Memory Management (Heap, Stack, GC)

Introduction

You don't malloc in Java — the JVM does it for you. But understanding where objects live and when they're cleaned up is the difference between a service that runs for months and one that OOMs every weekend.

Understanding the topic

JVM memory layout:

• Stack — one per thread; holds method frames, primitives & references. Tiny, fast, freed on return.
• Heap — shared; holds every new-allocated object. Garbage-collected.
• Metaspace — class metadata (replaced PermGen in Java 8).
• Code cache — JIT-compiled native code.

Syntax reference

Heap regions for the G1 GC (default):

┌─────────── HEAP ───────────┐
│  Young Gen                 │
│   ├─ Eden     (new objs)   │
│   ├─ Survivor S0           │
│   └─ Survivor S1           │
├────────────────────────────┤
│  Old Gen   (long-lived)    │
└────────────────────────────┘
   ← Minor GC sweeps Young
   ← Major / Full GC sweeps Old

Real-world use

Common production failures:
• Memory leak: a static Map caches user objects forever — heap grows until OOM. Fix: use a WeakHashMap or Caffeine cache with size/TTL bounds.
• GC pause storm: a 32 GB heap with the default G1 occasionally pauses 4 seconds. Fix: switch to ZGC (-XX:+UseZGC) for sub-millisecond pauses.

Best practices

• Size the heap explicitly: -Xms2g -Xmx2g. Same min/max avoids resize jitter.
• Pick a GC: G1 (default), ZGC (low pause), Parallel (throughput).
• Profile with jcmd, JFR or async-profiler before tuning blindly.

Purpose of this lesson

Master Memory Management (Heap, Stack, GC) so you can apply it confidently in production Java code, technical interviews, and code reviews.

Step-by-step explanation

1. Understand the core idea behind Memory Management (Heap, Stack, GC).
2. Walk through the runnable example and tweak it in the playground.
3. Apply the pattern in a small Spring Boot or CLI exercise of your own.
4. Re-read the common mistakes and interview Q&A to lock the concept in.

Interactive workflow diagram

Debugging tips

• Read the full stack trace — Java's exception messages name the offending class and line.
• Reproduce in the smallest possible main() method before fixing in the real app.
• Use IntelliJ's debugger breakpoints and 'Evaluate Expression' rather than scattering System.out.

Optimization strategies

• Tune heap with -Xms/-Xmx — match Xms to Xmx in production to avoid resize pauses.
• G1 is the default since Java 9; switch to ZGC/Shenandoah for sub-ms pauses on huge heaps.
• Use JFR continuously in prod — overhead is <1%.

Enterprise example

Teams at Netflix, Uber and Goldman Sachs apply Memory Management (Heap, Stack, GC) daily — usually wrapped behind Spring Boot services with observability hooks (Micrometer + OpenTelemetry).

Interview questions & answers

Summary

In this lesson you learned Memory Management (Heap, Stack, GC) — the concept, syntax, a runnable example, and the production pitfalls to avoid. Apply it in the playground before moving on.