JVM memory na garbage collection hufanya kazi vipi?

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JVM **husimamia kumbukumbu kiotomatiki** — ikitenga objects na kuachilia zisizofikika kupitia **garbage collection (GC)** — ili usilazimike kuachilia kumbukumbu mwenyewe. Kuelewa maeneo ya kumbukumbu na tabia ya GC ni ufunguo wa kurekebisha utendaji na kutambua matatizo ya kumbukumbu. ## Maeneo ya kumbukumbu ya 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+) ``` Objects huishi kwenye **heap**; local variables na references huishi kwenye **stacks** za kila thread. GC hulenga heap. ## Dhana ya kizazi (generational hypothesis) ```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). ``` ## Jinsi GC inavyofanya kazi: mark na sweep ```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 ``` Object ni takataka inapokuwa haifikiki tena kutoka GC root yoyote. Huiachilii wewe — GC huipata na kuirudisha. ## 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 ``` ## Algorithms za GC (chagua kulingana na workload) ```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) ``` ## Memory leaks bado hutokea (licha ya GC) ```java // ❌ a leak: a static collection that only grows keeps objects REACHABLE forever static List

Maeneo ya kumbukumbu ya JVM

Dhana ya kizazi (generational hypothesis)

Jinsi GC inavyofanya kazi: mark na sweep

Minor vs Major GC

Algorithms za GC (chagua kulingana na workload)

Memory leaks bado hutokea (licha ya GC)

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