Kako radi memorija JVM-a i prikupljanje smeća?

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JVM **automatski upravlja memorijom** — dodjeljuje objekte i oslobađa one koji nisu dostupni putem **prikupljanja smeća (GC)** — zato ne trebate ručno oslobađati memoriju. Razumijevanje memorijskih regija i ponašanja GC-a ključno je za optimizaciju performansi i dijagnostiku problema s memorijom. ## Memorijske regije JVM-a ```text HEAP — where all OBJECTS live (shared across threads), GC operates here ├── Young Generation — new objects (most die young) │ ├── Eden — new allocations go here │ └── Survivor — objects that survived a young GC └── Old (Tenured) Gen — long-lived objects (promoted from young) STACK — per-thread; method frames, local variables, primitive locals, references Metaspace — class metadata (replaced PermGen in Java 8+) ``` Objekti žive na **heap**-u; lokalne varijable i reference žive na **stack** per-thread. GC se fokusira na heap. ## Generacijska hipoteza ```text Observation: MOST objects die young (short-lived). → GC splits the heap into generations and collects the young gen FREQUENTLY (cheap, fast) and the old gen RARELY (expensive). ``` ## Kako radi GC: označavanje i brisanje ```text 1. MARK — starting from GC roots (stack refs, static fields), mark all REACHABLE objects 2. SWEEP — reclaim memory of unmarked (unreachable) objects 3. COMPACT— (often) move survivors together to reduce fragmentation ``` Objekt je smeće kada više nije **dostupan** ni od jednog GC root-a. Vi ga ne oslobađate — GC ga pronalazi i vraća. ## Minor naspram Major GC ```text Minor GC — collects the Young Generation; frequent, fast, short pause Major/Full GC — collects the Old Generation (and more); rarer, longer pause → long Full GC pauses ("stop-the-world") are a common performance concern ``` ## GC algoritmi (odaberite prema radnom opterećenju) ```text G1 GC (default, Java 9+) — balanced, predictable pause times, good general choice Parallel GC — max throughput (batch jobs), longer pauses acceptable ZGC / Shenandoah — very low pause times for large heaps (latency-sensitive apps) ``` ## Curenja memorije još uvijek se događaju (unatoč GC-u) ```java // ❌ a leak: a static collection that only grows keeps objects REACHABLE forever static List