Finalization in the collector interface

Abstract

When a tracing garbage collector operates, it treats the objects and pointers in the system as nodes and edges in a directed graph. Most collectors simply use the graph to seek out objects that have become unreachable from the root objects and recycle the storage associated with them.

A few collector designers have hit on the idea of using the trace to gather other information about the connectivity of the graph, and notify user-level code when an object is reachable from the roots, but only in a restricted way. The user-level code typically uses this information to perform some final action on the object, and then destroys even the restricted access to the object, allowing the next pass of the garbage collector to recycle the storage. Finalization is useful for appropriating the power of garbage collection to manage non-memory resources. The resource in question can be embodied in a memory object with finalization enabled. When the memory resource is reachable only through restricted paths, the non-memory resource can be recycled and the restricted access destroyed. The users of the resource need not coordinate to manage, nor do they need to know that the resource is precious or needs finalization.

This paper presents system-level details of five different implementations of finalization in five different systems, and language-level details of several languages that have defined similar mechanisms. These comparisons highlight several areas of concern when designing a system with finalization.