Garbage Collection

How GC Decides What to Free

The garbage collector frees objects that are no longer reachable from a GC root: local variables on thread stacks, static fields, active threads, JNI references. Reachability, not reference counting, so circular references are collected fine.

GC roots ──► Order ──► Customer          reachable: kept
          └─► Cart  ──► Item
                 X  ──► OldSession ◄──► OldToken   unreachable (even with a cycle): freed

Generational Heap

Most objects die young (request DTOs, temporary strings). So the heap is split into generations:

New objects go to Eden.
A minor GC copies live Eden objects to a Survivor space; dead ones are dropped for free.
Objects that survive several minor GCs are promoted to the Old generation.
A major / full GC cleans the old generation — slower, longer pauses.

Collectors

Collector	Flag	Good for
Serial	`-XX:+UseSerialGC`	tiny heaps, single-core containers
Parallel	`-XX:+UseParallelGC`	batch jobs: max throughput, pauses OK
G1 (default since Java 9)	`-XX:+UseG1GC`	general servers, pause target `-XX:MaxGCPauseMillis=200`
ZGC (generational in 21)	`-XX:+UseZGC -XX:+ZGenerational`	big heaps, sub-millisecond pauses
Shenandoah	`-XX:+UseShenandoahGC`	low pause (OpenJDK builds)

Example

Turn on GC logging to see collections:

java -Xms512m -Xmx512m -Xlog:gc -jar shop.jar

Example output

[0.012s][info][gc] Using G1
[3.481s][info][gc] GC(0) Pause Young (Normal) (G1 Evacuation Pause) 25M->4M(512M) 3.211ms
[7.902s][info][gc] GC(1) Pause Young (Normal) (G1 Evacuation Pause) 31M->6M(512M) 2.874ms

Reference Types

Type	Collected when	Use
Strong (normal)	never while reachable	everything
`SoftReference`	memory is low	memory-sensitive caches
`WeakReference`	next GC if only weakly reachable	`WeakHashMap`, listeners
`PhantomReference`	after finalization, for cleanup	`Cleaner` API

Note: System.gc() is only a hint. Never rely on it. finalize() is deprecated; use try-with-resources or Cleaner.