Getting rid of Json Recursion

Ever Fall into Infinite Recursion?

I have!

During a project, I was referencing UrlEntity and ClickHistory entity through a bidirectional relation without caring about the Jackson serialization process. And while testing it, I saw that Jackson keeps serializing back and forth forever — and it caused infinite recursion!

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How Jackson Works?

You write your POJO obviously for a reason — to convert it to a JSON object! This conversion is called serialization, and when you convert JSON back into a Java object, that’s deserialization .

Here comes Jackson — a Java library that handles serialization and deserialization .

But there are some critical points:

When Jackson serializes objects, it follows all fields recursively until:

1. All fields are visited, or
2. It finds a break (null)

Think of them like base cases in recursion.

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What Happened in My Case

@Entity
public class Url {
    @OneToMany(mappedBy = "url")
    private List<ClickHistory> clicks;
}

@Entity
public class ClickHistory {
    @ManyToOne
    private Url url;
}

I had a bidirectional relation. In this case, think Url as a parent class and ClickHistory as a child class.

• Jackson started serializing objects from the parent class (Url).
• It saw the relation with the child class and went there.
• While serializing the child (ClickHistory), it found a relation back to the parent (Url).
• Then it started serializing the parent again… and again…

That caused an infinite loop, or you could say infinite recursion!

This kind of thing often happens while creating relations among different entities.

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Mitigating Infinite Recursion Problem

Normally, I use @JsonManagedReference and @JsonBackReference annotations.

What do they mean?

• If you use @JsonManagedReference on the parent class, Jackson will serialize it normally.
• Then it goes to the child class as referenced.
• When it finds the parent field again with a @JsonBackReference, it will tell Jackson: “Don’t serialize this — it’s already being handled on the parent side.”

Example

@Entity
public class Url {
    @OneToMany(mappedBy = "url")
    @JsonManagedReference
    private List<ClickHistory> clicks;
}

@Entity
public class ClickHistory {
    @ManyToOne
    @JsonBackReference
    private Url url;
}

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What Happens Now

• JSON for Url will include clicks.
• JSON for ClickHistory will not include the url field.
• The infinite loop is gone!