tstorms tstorms - 3 months ago 27
Java Question

Cyclic references in a bidirectional many to many relationship

I'm having a bidirectional many to many relationship in my entities. See the example below:

public class Collaboration {

@JsonManagedReference("COLLABORATION_TAG")
private Set<Tag> tags;

}

public class Tag {

@JsonBackReference("COLLABORATION_TAG")
private Set<Collaboration> collaborations;

}


When I try to serialize this to JSON, I'm getting the following exception: `


"java.lang.IllegalArgumentException: Can not handle managed/back
reference 'COLLABORATION_TAG': back reference type (java.util.Set) not
compatible with managed type (foo.Collaboration).


Actually, I know this makes sense because the javadoc explicitly states that you can't use @JsonBackReference on Collections. My question is, how should I address this problem? What I've done for now is remove the @JsonManagedReference annotation on the parent side, and added the @JsonIgnore on the child side. Could someone tell me what the side effects are of this approach? Are there any other suggestions?

Answer

I ended up implementing the following solution.

One end of the relationship is considered to be the parent. It does not need any Jackson related annotation.

public class Collaboration {

    private Set<Tag> tags;

}

The other side of the relationship is implemented as follows.

public class Tag {

    @JsonSerialize(using = SimpleCollaborationSerializer.class)
    private Set<Collaboration> collaborations;

}

I'm using a custom serializer to will make sure that no cyclic references will occur. The serializer could be implemented like this:

public class SimpleCollaborationSerializer extends JsonSerializer<Set<Collaboration>> {

    @Override
    public void serialize(final Set<Collaboration> collaborations, final JsonGenerator generator,
        final SerializerProvider provider) throws IOException, JsonProcessingException {
        final Set<SimpleCollaboration> simpleCollaborations = Sets.newHashSet();
        for (final Collaboration collaboration : collaborations) {
            simpleCollaborations.add(new SimpleCollaboration(collaboration.getId(), collaboration.getName()));                
        }
        generator.writeObject(simpleCollaborations);
    }

    static class SimpleCollaboration {

        private Long id;

        private String name;

        // constructors, getters/setters

    }

}

This serializer will only show a limited set of the properties of the Collaboration entity. Because the "tags" property is omited, no cyclic references will occur.

A good read about this topic can be found here. It explains all possibilities when you're having a similar scenario.

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