Evolution of Garbage Collection in Java: From Java 8 to Java 25

Garbage Collection (GC) has been one of Java’s defining features since its inception. It frees developers from manual memory management and helps applications run safely and reliably. However, as systems have scaled and low-latency requirements have grown, GC behavior has evolved significantly — especially between Java 8 and Java 25.

This post explores how garbage collection in Java has changed over the years, what collectors existed in Java 8, and what the modern JVM offers today.

Garbage Collectors in Java 8

When Java 8 was released in 2014, developers had access to several garbage collectors. Each had a different goal — some focused on throughput, while others aimed to minimize pause times.

1. Serial Garbage Collector

The simplest GC implementation. It uses a single thread for both minor and major collections. Best suited for small applications or environments with single-core processors.

2. Parallel Garbage Collector (Throughput Collector)

The default GC in Java 8. It uses multiple threads to perform stop-the-world collections, optimizing for maximum application throughput. Suitable for batch and server-side workloads where small pauses are acceptable.

3. CMS (Concurrent Mark-Sweep) Collector

Designed for low-latency applications. CMS performs most of its work concurrently with the application threads to reduce pause times. However, it can lead to fragmentation and requires occasional full GCs, which can still cause pauses.

4. G1 (Garbage First) Collector

Introduced in Java 7 and fully supported in Java 8, G1 was built to replace CMS. It divides the heap into regions and collects them incrementally. G1 aims for predictable pause times by prioritizing regions with the most garbage.

5. Experimental Collectors

Java 8 also included experimental collectors like the Epsilon (no-op GC) and early work on Shenandoah and ZGC, though these were not production-ready in Java 8. They hinted at the direction future collectors would take: concurrent and low-pause designs.

Transition After Java 8

As Java evolved, CMS was deprecated in Java 9 and removed in later releases. The JVM introduced new collectors aimed at scalability and ultra-low pause times:

• Java 9–14: G1 became the default GC, offering predictable pause times and better heap compaction.
• Java 11: ZGC was introduced as an experimental low-latency collector capable of handling multi-terabyte heaps with sub-10ms pauses.
• Java 12: Shenandoah GC appeared, another concurrent collector developed by Red Hat, targeting pause times independent of heap size.

Modern Garbage Collectors (Java 17–25)

By Java 17 and beyond, the JVM GC landscape had transformed. Each collector is now designed for a specific performance goal.

1. G1 GC (Default)

Balanced for most applications. G1 continues to evolve with improved concurrent phases, better heap compaction, and region-based memory management.

2. ZGC

A concurrent collector focused on sub-millisecond pause times, even on heaps reaching multiple terabytes. ZGC operates with colored pointers and concurrent marking, allowing near-continuous application execution.

3. Shenandoah GC

Similar goals to ZGC but with a different implementation approach. Shenandoah uses concurrent compaction to keep pause times minimal, often below 10ms, regardless of heap size.

4. Serial and Parallel GC (Still Available)

Although no longer defaults, Serial and Parallel GCs remain available for smaller systems or workloads where simplicity or throughput is more important than latency.

5. Generational ZGC (Java 21+)

A significant milestone. ZGC became generational in Java 21, adding the ability to separate young and old objects — improving efficiency without sacrificing pause-time guarantees.

GC Evolution Summary

Java Version	Default GC	Key Collectors	Focus
Java 8	Parallel GC	Serial, Parallel, CMS, G1	Throughput / Low Pause (CMS)
Java 11	G1 GC	G1, ZGC (experimental)	Predictable pauses, scalability
Java 17	G1 GC	G1, ZGC, Shenandoah	Low-latency, concurrent operation
Java 21–25	G1 / Generational ZGC	G1, ZGC, Generational ZGC, Shenandoah	Sub-millisecond pauses, large heaps

Conclusion

From Java 8’s CMS and Parallel GC to Java 25’s Generational ZGC, garbage collection has evolved from stop-the-world designs to highly concurrent, almost pause-free collectors. The JVM now offers developers multiple options tailored for throughput, latency, or scalability.

If you are still running Java 8, it’s worth upgrading — not just for language features but for the modern GC improvements that can dramatically change application performance and responsiveness.

Key takeaway: Modern Java GC is no longer a background task — it’s an integral, intelligent part of your application’s performance story.