Understanding Automatic Garbage Collection and Generational GC in Java

What is automatic garbage collection?

Automatic garbage collection is a mechanism that identifies used and unused objects in heap memory and removes the latter.

Used objects (referenced) have pointers pointing to them; unused objects (unreferenced) have no pointers and can be reclaimed.

In languages like C, programmers manually allocate and free memory, whereas Java relies on a garbage collector to handle memory release.

Step 1: Mark

The garbage collector marks which memory is in use and which is not.

Step 2: Sweep

This step deletes the objects that were marked as unreferenced.

Compaction

After deletion, the remaining referenced objects are compacted to simplify future allocations.

Why is generational garbage collection needed?

Marking and compacting the entire heap is inefficient; most objects die young, so separating the heap into generations improves performance.

JVM Generations

The heap is divided into Young (Eden + Survivor spaces), Old, and Permanent generations.

New objects are allocated in Eden; when Eden fills, a minor GC occurs, moving surviving objects to Survivor spaces and eventually promoting long‑living objects to the Old generation.

Minor GC triggers a "Stop‑the‑World" pause; Major GC (Old generation) also pauses all threads and is slower.

The Permanent generation holds class metadata, which can also be reclaimed when no longer needed.

Generational GC Process

Objects are first allocated to Eden; Survivor spaces start empty.

When Eden fills, a minor GC moves live objects to a Survivor space and clears Eden.

Subsequent minor GCs continue moving objects between Survivor spaces, increasing their age until they are promoted to the Old generation.

Eventually, the Old generation undergoes Major GC, cleaning and compacting the space.

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