📋 CHEAT SHEET

JVM Memory Model Diagram

Visual diagram of JVM memory areas — Heap, Young/Old generation, Metaspace, Stack, PC Register and Code Cache explained.

JVM Memory Architecture

Shared — all threads

Heap · GC managed

Heap

Objects and arrays · tuned via -Xms / -Xmx

Young generation · Minor GC (milliseconds)

Eden space

New objects

All allocations start here

Survivor S0

From space

Survived 1+ GC

Survivor S1

To space

Copy target

Old generation (tenured) · Major GC (seconds)

Long-lived objects · promoted after N GC cycles · -XX:MaxTenuringThreshold

G1GC · ZGC · Shenandoah

No GC · native memory

Metaspace

Class metadata

Class bytecodes · method info · constant pool · replaced PermGen in Java 8+

-XX:MaxMetaspaceSize

Per-thread — not shared

JVM stack

Stack frames

One frame per method · local vars · operand stack · return address

StackOverflowError

PC register

Program counter

Current bytecode instruction address

Native stack

JNI calls

C/C++ native method frames

JIT only · no GC

Code cache

JIT-compiled native code

Hotspot methods compiled to machine code · -XX:ReservedCodeCacheSize

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