Bagaimana memori JVM dan pengumpulan sampah bekerja?

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JVM **secara otomatis mengelola memori** — mengalokasikan objek dan membebaskan yang tidak dapat dijangkau melalui **pengumpulan sampah (GC)** — jadi Anda tidak perlu membebaskan memori secara manual. Memahami wilayah memori dan perilaku GC sangat penting untuk menyetel kinerja dan mendiagnosis masalah memori. ## Wilayah memori 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+) ``` Objek hidup di **heap**; variabel lokal dan referensi hidup di **stack** per-thread. GC berfokus pada heap. ## Hipotesis generasional ```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). ``` ## Cara kerja GC: tandai dan bersihkan ```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 ``` Objek adalah sampah ketika tidak lagi **dapat dijangkau** dari akar GC mana pun. Anda tidak membebaskannya — GC menemukannya dan mengklaimnya. ## Minor 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 ``` ## Algoritma GC (pilih sesuai beban kerja) ```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) ``` ## Kebocoran memori masih terjadi (terlepas dari GC) ```java // ❌ a leak: a static collection that only grows keeps objects REACHABLE forever static List

Wilayah memori JVM

Hipotesis generasional

Cara kerja GC: tandai dan bersihkan

Minor vs Major GC

Algoritma GC (pilih sesuai beban kerja)

Kebocoran memori masih terjadi (terlepas dari GC)

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