Garbage collection and finalizers

Reachability, not scope, decides collection. An object is collectable once no chain of references reaches it from a GC root — a live thread’s stack, a static field, or a JNI reference.

Why finalize() fails

• It may never run. The JVM is allowed to exit without finalising anything.
• It runs on an unspecified thread at an unspecified time.
• It costs at least two GC cycles, because a finalizable object must survive one collection to be finalised and another to be freed.
• Resurrecting this inside it is legal, which makes the whole design unsound.

// The replacement
class Handle implements AutoCloseable {
    @Override public void close() { /* release here */ }
}

try (Handle h = new Handle()) {
    // deterministic release at the end of the block
}

Reference types, weakest last

SoftReference is cleared under memory pressure, WeakReference at the next collection, PhantomReference only after the object is finalised — the last being the basis for Cleaner, the sanctioned post-mortem hook.