Parent-child relation between two objects causes JSON StackOverflowError

问题

I am trying to achieve a parent-child relation between some objects and I ran into a bit of trouble.

In my case, I am trying to store objects within other objects (e.g. container stores multiple items or other containers with items). The tricky part is that every object in the storage should be able to tell what it's outermost parent object is. While this seems to work in my in-memory database (using h2 at the moment), trying to get a JSON representation of all my storage items gives this (I return a List<StorageUnit> ):

Could not write JSON: Infinite recursion (StackOverflowError); nested exception is com.fasterxml.jackson.databind.JsonMappingException: Infinite recursion (StackOverflowError) (through reference chain: java.util.ArrayList[0]->com.warehousing.storage.FixedContentsCase["contents"]->java.util.ArrayList[0]->com.warehousing.storage.FixedContentsCase["contents"]->...

Here are the classes:

StorageUnit

@Entity
@Inheritance
public abstract class StorageUnit {

   @Id
   @GeneratedValue(strategy=GenerationType.IDENTITY)
   private Long id;
   @ManyToOne
   private Location location;
   protected Long parentContainerId;
   // <getters & setters> 
   public abstract List<StorageUnit> getContents();
}

FixedContentCase


@Entity
public class FixedContentsCase extends StorageUnit {
    @OneToMany
    private List<Item> items; 

    public FixedContentsCase() {
        super();
        items = new ArrayList<>();
    }
    // <getters & setters> 
    @Override
    public List<StorageUnit> getContents() {
        // Return the case itself and its contents
        List<StorageUnit> contents = new ArrayList<>(Arrays.asList(this));
        for (StorageUnit item : items) 
            contents.addAll(item.getContents());
        return contents;
    }   
}

Item

@Entity
public class Item extends StorageUnit {

    private String description;

    public Item() {
        super();
        this.description = "";
    }
    // <getters & setters> 
    @Override
    public List<StorageUnit> getContents() {
        return Arrays.asList(this);
    }   
}

I have tried to annotate the StorageUnit class with @JsonIgnoreProperties("parentContainerId") but it didn't work. Annotating parentContainerId with @JsonIgnore didn't help either. I also tried annotating the getters instead of the attributes themselves (as per following). Is there a way to work around this or is some kind of design change necessary? Thanks!

回答1:

Your problem is that you add the storage unit itself to its list of contents, leading to infinite recursion if you traverse the tree downwards. The solution: Use a reference and only serialize the object once, using @JsonIdentityInfo and @JsonIdentityReference:

public class MyTest {

    @Test
    public void myTest() throws JsonProcessingException {
        final FixedContentsCase fcc = new FixedContentsCase();
        fcc.setId(Long.valueOf(1));
        final Item item = new Item();
        item.setId(Long.valueOf(2));
        item.setDescription("item 1");
        fcc.getItems().add(item);
        final ObjectMapper om = new ObjectMapper();
        System.out.println(om.writeValueAsString(fcc));
    }
}

@JsonIdentityInfo(generator = ObjectIdGenerators.PropertyGenerator.class, property = "id")
@JsonIdentityReference(alwaysAsId = false)
class Item extends StorageUnit {

    ...
}

@JsonIdentityInfo(generator = ObjectIdGenerators.PropertyGenerator.class, property = "id")
@JsonIdentityReference(alwaysAsId = false)
class FixedContentsCase extends StorageUnit {

    ...
}

abstract class StorageUnit {

    ...
}

回答2:

Using Jackson this is definitely possible by annotations like @JsonIgnore or the DTO approach BugsForBreakfast mentioned.

I created a jackson MixIn handler to allow dynamic filtering which i use to avoid the boilerplate of DTOs

https://github.com/Antibrumm/jackson-antpathfilter

The examples in the readme should show how it works and if it‘s a possible solution for you.

来源：https://stackoverflow.com/questions/60265882/parent-child-relation-between-two-objects-causes-json-stackoverflowerror

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