CommonDelegatingErrorHandler: How to traverse exceptions in order to find a correct delegate

[amseager]

Currently, everything in this handler uses this private method:

@Nullable
	private CommonErrorHandler findDelegate(Throwable thrownException) {
		Throwable cause = thrownException;
		if (cause instanceof ListenerExecutionFailedException) {
			cause = thrownException.getCause();
		}
		if (cause != null) {
			Class<? extends Throwable> causeClass = cause.getClass();
			for (Entry<Class<? extends Throwable>, CommonErrorHandler> entry : this.delegates.entrySet()) {
				if (entry.getKey().isAssignableFrom(causeClass)) {
					return entry.getValue();
				}
			}
		}
		return null;
	}

As you can see, if we have ListenerExecutionFailedException, we'll get its cause and find an appropriate delegate.

However, this wouldn't work in a lot of cases when we have our set of exceptions wrapped in something else, e.g. in MessageConversionException (I think it's the most popular case). And we cannot use MessageConversionException as a delegates map key since we want to have a different logic for each underlying exception (let's say we want custom DefaultErrorHandler for ConstraintViolationException with a fixed backoff and its own recoverer and the another one for any other exception with ExponentialBackOff without a recoverer at all - but both are wrapped in the same MessageConversionException).

How to solve this problem? I don't want to write a custom DelegatingErrorHandler because I'll need to copy almost all the code from the CommonDelegatingErrorHandler.
Previously, we had ConditionalDelegatingErrorHandler which had handle method being public so it could've been ok to override it but now it's deprecated in favor of CommonDelegatingErrorHandler.

[garyrussell]

I converted this to an issue; feel free to submit a PR.