How to serialize nested objects limiting depth of serialization?

Question

There is a simple POJO - Category with Set as subcategories inside. Nesting might be quite deep as each of subcategories may contai

Answer 1

If you can get current depth from a POJO you can do it with a ThreadLocal variable holding a limit. In a controller, before you return a Category instance set a depth limit on a ThreadLocal integer.

@RequestMapping("/categories")
@ResponseBody
public Category categories() {
    Category.limitSubCategoryDepth(2);
    return root;
}

In a subcategory getter you check depth limit against current depth of a category, if it's over the limit return null.

You'll need to clean up thread local somehow, perhaps with a spring's HandlerInteceptor::afterCompletition.

private Category parent;
private Set<Category> subCategories;

public Set<Category> getSubCategories() {
    Set<Category> result;
    if (depthLimit.get() == null || getDepth() < depthLimit.get()) {
        result = subCategories;
    } else {
        result = null;
    }
    return result;
}

public int getDepth() {
    return parent != null? parent.getDepth() + 1 : 0;
}

private static ThreadLocal<Integer> depthLimit = new ThreadLocal<>();

public static void limitSubCategoryDepth(int max) {
    depthLimit.set(max);
}

public static void unlimitSubCategory() {
    depthLimit.remove();
}

If you can't get depth from a POJO, you'll need to either make a tree copy with limited depth or learn how to code a custom Jackson serializer.