CODEWITHSUNDEEP
javaspringspring-data-rest
dcb
dcb

Reputation: 2292

How to Load an Entity Inside a Spring Data REST Server by Self Link?

Given a Spring Data REST (SDR) server built with Spring Boot Gradle Plugin 2.2.5.RELEASE, is it possible to load an @Entity by self link within the server application?

I'm aware how to access it with an HTTP client, e.g. using curl:

$ curl localhost/users/1 # Responds with 200 OK and JSON representation

What I'm searching for is a mechanism to do this in the server using Java only, ideally using a standard SDR mechanism:

@Service
public class SelfLinkResolver {

    public Object findBySelfLink(Link self) {
        if (self == null || !self.getRel().equals(SELF)) {
            throw new IllegalArgumentException("Non-null self link expected");
        }
        return null; // How to return the entity using a standard SDR mechanism?
    }

    public void exampleCall() {
        Link self = new Link("localhost/users/1");
        Object entity = findBySelfLink(self);
        requireNonNull(entity, "Failed to load entity by self link");
    }

}

Upvotes: 2

Views: 435

Answers (2)

dcb
dcb

Reputation: 2292

I finally came up with this solution, which uses SDR's UriToEntityConverter. In contrast to my question, it requires not only the self link, but also the entity class. It therefore doesn't fully answer my initial question.

I guess that there is no SDR solution that does not require the entity class, since there is no need for this within the framework, at least for usual API calls. SDR is always provided with the type information through the Repository, to which the self link refers. However, I didn't dive into other classes such as PersistentEntities, RepositoryInvokerFactory or Repositories, which might provide a solution for this.

WARNING: My tested implementation differs from this. This code is untested, but should illustrate the idea.

import lombok.NonNull;
import org.springframework.core.convert.TypeDescriptor;
import org.springframework.data.mapping.context.PersistentEntities;
import org.springframework.data.repository.support.Repositories;
import org.springframework.data.repository.support.RepositoryInvokerFactory;
import org.springframework.data.rest.core.UriToEntityConverter;
import org.springframework.hateoas.Link;
import org.springframework.stereotype.Component;

import java.net.URI;
import java.util.Optional;

import static com.google.common.base.Preconditions.checkArgument;
import static java.lang.String.format;
import static org.springframework.hateoas.IanaLinkRelations.SELF;

@Component
public class SelfLinkToEntityConverter extends UriToEntityConverter {

    private static final TypeDescriptor URI_DESCRIPTOR = TypeDescriptor.valueOf(URI.class);

    SelfLinkToEntityConverter(@NonNull PersistentEntities entities,
                              @NonNull RepositoryInvokerFactory invokerFactory,
                              @NonNull Repositories repositories) {
        super(entities, invokerFactory, repositories);
    }

    @NonNull
    public <T> Optional<T> findBySelfLink(@NonNull Link self, @NonNull Class<T> entityClass) {
        checkArgument(self.getRel().equals(SELF), "Non-null self link expected");

        URI uri = self.expand().toUri();
        TypeDescriptor typeDescriptor = TypeDescriptor.valueOf(entityClass);

        try {
            @SuppressWarnings("unchecked")
            T entity = (T) super.convert(uri, URI_DESCRIPTOR, typeDescriptor);

            return Optional.ofNullable(entity);
        } catch (IllegalArgumentException o_O) {
            throw new IllegalArgumentException(format("Failed to load %s: %s",
                    entityClass.getSimpleName(), self.getHref()));
        }
    }

}

Upvotes: 0

yejianfengblue
yejianfengblue

Reputation: 2419

An internal solution is parse your link and extract the ID (1 in your example), the call repository.findById(id).

Another solution would be new a RestTemplate, call your own API.

Upvotes: 1

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