Error Handling Operators

This section explains operators that handle errors and exceptions encountered by Observables.

• onErrorResumeNext( ) — instructs an Observable to continue emitting items after it encounters an error
• onErrorReturn( ) — instructs an Observable to emit a particular item when it encounters an error
• onExceptionResumeNextViaObservable( ) — instructs an Observable to continue emitting items after it encounters an exception (but not another variety of throwable)

onErrorResumeNext( )

instructs an Observable to attempt to continue emitting items after it encounters an error

The onErrorResumeNext( ) method returns an Observable that mirrors the behavior of the source Observable, unless that Observable invokes onError( ) in which case, rather than propagating that error to the Observer, onErrorResumeNext( ) will instead begin mirroring a second, backup Observable, as shown in the following sample code:

def myObservable = Observable.create({ anObserver ->
  anObserver.onNext('Three');
  anObserver.onNext('Two');
  anObserver.onNext('One');
  anObserver.onError();
});
def myFallback = Observable.create({ anObserver ->
  anObserver.onNext('0');
  anObserver.onNext('1');
  anObserver.onNext('2');
  anObserver.onCompleted();
});

myObservable.onErrorResumeNext(myFallback).subscribe(
  [ onNext:{ myWriter.println(it); },
    onCompleted:{ myWriter.println("Sequence complete"); },
    onError:{ myWriter.println("Error encountered"); } ]
);

Three
Two
One
0
1
2
Sequence complete

onErrorReturn( )

instructs an Observable to emit a particular item to an Observer’s onNext method when it encounters an error

The onErrorReturn( ) method returns an Observable that mirrors the behavior of the source Observable, unless that Observable invokes onError( ) in which case, rather than propagating that error to the Observer, onErrorReturn( ) will instead emit a specified item and invoke the Observer's onCompleted( ) method, as shown in the following sample code:

def myObservable = Observable.create({ anObserver ->
  anObserver.onNext('Four');
  anObserver.onNext('Three');
  anObserver.onNext('Two');
  anObserver.onNext('One');
  anObserver.onError();
});

myObservable.onErrorReturn({ return('Blastoff!'); }).subscribe(
  [ onNext:{ myWriter.println(it); },
    onCompleted:{ myWriter.println("Sequence complete"); },
    onError:{ myWriter.println("Error encountered"); } ]
);

Four
Three
Two
One
Blastoff!
Sequence complete

onExceptionResumeNextViaObservable( )

instructs an Observable to continue emitting items after it encounters an exception (but not another variety of throwable)

Much like onErrorResumeNext( ) method, this returns an Observable that mirrors the behavior of the source Observable, unless that Observable invokes onError( ) in which case, if the Throwable passed to onError( ) is an Exception, rather than propagating that Exception to the Observer, onExceptionResumeNextViaObservable( ) will instead begin mirroring a second, backup Observable. If the Throwable is not an Exception, the Observable returned by onExceptionResumeNextViaObservable( ) will propagate it to its observers' onError( ) method and will not invoke its backup Observable.