As far as I can tell by looking at the code, this class seems thread safe, with synchronized in the right place. But there's no comments/javadoc so it's not very explicit.
I'm wondering if it's safe to use one instance per schema registry server.
The code is here: https://github.com/confluentinc/schema-registry/blob/master/client/src/main/java/io/confluent/kafka/schemaregistry/client/CachedSchemaRegistryClient.java