Doppio uses JavaScript's garbage collector. (As a result, it cannot support weak references.) As for concurrency, thread quanta are mapped to JavaScript events, so Doppio can emulate preemptive multithreading. Doppio potentially preempts a "thread" at each function call.

The nitty gritty details are in the PLDI 2014 paper [0]. Some details have slightly changed, though (e.g. DoppioJVM supports JDK8 now).

[0] https://plasma-umass.github.io/doppio-demo/paper.pdf