Java – use spring data rest to expose hypermedia links on collections, even if they are empty

First, I read the previous question: exposing link on collection entity in spring data rest

But the problem still exists, there is no trick

In fact, if I want to expose a link to a collection resource, I'm using the following code:

@Component
public class FooProcessor implements ResourceProcessor<PagedResources<Resource<Foo>>> {

    private final FooLinks fooLinks;

    @Inject
    public FooProcessor(FooLinks fooLinks) {
        this.FooLinks = fooLinks;
    }

    @Override
    public PagedResources<Resource<Foo>> process(PagedResources<Resource<Foo>> resource) {
        resource.add(fooLinks.getMyCustomLink());
        return resource;
    }
}

This is normal unless the collection is empty

The only feasible method is to replace the following code with:

@Component
public class FooProcessor implements ResourceProcessor<PagedResources> {

    private final FooLinks fooLinks;

    @Inject
    public FooProcessor(FooLinks fooLinks) {
        this.FooLinks = fooLinks;
    }

    @Override
    public PagedResources process(PagedResources resource) {
        resource.add(fooLinks.getMyCustomLink());
        return resource;
    }
}

But by doing so, the link will be exposed to all collections

I can create conditions to expose only what I want, but I don't think it's clean

Solution

I think spring has some magic when trying to find the type of collection - you can't tell what type it is on an empty collection - so spring data rest can't determine which resourceprocessor to use

I think I've seen org springframework. data. rest. webmvc. ResourceProcessorHandlerMethodReturnValueHandler. Resourcesprocessorwrapper #isvaluetypematch they try to determine the type by looking at the first element in the collection, otherwise they just stop processing:

if (content.isEmpty()) {
    return false;
}

So I don't think you can use spring data rest to solve this problem For your controller, you can go back and write a custom controller and use spring HATEOAS and implement your own resourceassemblysupport to view links on empty collections