Runtime Semantics

Garbage Collection Does Not Close External Resources

An object can become unreachable while a file descriptor, socket, database transaction, or operating-system lock associated with it still has a meaningful external lifetime. Garbage collection can reclaim managed memory after reachability disappears. It does not, by that fact alone, perform the protocol operation that releases an external resource. The distinction is structural. A collector reasons about references inside a managed heap. A resource such as a file descriptor is an entry maintained by the operating system, and a database transaction is state maintained by another component. Their release semantics come from APIs and protocols outside the collector’s reachability model.