Spring boot @ResponseBody doesn't serialize entity id

I recently had the same problem and it's because that's how spring-boot-starter-data-rest works by default. See my SO question -> While using Spring Data Rest after migrating an app to Spring Boot, I have observed that entity properties with @Id are no longer marshalled to JSON

To customize how it behaves, you can extend RepositoryRestConfigurerAdapter to expose IDs for specific classes.

import org.springframework.context.annotation.Configuration;
import org.springframework.data.rest.core.config.RepositoryRestConfiguration;
import org.springframework.data.rest.webmvc.config.RepositoryRestConfigurerAdapter;

@Configuration
public class RepositoryConfig extends RepositoryRestConfigurerAdapter {
    @Override
    public void configureRepositoryRestConfiguration(RepositoryRestConfiguration config) {
        config.exposeIdsFor(Person.class);
    }
}

In case you need to expose the identifiers for all entities:

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.context.annotation.Configuration;
import org.springframework.data.rest.core.config.RepositoryRestConfiguration;
import org.springframework.data.rest.webmvc.config.RepositoryRestConfigurer;

import javax.persistence.EntityManager;
import javax.persistence.metamodel.Type;

@Configuration
public class RestConfiguration implements RepositoryRestConfigurer {

    @Autowired
    private EntityManager entityManager;

    @Override
    public void configureRepositoryRestConfiguration(RepositoryRestConfiguration config) {
        config.exposeIdsFor(
                entityManager.getMetamodel().getEntities().stream()
                .map(Type::getJavaType)
                .toArray(Class[]::new));
    }
}

Note that in versions of Spring Boot prior to 2.1.0.RELEASE you must extend the (now deprecated) org.springframework.data.rest.webmvc.config.RepositoryRestConfigurerAdapter instead of implement RepositoryRestConfigurer directly.

---

If you only want to expose the identifiers of entities that extends or implements specific super class or interface:

    ...
    @Override
    public void configureRepositoryRestConfiguration(RepositoryRestConfiguration config) {
        config.exposeIdsFor(
                entityManager.getMetamodel().getEntities().stream()
                .map(Type::getJavaType)
                .filter(Identifiable.class::isAssignableFrom)
                .toArray(Class[]::new));
    }

---

If you only want to expose the identifiers of entities with a specific annotation:

    ...
    @Override
    public void configureRepositoryRestConfiguration(RepositoryRestConfiguration config) {
        config.exposeIdsFor(
                entityManager.getMetamodel().getEntities().stream()
                .map(Type::getJavaType)
                .filter(c -> c.isAnnotationPresent(ExposeId.class))
                .toArray(Class[]::new));
    }

Sample annotation:

import java.lang.annotation.*;

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
public @interface ExposeId {}

Answer from @eric-peladan didn't work out of the box, but was pretty close, maybe that worked for previous versions of Spring Boot. Now this is how it is supposed to be configured instead, correct me if I'm wrong:

import org.springframework.context.annotation.Configuration;
import org.springframework.data.rest.core.config.RepositoryRestConfiguration;
import org.springframework.data.rest.webmvc.config.RepositoryRestConfigurerAdapter;

@Configuration
public class RepositoryConfiguration extends RepositoryRestConfigurerAdapter {

    @Override
    public void configureRepositoryRestConfiguration(RepositoryRestConfiguration config) {
        config.exposeIdsFor(User.class);
        config.exposeIdsFor(Comment.class);
    }
}