Adds fallback implementation configuration to the HystrixFeign builder

[codefromthecrypt]

Fallbacks are known values, which you return when there's an error invoking an http method.
For example, you can return a cached result as opposed to raising an error to the caller. To use
this feature, pass a safe implementation of your target interface as the last parameter to HystrixFeign.Builder.target.

Here's an example:

// When dealing with fallbacks, it is less tedious to keep interfaces small.
interface GitHub {
  @RequestLine("GET /repos/{owner}/{repo}/contributors")
  List<String> contributors(@Param("owner") String owner, @Param("repo") String repo);
}

// This instance will be invoked if there are errors of any kind.
GitHub fallback = (owner, repo) -> {
  if (owner.equals("Netflix") && repo.equals("feign")) {
    return Arrays.asList("stuarthendren"); // inspired this approach!
  } else {
    return Collections.emptyList();
  }
};

GitHub github = HystrixFeign.builder()
                            ...
                            .target(GitHub.class, "https://api.github.com", fallback);

Credit to the idea goes to @stuarthendren!

Fixes #298

[cloudbees-pull-request-builder]

NetflixOSS » feign » feign-pull-requests #195 SUCCESS
This pull request looks good

[codefromthecrypt]

ps @spencergibb at the level of abstraction of feign, it doesn't care how the fallback instance is created. so, it could be created by a dynamic proxy or whatever (which I suspect would be the way to integrate with custom annotation processing)

[codefromthecrypt]

will merge this in 8hrs unless there's blocking feedback, as there are folks waiting on 8.14

[codefromthecrypt]

scrubbed covariants after seeing this! https://gist.github.com/JakeWharton/14547311b1f145cb0695

[cloudbees-pull-request-builder]

NetflixOSS » feign » feign-pull-requests #198 SUCCESS
This pull request looks good

[codefromthecrypt]

this is now released as 8.14 cc @christopherlakey