Q: How does Garbage Collection work in Java?
Answer:
Garbage Collection (GC) automatically frees heap memory by reclaiming objects that are no longer reachable.
How Objects Become Eligible for GC
An object is eligible when no live thread can reach it through any chain of references.
Object a = new Object(); // Object created, referenced by 'a'
a = null; // Reference removed → object is now unreachable → GC eligible
GC Process: Generational Collection
1. Minor GC (Young Generation)
- New objects are allocated in Eden.
- When Eden fills up, a minor GC runs.
- Live objects are copied to a Survivor space (S0 or S1).
- Dead objects are discarded (Eden is cleared).
- Objects that survive multiple minor GCs are promoted to Old Gen.
2. Major GC / Full GC (Old Generation)
- Triggered when Old Gen fills up.
- Much slower than minor GC (scans the entire heap).
- "Stop the world" — all application threads are paused.
GC Algorithms
| Collector | Type | Pause | Best For |
|---|---|---|---|
| Serial GC | Single-threaded | Long STW | Small apps, single-core |
| Parallel GC (default < Java 9) | Multi-threaded | Medium STW | Batch processing, throughput |
| G1 GC (default Java 9+) | Region-based | Short STW | General purpose, balanced |
| ZGC (Java 15+) | Concurrent | < 1ms STW | Ultra-low latency |
| Shenandoah (Java 12+) | Concurrent | < 1ms STW | Low latency, Red Hat |
G1 GC (Garbage-First)
The default collector since Java 9. Divides the heap into equal-sized regions and prioritizes collecting regions with the most garbage first.
java -XX:+UseG1GC -XX:MaxGCPauseMillis=200 MyApp
ZGC (Z Garbage Collector)
Designed for sub-millisecond pauses regardless of heap size (supports up to 16 TB heaps).
java -XX:+UseZGC MyApp
GC Tuning Tips
# Enable GC logging (Java 9+)
java -Xlog:gc*:file=gc.log:time,uptime,level,tags
# Set target pause time for G1
java -XX:MaxGCPauseMillis=100
# Monitor with jstat
jstat -gc <pid> 1000 # GC stats every 1 second
[!TIP] In interviews, mentioning G1 GC (default, balanced) and ZGC (ultra-low latency) shows you understand modern Java. The key insight: "GC is a trade-off between throughput (total work done) and latency (pause duration). G1 balances both; ZGC minimizes latency at some throughput cost."