Kako deluje pomnilnik JVM in čiščenje garbage?

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JVM **avtomatično upravlja pomnilnik** — dodeljuje objekte in sprošča nedosegljive preko **garbage collection (GC)** — tako da ne morate ročno sproščati pomnilnika. Razumevanje pomnilniških regij in vedenja GC je ključno za nastavljanje delovanja in diagnosticiranje problemov s pomnilnikom. ## Pomnilniške regije JVM ```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 živijo na **heap**; lokalne spremenljivke in reference živijo na per-nit **stack**. GC se osredotoča 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 deluje GC: označevanje in čiščenje ```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 smeti, ko ni več **dosegljiv** iz nobenega GC root. Ga ne sproščate — GC ga najde in spet pridobi. ## Manjše in večje 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 (izbira glede na delovno obremenitev) ```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) ``` ## Puščanje pomnilnika se še vedno dogaja (kljub GC) ```java // ❌ a leak: a static collection that only grows keeps objects REACHABLE forever static List

Pomnilniške regije JVM

Generacijska hipoteza

Kako deluje GC: označevanje in čiščenje

Manjše in večje GC

GC algoritmi (izbira glede na delovno obremenitev)

Puščanje pomnilnika se še vedno dogaja (kljub GC)

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