This is an important release which comes with two new major features and a bunch of performance improvements:
#1764 New Feature: Interfaces can implement other interfaces
This change allows interfaces to be implement by other interfaces instead of just objects.
This post explains the motivation and the background for it
#1772 New Feature: Custom Scalar specification URL
This lets you define a specification URL for a custom Scalar, which documents how this Scalar works.
This talk explains some background.
Performance improvements
We have a lot of performance improvements in this release. Some of them are significant improvements for systems at high scale.
#1815 More efficient DataFetcher
#1891 Improve ExecutionPath performance
#1792 Reduce NonNullable Exception overhead
Directives on variable declaration
It is now allowed to use directives on variable declaration.
Subscription improvements
#1804 DataFethcerResult can now be returned from subscription fetchers
#1801 Fixes the Subscription publisher so that it does not drop values at scale.
Non standard Scalar handling (Breaking change)
Any special handling for non standard Scalars got removed. This means you have to declare any Long, Char, etc Scalar explicitly in your SDL and provide an implementation.
The implementation for these non standard Scalars are still available in GraphQL Java itself (in the Scalars class) but they will be removed in the next release. The extended scalars project is the recommend way to migrate.