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There are a variety of operators that you can use to react to or recover from `onError` notifications from reactive sources, such as `Observable`s. For example, you might:
1. swallow the error and switch over to a backup Observable to continue the sequence
1. swallow the error and emit a default item
1. swallow the error and immediately try to restart the failed Observable
1. swallow the error and try to restart the failed Observable after some back-off interval
# Outline
- [`doOnError`](#doonerror)
- [`onErrorComplete`](#onerrorcomplete)
- [`onErrorResumeNext`](#onerrorresumenext)
- [`onErrorReturn`](#onerrorreturn)
- [`onErrorReturnItem`](#onerrorreturnitem)
- [`onExceptionResumeNext`](#onexceptionresumenext)
- [`retry`](#retry)
- [`retryUntil`](#retryuntil)
- [`retryWhen`](#retrywhen)
## doOnError
**Available in:**  `Flowable`,  `Observable`,  `Maybe`,  `Single`,  `Completable`
**ReactiveX documentation:** [http://reactivex.io/documentation/operators/do.html](http://reactivex.io/documentation/operators/do.html)
Instructs a reactive type to invoke the given `io.reactivex.functions.Consumer` when it encounters an error.
### doOnError example
```java
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!"));
```
## onErrorComplete
**Available in:**  `Flowable`,  `Observable`,  `Maybe`,  `Single`,  `Completable`
**ReactiveX documentation:** [http://reactivex.io/documentation/operators/catch.html](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.
### onErrorComplete example
```java
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!"));
```
## onErrorResumeNext
**Available in:**  `Flowable`,  `Observable`,  `Maybe`,  `Single`,  `Completable`
**ReactiveX documentation:** [http://reactivex.io/documentation/operators/catch.html](http://reactivex.io/documentation/operators/catch.html)
Instructs a reactive type to emit a sequence of items if it encounters an error.
### onErrorResumeNext example
```java
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
```
## onErrorReturn
**Available in:**  `Flowable`,  `Observable`,  `Maybe`,  `Single`,  `Completable`
**ReactiveX documentation:** [http://reactivex.io/documentation/operators/catch.html](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.
### onErrorReturn example
```java
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
```
## onErrorReturnItem
**Available in:**  `Flowable`,  `Observable`,  `Maybe`,  `Single`,  `Completable`
**ReactiveX documentation:** [http://reactivex.io/documentation/operators/catch.html](http://reactivex.io/documentation/operators/catch.html)
Instructs a reactive type to emit a particular item when it encounters an error.
### onErrorReturnItem example
```java
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
```
## onExceptionResumeNext
**Available in:**  `Flowable`,  `Observable`,  `Maybe`,  `Single`,  `Completable`
**ReactiveX documentation:** [http://reactivex.io/documentation/operators/catch.html](http://reactivex.io/documentation/operators/catch.html)
Instructs a reactive type to continue emitting items after it encounters an `java.lang.Exception`. Unlike [`onErrorResumeNext`](#onerrorresumenext), this one lets other types of `Throwable` continue through.
### onExceptionResumeNext example
```java
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
```
## retry
**Available in:**  `Flowable`,  `Observable`,  `Maybe`,  `Single`,  `Completable`
**ReactiveX documentation:** [http://reactivex.io/documentation/operators/retry.html](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.
### retry example
```java
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!
```
## retryUntil
**Available in:**  `Flowable`,  `Observable`,  `Maybe`,  `Single`,  `Completable`
**ReactiveX documentation:** [http://reactivex.io/documentation/operators/retry.html](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`.
### retryUntil example
```java
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!
```
## retryWhen
**Available in:**  `Flowable`,  `Observable`,  `Maybe`,  `Single`,  `Completable`
**ReactiveX documentation:** [http://reactivex.io/documentation/operators/retry.html](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.
### retryWhen example
```java
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
```
|
