LXR: Low-latency, high-throughput GC for JVM

> LXR takes a very different approach to existing production collectors, using advanced reference counting for prompt reclamation, carefully avoiding the overhead of concurrent copying.

Curious, why hasn't this been done before, like in Objective-C? Java has some of most advanced garbage collection of any language with fancy escape analysis and a plethora of collectors available, including some unique ones like G1 and Shenandoah.