Why is the Java runtime partially thread safe?

[lppedd]

Rewriting the Kotlin runtime taking the Java runtime as starting point, I've noticed that here and there it uses thread-safe utilities like synchronized blocks or CopyOnWriteArrayLists.

Looking for answers to "Is ANTLR thread safe?", we find:

In ANTLR 4, instance methods are not safe for multi-threaded operations, but you may create two separate instances of the parser and use those separate instances on multiple threads

So what's the point of using, for example, a CopyOnWriteArrayList?
What's the ANTLR team opinion on this subject?

[KvanTTT]

It could be just a bug or overengineering.

Some time ago I enountered that sometimes ANTLR is freezing in multithread mode. I started investigating and noticed that IDEA warns that some code may cause deadlock. I fixed the code and it fixed the original problem.