lysine-dev/retrofit · error · NullPointerException

scheduler == null

Error message

scheduler == null

What it means

RxJava3CallAdapterFactory.createWithScheduler(Scheduler) builds an adapter factory that subscribeOn's the supplied scheduler by default. Passing null is rejected because the factory would then hold no scheduler to apply; for the no-scheduler case the library provides dedicated factory methods (createSynchronous() for synchronous, createAsync() for asynchronous). It throws NullPointerException ('scheduler == null') behind a ConstantConditions suppression.

Solutions

1. If you want no custom scheduler, use the dedicated factory: RxJava3CallAdapterFactory.createSynchronous()() or RxJava3CallAdapterFactory.createAsync().
2. Otherwise supply a real Scheduler such as Schedulers.io(), Schedulers.computation(), or AndroidSchedulers.mainThread().
3. Resolve the null at its source (DI binding / config property) rather than passing it through.

Example fix

// before
RxJavaCallAdapterFactory factory = RxJava3CallAdapterFactory.createWithScheduler(configScheduler);
// after
RxJava3CallAdapterFactory factory = configScheduler != null
    ? RxJava3CallAdapterFactory.createWithScheduler(configScheduler)
    : RxJava3CallAdapterFactory.createAsync();

Scheduler s = resolveScheduler(); // may be null when unconfigured
RxJava3CallAdapterFactory factory =
    s != null
      ? RxJava3CallAdapterFactory.createWithScheduler(s)
      : RxJava3CallAdapterFactory.createAsync();

try {
  factory = RxJava3CallAdapterFactory.createWithScheduler(s);
} catch (NullPointerException e) {
  // s was null; fall back to the async factory
  factory = RxJava3CallAdapterFactory.createAsync();
}

When it happens

Trigger: Calling RxJava3CallAdapterFactory.createWithScheduler(null), most often when the scheduler is read from DI/configuration/env that was not bound and resolved to null. Thrown immediately at the top of createWithScheduler.

Common situations: Reading a Scheduler from a DI graph or config property that resolved to null; conditional scheduler selection that falls through to null; copy-paste of createWithScheduler without choosing the right base factory method; environment differences (e.g. Android main thread scheduler absent in unit tests).

Related errors

• scheduler == null
• scheduler == null
• error == null
• response == null
• error == null

AI-assisted analysis of lysine-dev/retrofit@d0b112dad0 (2026-08-13). Data as JSON: /api/errors/af8148187767b3a8. Report an issue: GitHub.

Appendix: source

Thrown at retrofit-adapters/rxjava3/src/main/java/retrofit2/adapter/rxjava3/RxJava3CallAdapterFactory.java:86

    return new RxJava3CallAdapterFactory(null, true);
  }

  /**
   * Returns an instance which creates synchronous observables that do not operate on any scheduler
   * by default. Applying {@code subscribeOn(..)} will change the scheduler on which the HTTP calls
   * are made.
   */
  public static RxJava3CallAdapterFactory createSynchronous() {
    return new RxJava3CallAdapterFactory(null, false);
  }

  /**
   * Returns an instance which creates synchronous observables that {@code subscribeOn(..)} the
   * supplied {@code scheduler} by default.
   */
  @SuppressWarnings("ConstantConditions") // Guarding public API nullability.
  public static RxJava3CallAdapterFactory createWithScheduler(Scheduler scheduler) {
    if (scheduler == null) throw new NullPointerException("scheduler == null");
    return new RxJava3CallAdapterFactory(scheduler, false);
  }

  private final @Nullable Scheduler scheduler;
  private final boolean isAsync;

  private RxJava3CallAdapterFactory(@Nullable Scheduler scheduler, boolean isAsync) {
    this.scheduler = scheduler;
    this.isAsync = isAsync;
  }

  @Override
  public @Nullable CallAdapter<?, ?> get(
      Type returnType, Annotation[] annotations, Retrofit retrofit) {
    Class<?> rawType = getRawType(returnType);

    if (rawType == Completable.class) {
      // Completable is not parameterized (which is what the rest of this method deals with) so it

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