# Reactive Programming with Reactor 3

Reactor
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## Error

### Description

Reactor ships with several operators that can be used to deal with errors: propagate errors but also recover from it (eg. by falling back to a different sequence or by retrying a new Subscription).

### Practice

In the first example, we will return a Mono containing default user Saul when an error occurs in the original Mono, using the method onErrorReturn. If you want, you can even limit that fallback to the IllegalStateException class. Use the User#SAUL constant.

onErrorReturn

Let's try the same thing with Flux. In this case, we don't just want a single fallback value, but a totally separate sequence (think getting stale data from a cache). This can be achieved with onErrorResume, which falls back to a Publisher<T>.

Emit bothUser#SAUL and User#JESSE whenever there is an error in the original FLux:

OnErrorResumeWith on flux

Dealing with checked exceptions is a bit more complicated. Whenever some code that throws checked exceptions is used in an operator (eg. the transformation function of a map), you will need to deal with it. The most straightforward way is to make a more complex lambda with a try-catch block that will transform the checked exception into a RuntimeException that can be signalled downstream.

There is a Exceptions#propagate utility that will wrap a checked exception into a special runtime exception that can be automatically unwrapped by Reactor subscribers and StepVerifier: this avoid seeing an irrelevant RuntimeException in the stacktrace.

Try to use that on the capitalizeMany method within a map: you'll need to catch a GetOutOfHereException, which is checked, but the corresponding test still expects the GetOutOfHereException directly.

Checked exception
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