Java Garbage Collection

Garbage Collection (GC) is one of the most powerful features of Java. It frees developers from manual memory management by automatically reclaiming unused objects. However, poorly tuned GC can lead to performance bottlenecks, long pause times, and even application crashes. In this blog, we’ll explore how Java Garbage Collection works, compare different collectors, and highlight best practices for choosing and tuning GC policies.

🔑 What is Garbage Collection in Java?

• Definition: Garbage Collection is the process of automatically identifying and removing objects that are no longer referenced.
• Goal: Free up heap memory and prevent memory leaks.
• How it works: The JVM uses algorithms like mark-and-sweep to detect unused objects and reclaim memory.

🧩 Types of Java Garbage Collectors

Collector	Features	Best Use Cases
Serial GC	Single-threaded, simple mark-and-sweep.	Small desktop apps, single-core systems.
Parallel GC	Multi-threaded, throughput-focused.	Batch jobs, data processing pipelines.
G1 GC	Region-based, predictable pause times.	Web servers, e-commerce platforms.
ZGC	Ultra-low latency, supports heaps >32GB.	Financial systems, trading platforms.
Shenandoah GC	Concurrent compaction, low pause times.	Real-time gaming, simulations.
CMS (Deprecated)	Concurrent mark-sweep, low latency.	Legacy enterprise apps.

📊 Example: Garbage Collection in Action

public class GCExample {

    public static void main(String[] args) {

        GCExample obj1 = new GCExample();

        GCExample obj2 = new GCExample();

        obj1 = null;  // Eligible for GC

        obj2 = null;

        System.gc();  // Request GC

    }

    @Override

    protected void finalize() throws Throwable {

        System.out.println(“Object collected!”);

    }

}

🧠 Key Factors to Consider When Choosing a Java Garbage Collector

Selecting the right garbage collector (GC) can dramatically affect your application’s performance, responsiveness, and scalability. Here’s a breakdown of the most important factors to guide your decision:

⚙️ 1. Application Type

• Batch processing or background jobs → prioritize throughput (Parallel GC).
• Interactive or real-time systems → prioritize low latency (ZGC, Shenandoah).
• Web applications or microservices → need balanced performance (G1 GC).

🚀 2. Performance Goals

Throughput: Maximize application work vs. GC time. Use Parallel GC.

• Latency: Minimize pause times. Use ZGC or Shenandoah.
• Balanced: Acceptable throughput and latency. Use G1 GC.

🧮 3. Heap Size

• Small heaps (<4GB): Serial or Parallel GC may suffice.
• Medium heaps (4–32GB): G1 GC is often optimal.
• Large heaps (>32GB): ZGC or Shenandoah are designed for scalability.

🧵 4. Pause Time Sensitivity

• If your app can’t tolerate long GC pauses (e.g., trading platforms, gaming), choose collectors with concurrent compaction like ZGC or Shenandoah.

🧑‍💻 5. Hardware Resources

• Multi-core CPUs: Use Parallel or G1 GC to leverage concurrency.
• Limited CPU: Serial GC may be more predictable.

📊 6. GC Tuning and Monitoring

• Choose a collector that offers predictable behavior and easy tuning.
• G1 GC provides fine-grained control over pause times and region sizes.
• Use GC logs (-Xlog:gc*) and tools like JFR, VisualVM, or GCViewer for analysis.

🧱 7. JVM Version Compatibility

• ZGC and Shenandoah are available in newer JVM versions (Java 11+).
• CMS is deprecated since Java 9—avoid it for new projects.

📋 Comparison Table

Factor	Serial GC	Parallel GC	G1 GC	ZGC	Shenandoah GC
Latency	High	Moderate	Low	Very Low	Very Low
Throughput	Low	High	Moderate	Moderate	Moderate
Heap Size Support	Small	Medium	Medium	Huge	Huge
Pause Time Control	None	Limited	Good	Excellent	Excellent
JVM Version	All	All	Java 7+	Java 11+	Java 12+
Tuning Complexity	Low	Moderate	Moderate	High	High

⚠️ Risks and Trade-offs

• ZGC and Shenandoah require more CPU and are harder to tune.
• Parallel GC may cause long pauses in latency-sensitive apps.
• Serial GC is outdated for modern multi-threaded workloads.

🚀 Best Policies for Java Garbage Collection

1. Choose the Right Collector
   • For low latency → ZGC or Shenandoah.
   • For high throughput → Parallel GC.
   • For balanced workloads → G1 GC.
2. Tune Heap Sizes
   • Use -Xms and -Xmx to set initial and maximum heap size.
   • Avoid frequent resizing by keeping them equal.
3. Monitor GC Logs
   • Enable GC logging with -Xlog:gc*.
   • Analyze pause times and frequency to detect bottlenecks.
4. Minimize Object Creation
   • Reuse objects where possible.
   • Prefer primitive types over wrappers to reduce overhead.
5. Avoid Memory Leaks
   • Nullify references when objects are no longer needed.
   • Use tools like VisualVM or JConsole to detect leaks.
6. Leverage Generational GC
   • Understand Young, Old, and Permanent generations.
   • Optimize object lifecycles to reduce promotion overhead.
7. Test Under Load
   • Simulate production traffic to evaluate GC performance.
   • Adjust policies based on real-world metrics.

⚠️ Risks and Trade-offs

• Serial GC → Simple but unsuitable for multi-threaded apps.
• Parallel GC → High throughput but longer pause times.
• G1 GC → Balanced but requires careful tuning.
• ZGC/Shenandoah → Excellent latency but higher CPU usage.

🎯 Conclusion

Java Garbage Collection is not one-size-fits-all. The best policy depends on your application’s workload, latency requirements, and heap size. By carefully selecting the right GC, tuning heap parameters, and monitoring logs, you can achieve optimal performance and stability.

👉 If you’re building a real-time system, go for ZGC or Shenandoah.
👉 If you need throughput, choose Parallel GC.
👉 For balanced workloads, G1 GC is often the safest bet.

---