SubscriptionTest.java

package org.zalando.fahrschein.domain;

import com.fasterxml.jackson.annotation.JsonInclude;
import com.fasterxml.jackson.databind.ObjectMapper;
import java.io.ByteArrayOutputStream;
import java.util.Collections;
import org.junit.Test;

import static org.junit.Assert.assertFalse;

public class SubscriptionTest {

  @Test
  public void serializationFailsUnlessFieldsAreMarkedIgnorable() throws Exception {

    ObjectMapper objectMapper = new ObjectMapper();
    // emulates NakadiClient#subscribe
    final Subscription subscription = new Subscription("ss", Collections.singleton("s"), "cg");
    ByteArrayOutputStream baos = new ByteArrayOutputStream();
    objectMapper.writeValue(baos, subscription);

    // bare object mappers serialize nulls
    assertFalse(baos.toString().contains("id"));
    assertFalse(baos.toString().contains("createdAt"));
  }

  @Test
  public void passesWithConfiguredObjectMapper() throws Exception {

    ObjectMapper objectMapper = new ObjectMapper();
    objectMapper.setSerializationInclusion(JsonInclude.Include.NON_NULL);

    // emulates NakadiClient#subscribe
    final Subscription subscription = new Subscription("ss", Collections.singleton("s"), "cg");
    ByteArrayOutputStream baos = new ByteArrayOutputStream();
    objectMapper.writeValue(baos, subscription);

    // nulls elided
    assertFalse(baos.toString().contains("id"));
    assertFalse(baos.toString().contains("createdAt"));
  }
}

If the sub object is changed to use ignorables (see // mark below), the bare object mapper will pass

package org.zalando.fahrschein.domain;

import com.fasterxml.jackson.annotation.JsonCreator;

import com.fasterxml.jackson.annotation.JsonInclude;
import javax.annotation.Nullable;
import javax.annotation.concurrent.Immutable;
import java.time.OffsetDateTime;
import java.util.Collections;
import java.util.HashSet;
import java.util.Set;

@Immutable
public class Subscription {
    @Nullable
    @JsonInclude(JsonInclude.Include.NON_NULL) // mark
    private final String id;
    private final String owningApplication;
    private final Set<String> eventTypes;
    private final String consumerGroup;
    @Nullable
    @JsonInclude(JsonInclude.Include.NON_NULL)  // mark
    private final OffsetDateTime createdAt;

   ...
}