Kaip veikia JVM atmintis ir atliekų rinkimas?

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JVM **automatiškai valdo atmintį** — paskirstydama objektus ir atlaisydama nepasiekiamus **atliekų rinkimo (GC)** būdu — todėl jums nereikia rankiniu būdu atlaisvinti atminties. Atmintis sričių ir GC elgsenos supratimas yra būtinas našumui reguliuoti ir atmintiesprobleмoms diagnozuoti. ## JVM atmintisregionai ```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+) ``` Objektai gyvena **krūvoje**; lokalūs kintamieji ir nuorodos gyvena per giją **kaupimuose**. GC sutelktas į krūvą. ## Generacine hipoteze ```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). ``` ## Kaip veikia GC: žymėjimas ir valymas ```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 ``` Objektas tampa atliekus, kai jis nebėra **pasiekiamas** iš jokios GC šaknies. Jūs jo neatlaisvaite — GC jį randa ir atgauna. ## Minor GC vs 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 algoritmai (pasirinkite pagal darbo krūvį) ```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) ``` ## Atmintiesžudytės vis dar nutinka (nepaisant GC) ```java // ❌ a leak: a static collection that only grows keeps objects REACHABLE forever static List