Java Specialists is the long-running training company of Heinz Kabutz, a South African Java performance specialist who has been publishing the Java Specialists' Newsletter since 2000 — one of the longest continuously running developer publications anywhere. His material is widely respected for depth on the JVM internals and concurrency that most Java tutorials skip.
The CourseFlix listing carries three Java Specialists courses: Mastering Java 17, Java Design Patterns, and Data Structures in Java. Material is paid and aimed at experienced Java developers who want depth on the modern language and its idiomatic patterns rather than yet another framework tutorial.