Memory Management & GC

JVM memory: Heap (shared objects, GC'd), Stack (per-thread locals), Metaspace (class metadata, native memory), PC registers, and native method stacks.

Heap = Young Gen (Eden + 2 Survivors) + Old Gen. New objects go to Eden. Survivors hold objects that survived GC. Old Gen holds long-lived objects.

Young Gen holds new short-lived objects, collected frequently and cheaply (Minor GC). Old Gen holds long-lived objects, collected rarely and expensively (Major/Full GC).

A reference is a pointer to an object on the heap. Java has strong, soft, weak, and phantom references with different GC behavior.

Stack is faster because allocation is a pointer bump (no GC), data is cache-local, and access does not need indirection. Heap requires GC and has pointer chasing.

GC traces from roots, marks live objects, and reclaims dead ones. Most objects die young (generational hypothesis), so the young generation is collected frequently and cheaply.

See Q110 (JVM Internals). finalize() is deprecated because it is unpredictable, delays GC, and has security issues. Use try-with-resources and Cleaner.

GC only collects UNREACHABLE objects. A memory leak in Java means objects are reachable but no longer needed – growing static collections, unclosed resources, listener registries.

No. System.gc() is only a suggestion – the JVM may ignore it. It can cause a Full GC STW pause at the worst time. Let the GC decide when to collect.

Enable -XX:+HeapDumpOnOutOfMemoryError, analyze with Eclipse MAT, find the largest retained objects, and trace the GC root path to identify the leak.

StackOverflowError: thread stack exhausted (deep recursion). OutOfMemoryError: heap/metaspace/native memory exhausted.

See Q73 for the complete JVM memory model. Heap (Young+Old), Stack (per-thread), Metaspace (class metadata), PC registers, native method stacks.

PermGen was a fixed-size heap region for class metadata. It caused OOM when full. Metaspace (Java 8+) uses native memory that grows dynamically.

Minor GC copies live objects from Eden and one Survivor to the other Survivor. Dead objects are freed. Fast because most Eden objects are dead.

Old Gen full, Metaspace full, System.gc(), promotion failure (no space in Old Gen for promoted objects), or explicit GC request.

Minor GC: Young Gen only (fast). Major GC: Old Gen only (ambiguous term). Full GC: entire heap + Metaspace (slowest, longest pause).

Promotion: moving objects from Young Gen to Old Gen after surviving enough Minor GCs. Premature promotion: objects promoted too early, filling Old Gen with short-lived objects.

Shallow heap: memory of the object itself. Retained heap: total memory freed if this object were GC'd (includes all exclusively referenced objects).

Open the .hprof file in MAT. Use Leak Suspects for auto-analysis, Dominator Tree for biggest objects, and Path to GC Root to find who keeps objects alive.

Use jcmd inside the pod, or -XX:+HeapDumpOnOutOfMemoryError pre-configured, then kubectl cp the file out. Use ephemeral containers if no shell is available.

An object is eligible for GC when no live thread can reach it through any chain of strong references from GC roots.