Tail Call Optimisation in Java

Question

As of Java 8, Java does not provide Tail-Call Optimization (TCO). On researching about it, I came to know the reason which is:

In JDK classes [...] there are a number of security sensitive methods that rely on counting stack frames between JDK library code and calling code to figure out who's calling them.

However Scala, which is based on JVM, has support for Tail-Call Optimisation. Scala does tail recursion optimisation at compile time. Why can't Java use the same approach?

PS: Not sure whether the latest version (Java 11 as of now) of Java now has TCO. Would be great if some who knows can share this also.

Notes:

1. I know TCO is at backlog and is of lower priority but want to know why can't Java make changes in compile time similar to Scala.

2. Java doesn't have tail call optimization for the same reason most imperative languages don't have it. Imperative loops are the preferred style of the language, and the programmer can replace tail recursion with imperative loops. (Source)

Answer 1

Why can't Java use the same approach ?

I can't say which approach will be used, but it's better-explained in Project Loom's proposal:

As adding the ability to manipulate call stacks to the JVM will undoubtedly be required, it is also the goal of this project to add an even lighter-weight construct that will allow unwinding the stack to some point and then invoke a method with given arguments (basically, a generalization of efficient tail-calls). We will call that feature unwind-and-invoke, or UAI. It is not the goal of this project to add an automatic tail-call optimization to the JVM.

As far as I've heard, work has not yet begun on tail calls, as Fibers and Continuations seem to currently be a higher priority.

Answer 2

I read a very nice blog post here about how to achieve tail recursion in Java: Knoldus blog post on Java tail recursion

However, the code on their blog doesn't compile so I created a small repo with their code but fixed the syntax so it compiles. Github repo with working code

Hope this is useful to someone, I found the ideas presented at the Knoldus blog post very interesting.

EDIT: actually I found out later that the ideas presented in the blog post are originally Venkat Subramaniam's. He discusses these subjects in his talk here.