Garbage Collection (GC) in Java

Garbage Collection is the process of automatically reclaiming heap memory by removing objects that are no longer reachable.

In Java, garbage collection is fully automatic and handled by the JVM. Java developers do not explicitly free memory like in C/C++.

Definition:
Removing unused objects from heap memory so that new objects can occupy that memory is called Garbage Collection.

Why Garbage Collection Is Needed

Without garbage collection:

  • Memory leaks would be common
  • Applications would crash frequently
  • Developers would manually manage memory

GC provides:

  • Memory safety
  • Fewer crashes
  • Simpler application code

Important GC Concepts

Reachability

An object becomes eligible for GC when:

  • No active references point to it
  • It is not reachable from GC roots

GC Roots include:

  • Local variables
  • Static variables
  • Active threads
  • JNI references

GC does not delete objects immediately — it only deletes unreachable ones.

Types of Garbage Collectors in Java

Java provides multiple garbage collectors, each optimized for different needs. There are 7 major GC types. —

1️⃣ Serial Garbage Collector

  • Oldest and simplest GC
  • Uses single thread for GC
  • Uses Stop-The-World approach
  • Application pauses completely during GC

Best Suited For

  • Single-threaded applications
  • Small heap sizes (≤ 100MB)
  • Simple tools or batch jobs

JVM Option 

-XX:+UseSerialGC

Key Point

✔ No thread communication overhead
❌ Very long pause times

2️⃣ Parallel Garbage Collector

  • Uses multiple threads for GC
  • Heap divided into regions
  • GC work done in parallel
  • Still Stop-The-World

Goal

  • Maximize throughput
  • Reduce GC time using parallelism

Best Suited For

  • Medium to large heap
  • CPU-rich systems
  • Applications where pause time is acceptable

JVM Option 

-XX:+UseParallelGC

3️⃣ Concurrent Mark-Sweep (CMS) – Deprecated ❌

  • Most GC work done concurrently
  • Reduces pause time
  • Higher CPU usage
  • Poor performance with large heaps

Status

  • Deprecated in Java 9
  • Removed in Java 14

JVM Option (Legacy) 

-XX:+UseConcMarkSweepGC

4️⃣ G1 (Garbage First) Garbage Collector

  • Default GC since Java 9
  • Heap divided into equal-sized regions
  • Collects regions with most garbage first
  • Mix of concurrent + stop-the-world phases
  • Predictable pause times

Best Suited For

  • Large heap sizes (≥ 4GB)
  • Balanced throughput + low latency
  • Most enterprise applications

JVM Option 

-XX:+UseG1GC

Key Strength

✔ Predictable pauses
✔ Good default choice

5️⃣ Z Garbage Collector (ZGC)

  • Introduced in Java 11 (preview)
  • Production-ready from Java 15
  • Almost fully concurrent
  • Pause times < 10ms
  • Supports very large heaps (TBs)

Best Suited For

  • Real-time systems
  • Trading platforms
  • Low-latency services

JVM Option 

-XX:+UseZGC

6️⃣ Shenandoah Garbage Collector

  • Similar to ZGC
  • Ultra-low pause times
  • Concurrent compaction
  • Pause time independent of heap size

Best Suited For

  • Latency-critical applications
  • Alternative to ZGC

JVM Option 

-XX:+UseShenandoahGC

7️⃣ Epsilon Garbage Collector (Special Purpose)

  • Does NOT collect garbage
  • Allocates memory only

Used for:

  • Performance testing
  • GC overhead measurement
  • Memory allocation analysis

Result

  • Eventually throws OutOfMemoryError

JVM Option 

-XX:+UseEpsilonGC

Important: Epsilon GC is never used in production.

Throughput vs Latency

Throughput

  • % of time application is running
  • Higher throughput = better CPU utilization

Example:

  • App runs: 90s
  • GC runs: 10s
  • Throughput = 90%

Latency

  • Time application is paused during GC
  • Lower latency = better responsiveness

Example:

  • GC pause = 10ms
  • Latency = 10ms per GC
GC Type Focus Use Case
Serial Simplicity Small apps
Parallel Throughput Batch jobs
CMS Low latency Deprecated
G1 Balanced Default choice
ZGC Ultra-low latency Real-time apps
Shenandoah Ultra-low latency Large heaps
Epsilon No GC Testing only

GC Is Not Immediate

  • GC runs when JVM decides
  • System.gc() is only a request, not a command

GC Is Heap-Specific

  • GC manages heap memory
  • Stack memory is cleaned automatically

GC Tuning Is Advanced

  • Most apps should use default G1
  • Over-tuning GC often causes problems

Summary

  • GC is automatic in Java
  • Objects are removed based on reachability
  • Stop-the-world pauses impact latency
  • G1 is default since Java 9
  • ZGC & Shenandoah focus on ultra-low latency
  • Epsilon does not collect garbage

Choosing the right garbage collector is about balancing throughput, latency, and heap size — not using the latest GC blindly.

This site uses Just the Docs, a documentation theme for Jekyll.