[JAVA][SPRING-CLOUD] `spring-cloud` library generated code unusable with Java 8

Description

As the interfaces generated when using the Spring library builds an interface with a default method when using Java 8, the Feign integration is rendered inoperable as Feign will run the default method rather than initiating the HTTP call.

Swagger-codegen version

2.2.3

Swagger declaration file content or url

See https://github.com/wjam/swagger-codegen-spring-cloud for an example

Command line used for generation

Maven library used. Configuration is in the GitHub repo mentioned above

Steps to reproduce

Run mvn clean verify on the GitHub repo mentioned above.

Suggest a fix/enhancement

Without considering any backwards incompatibility issues, I'd say drop the default method as it is not only useless but also has a dangerous default return value of 200 rather than doing something like throwing java.lang.UnsupportedOperationException.

Another alternative, less backwards incompatible way, would be to allow the control over whether the default methods are generated or not.

Start by running mvn clean verify in the linked swagger-codegen-spring-cloud example and inspect the generated Java 8 interfaces used by Feign. Trace the Spring template or entry point that emits the default method, then verify that generated Feign calls initiate HTTP requests rather than returning the default response.