How to test deserialization

[jorgheymans]

We recently ran into an issue with this class:

/** Tracks the number of times a page is routed to in a counter. */
public class PageViewMeter implements VaadinAfterNavigationListener, MeterBinder {

  // 2 or more consecutive numbers
  private static final Pattern NUMERIC_ID_PATTERN = Pattern.compile("\\d{2,}");

  // kubernetes-kit will reinject this field after deserialization
  private transient MeterRegistry meterRegistry;

  @Override
  public void bindTo(@NonNull MeterRegistry meterRegistry) {
    this.meterRegistry = meterRegistry;
  }

  @Override
  public void afterNavigation(AfterNavigationEvent event) {
    Counter.builder(PAGE_VIEWS.getName())
        .description(PAGE_VIEWS.getDescription())
        .withRegistry(meterRegistry)
        .withTag("page", sanitize(event.getLocation().getPath()))
        .increment();
  }

  protected String sanitize(String url) {
    if (url == null || url.isEmpty()) {
      return "/";
    }
    return NUMERIC_ID_PATTERN.matcher(url).replaceAll("id");
  }
}

This meter will work fine, until the moment where the session is deserialized from the session store (redis):

java.lang.NullPointerException: Cannot invoke "io.micrometer.core.instrument.MeterRegistry.counter(io.micrometer.core.instrument.Meter$Id)" because "registry" is null
	at io.micrometer.core.instrument.Counter$Builder.register(Counter.java:150)
	at io.micrometer.core.instrument.Counter$Builder.lambda$withRegistry$0(Counter.java:134)
	at io.micrometer.core.instrument.Meter$MeterProvider.withTag(Meter.java:518)
	at ....internal.metrics.PageViewMeter.afterNavigation(PageViewMeter.java:32)

Refactoring the class to use constructor injection fixed the issue:

@Slf4j
public class PageViewMeter implements VaadinAfterNavigationListener {

  // vaadin kubernetes-kit will reinject this field after deserialization
  private final transient MeterRegistry meterRegistry;

  /**
   * Constructor injection purposely. If we implement MeterBinder instead, reinjection after
   * deserialization does not work
   */
  public PageViewMeter(@NonNull MeterRegistry meterRegistry) {
    this.meterRegistry = meterRegistry;
  }

  @Override
  public void afterNavigation(AfterNavigationEvent event) {
    if (meterRegistry == null) {
      log.warn("MeterRegistry is not set, unable to meter PageViews");
      return;
    }

    final Meter.MeterProvider<Counter> page =
        Counter.builder(PAGE_VIEWS.getName())
            .description(PAGE_VIEWS.getDescription())
            .withRegistry(meterRegistry);
.....
.....

This was not obvious to spot. So my question is how can we write a test for this? So that we know that the mechanism of kubernetes-kit reinjecting spring dependencies after deserialization works.

[mcollovati]

Strange that the transient field is handled by kubernetes-kit only with constructor injection. I wonder if the registry instance provided to the constructor and to the bindTo method are the same. The only reason I can imagine for the registry not being handled during deserialization when using bindTo is that the registry instance is not a spring bean (e.g. a wrapper over the bean)

[jorgheymans]

It would be nice if there was a supported way to test such class, pushing it through the serialization and deserialization mechanism. I thought at some point i'd seen a util class that provides such thing, not in kubernetes-kit but Vaadin core perhaps. But i'm unable to find it back.

[mcollovati]

Flow codebase has a utility class that simply serializes and deserializes an object. Useful to track classes that are not marked as Serializable of that have unserializable field instances, but, for example, it would not help in the above case, since transient injection should be involved, and this is a Kubernetes Kit feature.

You can perhaps get inspiration from SerializationDeserializationTest in Kubernets Kit codebase

kubernetes-kit/kubernetes-kit-starter/src/test/java/com/vaadin/kubernetes/starter/sessiontracker/serialization/SerializationDeserializationTest.java

Lines 57 to 72 in dbb9d59

	void processConstructorInjectedComponent(
	@Autowired TestConfig.CtorInjectionTarget target) throws Exception {
	ByteArrayOutputStream os = new ByteArrayOutputStream();
	TransientInjectableObjectOutputStream.newInstance(os, handler)
	.writeWithTransients(target);
	Object result = new TransientInjectableObjectInputStream(
	new ByteArrayInputStream(os.toByteArray()), handler)
	.readWithTransients();
	Assertions.assertThat(result).isNotSameAs(target)
	.isExactlyInstanceOf(TestConfig.CtorInjectionTarget.class)
	.asInstanceOf(InstanceOfAssertFactories
	.type(TestConfig.CtorInjectionTarget.class))
	.extracting(obj -> obj.defaultImpl, obj -> obj.alternative)
	.containsExactly(target.defaultImpl, target.alternative);
	}