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Error Handling Operators
There are a variety of operators that you can use to react to or recover from onError notifications from reactive sources, such as Observables. For example, you might:
- swallow the error and switch over to a backup Observable to continue the sequence
- swallow the error and emit a default item
- swallow the error and immediately try to restart the failed Observable
- swallow the error and try to restart the failed Observable after some back-off interval
doOnErroronErrorCompleteonErrorResumeNextonErrorReturnonErrorReturnItemonExceptionResumeNextretryretryUntilretryWhen
Available in: image Flowable, image Observable, image Maybe, image Single, image Completable
ReactiveX documentation: http://reactivex.io/documentation/operators/do.html
Instructs a reactive type to invoke the given io.reactivex.functions.Consumer when it encounters an error.
Observable.error(new IOException("Something went wrong")) .doOnError(error -> System.err.println("The error message is: " + error.getMessage())) .subscribe( x -> System.out.println("onNext should never be printed!"), Throwable::printStackTrace, () -> System.out.println("onComplete should never be printed!"));
Available in: image Flowable, image Observable, image Maybe, image Single, image Completable
ReactiveX documentation: http://reactivex.io/documentation/operators/catch.html
Instructs a reactive type to swallow an error event and replace it by a completion event.
Optionally, a io.reactivex.functions.Predicate can be specified that gives more control over when an error event should be replaced by a completion event, and when not.
Completable.fromAction(() -> { throw new IOException(); }).onErrorComplete(error -> { // Only ignore errors of type java.io.IOException. return error instanceof IOException; }).subscribe( () -> System.out.println("IOException was ignored"), error -> System.err.println("onError should not be printed!"));
Available in: image Flowable, image Observable, image Maybe, image Single, image Completable
ReactiveX documentation: http://reactivex.io/documentation/operators/catch.html
Instructs a reactive type to emit a sequence of items if it encounters an error.
Observable<Integer> numbers = Observable.generate(() -> 1, (state, emitter) -> { emitter.onNext(state); return state + 1; }); numbers.scan(Math::multiplyExact) .onErrorResumeNext(Observable.empty()) .subscribe( System.out::println, error -> System.err.println("onError should not be printed!")); // prints: // 1 // 2 // 6 // 24 // 120 // 720 // 5040 // 40320 // 362880 // 3628800 // 39916800 // 479001600
Available in: image Flowable, image Observable, image Maybe, image Single, image Completable
ReactiveX documentation: http://reactivex.io/documentation/operators/catch.html
Instructs a reactive type to emit the item returned by the specified io.reactivex.functions.Function when it encounters an error.
Single.just("2A") .map(v -> Integer.parseInt(v, 10)) .onErrorReturn(error -> { if (error instanceof NumberFormatException) return 0; else throw new IllegalArgumentException(); }) .subscribe( System.out::println, error -> System.err.println("onError should not be printed!")); // prints 0
Available in: image Flowable, image Observable, image Maybe, image Single, image Completable
ReactiveX documentation: http://reactivex.io/documentation/operators/catch.html
Instructs a reactive type to emit a particular item when it encounters an error.
Single.just("2A") .map(v -> Integer.parseInt(v, 10)) .onErrorReturnItem(0) .subscribe( System.out::println, error -> System.err.println("onError should not be printed!")); // prints 0
Available in: image Flowable, image Observable, image Maybe, image Single, image Completable
ReactiveX documentation: http://reactivex.io/documentation/operators/catch.html
Instructs a reactive type to continue emitting items after it encounters an java.lang.Exception. Unlike onErrorResumeNext, this one lets other types of Throwable continue through.
Observable<String> exception = Observable.<String>error(IOException::new) .onExceptionResumeNext(Observable.just("This value will be used to recover from the IOException")); Observable<String> error = Observable.<String>error(Error::new) .onExceptionResumeNext(Observable.just("This value will not be used")); Observable.concat(exception, error) .subscribe( message -> System.out.println("onNext: " + message), err -> System.err.println("onError: " + err)); // prints: // onNext: This value will be used to recover from the IOException // onError: java.lang.Error
Available in: image Flowable, image Observable, image Maybe, image Single, image Completable
ReactiveX documentation: http://reactivex.io/documentation/operators/retry.html
Instructs a reactive type to resubscribe to the source reactive type if it encounters an error in the hopes that it will complete without error.
Observable<Long> source = Observable.interval(0, 1, TimeUnit.SECONDS) .flatMap(x -> { if (x >= 2) return Observable.error(new IOException("Something went wrong!")); else return Observable.just(x); }); source.retry((retryCount, error) -> retryCount < 3) .blockingSubscribe( x -> System.out.println("onNext: " + x), error -> System.err.println("onError: " + error.getMessage())); // prints: // onNext: 0 // onNext: 1 // onNext: 0 // onNext: 1 // onNext: 0 // onNext: 1 // onError: Something went wrong!
Available in: image Flowable, image Observable, image Maybe, image Single, image Completable
ReactiveX documentation: http://reactivex.io/documentation/operators/retry.html
Instructs a reactive type to resubscribe to the source reactive type if it encounters an error until the given io.reactivex.functions.BooleanSupplier returns true.
LongAdder errorCounter = new LongAdder(); Observable<Long> source = Observable.interval(0, 1, TimeUnit.SECONDS) .flatMap(x -> { if (x >= 2) return Observable.error(new IOException("Something went wrong!")); else return Observable.just(x); }) .doOnError((error) -> errorCounter.increment()); source.retryUntil(() -> errorCounter.intValue() >= 3) .blockingSubscribe( x -> System.out.println("onNext: " + x), error -> System.err.println("onError: " + error.getMessage())); // prints: // onNext: 0 // onNext: 1 // onNext: 0 // onNext: 1 // onNext: 0 // onNext: 1 // onError: Something went wrong!
Available in: image Flowable, image Observable, image Maybe, image Single, image Completable
ReactiveX documentation: http://reactivex.io/documentation/operators/retry.html
Instructs a reactive type to pass any error to another Observable or Flowable to determine whether to resubscribe to the source.
Observable<Long> source = Observable.interval(0, 1, TimeUnit.SECONDS) .flatMap(x -> { if (x >= 2) return Observable.error(new IOException("Something went wrong!")); else return Observable.just(x); }); source.retryWhen(errors -> { return errors.map(error -> 1) // Count the number of errors. .scan(Math::addExact) .doOnNext(errorCount -> System.out.println("No. of errors: " + errorCount)) // Limit the maximum number of retries. .takeWhile(errorCount -> errorCount < 3) // Signal resubscribe event after some delay. .flatMapSingle(errorCount -> Single.timer(errorCount, TimeUnit.SECONDS)); }).blockingSubscribe( x -> System.out.println("onNext: " + x), Throwable::printStackTrace, () -> System.out.println("onComplete")); // prints: // onNext: 0 // onNext: 1 // No. of errors: 1 // onNext: 0 // onNext: 1 // No. of errors: 2 // onNext: 0 // onNext: 1 // No. of errors: 3 // onComplete