Garbage Collection (GC) is a core memory management feature in the Java Virtual Machine (JVM) that automatically reclaims unused heap memory. Rather than forcing developers to manually free objects, the GC tracks object lifecycle and cleans up memory that is no longer referenced — reducing memory leaks and improving application stability.
In this updated 2026 technical guide on javatechig.com, we’ll explore the internals of the garbage collector, how it works, the key algorithms, memory generations, performance implications, and modern JVM tuning.
What Is the Garbage Collector (GC)?
The Garbage Collector is a background process in the JVM responsible for:
- Identifying unused objects
- Reclaiming heap memory
- Compacting memory to reduce fragmentation
- Supporting efficient memory allocation
It removes the burden of manual memory management, preventing common bugs like use‑after‑free and dangling pointers.
Why Garbage Collection Matters
Manual memory management is complex and error‑prone. GC:
- Saves developers from manual memory freeing
- Reduces memory leaks
- Simplifies application logic
- Improves runtime safety
Modern applications — especially large enterprise and cloud microservices — rely on robust GC for predictable performance.
JVM Memory Overview
The JVM heap is typically divided into the following regions:
- Young Generation (Young GC)
- Old Generation (Old GC)
- Metaspace (Class metadata)
- Code Cache
The GC focuses primarily on the heap — the area where objects reside.
Young vs Old Generations
Young Generation
- Contains newly created objects
- Divided into: Eden, Survivor 0 (S0), Survivor 1 (S1)
- Most objects live briefly — short lifecycle
GC in this region is frequent and fast.
Old Generation
- Stores long‑lived objects
- Fewer GC cycles
- More expensive cleanup than Young GC
Objects promoted from young space are collected here.
How Garbage Collection Works
GC operates in multiple phases, typically:
1. Mark Phase
GC identifies all active (reachable) objects. Starting from roots:
- Local variables
- Static fields
- JNI references
Unreachable objects are candidates for removal.
2. Sweep Phase
Unreachable objects are removed from the heap.
3. Compact Phase
Remaining live objects are compacted to reduce fragmentation and improve allocation efficiency.
Popular Garbage Collection Algorithms
Modern JVMs support several GC algorithms:
1. Serial GC
- Single‑threaded
- Good for small heaps and client apps
- Not ideal for multi‑core servers
-XX:+UseSerialGC
2. Parallel GC (Throughput GC)
- Multi‑threaded GC
- High throughput focus
- Default for many server JVMs
-XX:+UseParallelGC
3. CMS (Concurrent Mark Sweep)
- Low pause GC
- Concurrent marking & sweeping
- Deprecated in newer JVMs
-XX:+UseConcMarkSweepGC
4. G1 (Garbage First) GC – Modern Default
- Region‑based GC
- Mix of concurrent and incremental phases
- Predictable pause times
-XX:+UseG1GC
G1 divides the heap into many small chunks (regions) and prioritizes collecting those with most garbage first.
5. ZGC and Shenandoah (Ultra‑Low Pause)
For large heaps and latency‑sensitive systems:
- ZGC: Scalable, sub‑millisecond pauses
- Shenandoah: Concurrent compaction
-XX:+UseZGC
-XX:+UseShenandoahGC
These are ideal for high‑throughput microservices and real‑time applications.
GC Process in Action
A typical GC cycle:
- JVM allocates objects in the Eden space
- Surviving objects move to Survivor spaces
- After multiple collections, objects are promoted to Old Generation
- When Old Gen fills, a full GC runs
- Live objects are compacted and unreachable ones removed
Identifying GC Behavior
Use the following flags to analyze GC:
-XX:+PrintGCDetails -XX:+PrintGCTimeStamps
Modern tools like JFR (Java Flight Recorder) and GC logs help diagnose:
- Pause duration
- Heap utilization
- Promotion rates
GC Tuning Essentials
Heap Size Configuration
-Xms512m -Xmx4g
-Xms: Initial heap size-Xmx: Maximum heap size
Balance these based on your app’s memory needs.
Selecting the Right GC
| Workload | Recommended GC |
|---|---|
| Small app | Serial GC |
| High throughput | Parallel GC |
| Low latency | G1 GC |
| Large heap, ultra low pause | ZGC/Shenandoah |
GC & Performance Impact
While GC simplifies memory management, poor GC tuning can lead to:
- Long pause times
- CPU overhead
- Memory fragmentation
Modern JVMs provide robust defaults, but tuning pays off for high‑performance and real‑time systems.
Tools for Garbage Collection Monitoring
| Tool | Purpose |
|---|---|
| JVisualVM | Heap analysis |
| JFR | Profiling & event capture |
| GC Logs | Raw GC behavior |
| GarbageCat | GC log analyzer |
Integrating these into performance workflows helps prevent GC bottlenecks before they impact users.
Common Misunderstandings
“GC Frees All Unused Memory Immediately”
Not true — GC runs at JVM’s discretion based on heap usage and thresholds.
“You Don’t Need to Think About Memory”
Even with GC, memory footprint planning prevents out‑of‑memory and poor performance.
Best Practices (2026 Updated)
✔ Use G1GC for most server apps
✔ Choose ZGC for latency‑sensitive systems
✔ Monitor GC activity regularly
✔ Avoid large object allocations in tight loops
✔ Use escape analysis‑friendly code
✔ Combine GC logs with observability tools
Summary
Garbage Collection is a foundation of Java’s memory management, relieving developers from manual deallocation and improving application safety. Understanding how it works — from generational heaps to modern GC algorithms — empowers you to tune performance, minimize pauses, and build resilient applications for a wide range of workloads.
