There was an issue filed about this, to which one of the Retrofit maintainers responded:
The reflection works around a bug in the JDK which was fixed in 14 but it’s only used for default methods. As it’s only a warning, it’s not preventing your call from working.
So your options are either to
- stick with Retrofit 2.8.x, and
- ignore the warning, or
- upgrade to Java 14
- downgrade to Retrofit 2.7.*