            e.printStackTrace();
        }           
    });
    

    return ResponseEntity.ok().build();
}

This endpoint accepts multiple images and form data which corresponds to the image(Product Details etc)

However, the problem occurs when I call the endpoint to get all products based on a particular categoryId:

    @RequestMapping(value = "getProductsByCategory", produces = MediaType.APPLICATION_JSON_VALUE)
public List<Products> getProductsByCategory(@RequestBody HashMap<String, String> request) {
    String category_id = request.get("cat_id");
    List<Products> list = productService.getProductsByCategory(category_id);
    return list;
}

This then calls into the service class which then calls the repository code:

    @Query("Select pro FROM Products pro WHERE pro.category_id=:cat_id")
List<Products> getByCategoryId(@Param("cat_id")String cat_id);

When I run the app in debug mode, I get the following data (At this time there is only one product for that particular categoryID):

Notice how 'ImageModel' is of a PersistentBag type. When i dig deeper into this I get the mapped images to the particular product. In this instance there is 4 product images for the product. When i dig even deeper I notice there is just a continuous loop:

The error is as follows

java.lang.StackOverflowError: null
at java.lang.ClassLoader.defineClass1(Native Method) ~[na:1.8.0_251]
at java.lang.ClassLoader.defineClass(ClassLoader.java:756) ~[na:1.8.0_251]
at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142) ~[na:1.8.0_251]
at java.net.URLClassLoader.defineClass(URLClassLoader.java:468) ~[na:1.8.0_251]
at java.net.URLClassLoader.access$100(URLClassLoader.java:74) ~[na:1.8.0_251]
at java.net.URLClassLoader$1.run(URLClassLoader.java:369) ~[na:1.8.0_251]

Could not write JSON: Infinite recursion (StackOverflowError); nested exception is com.fasterxml.jackson.databind.JsonMappingException: Infinite recursion (StackOverflowError) (through reference chain: com.youtube.ecommerce.model.Products["imageModel"]->org.hibernate.collection.internal.PersistentBag[0]->com.youtube.ecommerce.model.ImageModel["product"]->com.youtube.ecommerce.model.Products["imageModel"]->org.hibernate.collection.internal.PersistentBag[0]->com.youtube.ecommerce.model.ImageModel["product"]->com.youtube.ecommerce.model.Products["imageModel"]->org.hibernate.collection.internal.PersistentBag[0]->com.youtube.ecommerce.model.ImageModel["product"]->com.youtube.ecommerce.model.Products["imageModel"]->org.hibernate.collection.internal.PersistentBag[0]->com.youtube.ecommerce.model.ImageModel["product"]->com.youtube.ecommerce.model.Products["imageModel"]->org.hibernate.collection.internal.PersistentBag[0]

The error just keeps going and going so i didnt post the full thing but it continuously says the same thing over and over again.

Im really struggling to understand whats gone wrong here!!

Answer 1

Its because when you will serialize your entity, you will have graph like

Product->Image[]->Product->Image[]->Product-> and so on

so you have to cut the recursion somewhere, eg using @JsonIgnore or use @JsonManagedReference, @JsonBackReference`

This is exactly depicted on one of your images that you peek into the entity.