fallback.md

layout	title
default	Fallback

Fallback

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[Fallbacks][Fallback] allow you to provide an alternative result for a failed execution. They can also be used to suppress exceptions and provide a default result:

Fallback<Object> fallback = Fallback.of(defaultResult);

Throw a custom exception:

Fallback<Object> fallback = Fallback.ofException(e -> new CustomException(e.getException()));

Or compute an alternative result such as from a backup resource:

Fallback<Object> fallback = Fallback.of(this::connectToBackup);

A [CompletionStage] can be supplied as a fallback:

Fallback<Object> fallback = Fallback.ofStage(this::connectToBackup);

For computations that block, a Fallback can be configured to run asynchronously:

Fallback<Object> fallback = Fallback.builder(this::blockingCall).withAsync().build();

Failure Handling

[Fallbacks][Fallback] can be configured to handle only [certain results or exceptions][failure-handling]:

builder
  .handle(ConnectException.class)
  .handleResult(null);

When using a Fallback in combination with another policy, it's common to configure both to handle the same failures.

Event Listeners

[Fallbacks][Fallback] support event listeners that can tell you when the last execution attempt failed:

builder.onFailedAttempt(e -> log.error("Connection failed", e.getException()))

When the fallback attempt failed:

builder.onFailure(e -> log.error("Failed to connect to backup", e.getException()));

Or when the execution or fallback attempt succeeded:

builder.onSuccess(e -> log.info("Connection established"));

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