How does JVM memory and garbage collection work?

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The JVM **automatically manages memory** — allocating objects and freeing unreachable ones via **garbage collection (GC)** — so you don't manually free memory. Understanding the memory regions and GC behavior is key to tuning performance and diagnosing memory issues. ## JVM memory regions ```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 live on the **heap**; local variables and references live on per-thread **stacks**. GC focuses on the heap. ## The 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). ``` ## How GC works: mark and 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 ``` An object is garbage when it's no longer **reachable** from any GC root. You don't free it — the GC finds and reclaims it. ## 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 ``` ## GC algorithms (choose per 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 still happen (despite GC) ```java // ❌ a leak: a static collection that only grows keeps objects REACHABLE forever static List

JVM memory regions

The generational hypothesis

How GC works: mark and sweep

Minor vs Major GC

GC algorithms (choose per workload)

Memory leaks still happen (despite GC)

Why it matters