@Beta
public interface ImplicitOptionalConverter
extends com.google.common.base.Function<InvocationSuccess,com.google.common.base.Optional<Object>>

When a client marked @Delegate is optional, the implementation of this is responsible for creating the optional object. For example.

 interface MyCloud {
    @Delegate
    Optional<KeyPairClient> getKeyPairExtensionForRegion(String region);
 }
 

The input object of type InvocationSuccess will include the following.

1. the class declaring the method that returns optional: InvocationSuccess#getClazz; in the example above, MyCloud
2. the method returning the optional: InvocationSuccess#getMethod; in the example above, getKeyPairExtensionForRegion
3. the args passed to that method at runtime: InvocationSuccess#getArgs; for example North
4. the rest client to be enclosed in the optional, should you choose to return it: InvocationSuccess#getReturnVal; in the example above, an implementation of KeyPairClient

Using this context, your implementation of ImplicitOptionalConverter can perform whatever you need, when deciding if the the returnVal is present and available. Here are some ideas:

• call a smoke test command
• look at the annotations on the class and compare those against a configuration switch enabling the extension
• inspect the health of the client, perhaps looking for error status
• call another api which can validate the feature can be presented

The default implementation returns present if no SinceApiVersion annotation is assigned, or the value is less than or equal to the current ApiVersion. To override this, add the following in your subclass override of RestClientModule#configure method:

 bind(ImplicitOptionalConverter.class).to(MyCustomOptionalConverter.class);