JEP 521 Generational Shenandoah Garbage Collector in Java 25 Explained

JEP 521 Generational Shenandoah in Java 25 finalizes generational support for the Shenandoah garbage collector, improving efficiency, throughput, and low-latency behavior for modern Java applications.

• 1. Introduction to JEP 521: Generational Shenandoah
• 2. Shenandoah Garbage Collector Before Generations
• 3. JEP 404: Experimental Generational Shenandoah
• 4. What JEP 521 Finalizes in Generational Shenandoah
• 5. How Generational Shenandoah Works in Java 25
• 6. Performance and Practical Impact of JEP 521
• 7. Why JEP 521 Matters for Java’s Garbage Collection Evolution
• 8. Conclusion

1. Introduction to JEP 521: Generational Shenandoah

JEP 521 improves the Shenandoah garbage collector by adding generational support. Earlier versions of Shenandoah used a single heap. They treated all objects in the same way. With this JEP, Shenandoah follows a generational model. It groups objects based on how long they live.

This change is part of Java 25’s performance improvements. It builds on earlier experimental work. It also turns generational Shenandoah into a stable and supported feature. Developers can now use it safely in production systems.

1.1. What Is JEP 521 About

JEP 521 introduces Generational Shenandoah. The JVM divides the heap into a young generation and an old generation. New objects start in the young generation. Most garbage appears there. Objects that survive longer move to the old generation.

The generational idea itself is not new in garbage collection. However, it is new for Shenandoah. Earlier versions did not use generations. JEP 521 fills this gap. It allows Shenandoah to benefit from generational behavior while keeping its low pause-time design.

1.2. Why Generational Shenandoah Matters Now

Modern Java applications create many short-lived objects. This is common in microservices, APIs, and message-driven systems. Most of these objects exist only for a short time. They often live for a single request.

Without generations, Shenandoah checked short-lived and long-lived objects together. This increased GC work. With generational support, the garbage collector focuses mainly on the young generation. Most objects die there. This reduces unnecessary work and improves efficiency.

Java 25 is the right time for this change. The JVM already provides strong low-latency garbage collection support. JEP 521 makes Shenandoah a better fit for modern workloads. It keeps the main goal unchanged: short and predictable pause times.

2. Shenandoah Garbage Collector Before Generations

Before JEP 521, the Shenandoah garbage collector followed a single-generation design. Shenandoah treated the heap as one large space. It did not separate objects based on how long they lived. During garbage collection, it handled short-lived and long-lived objects in the same way.

The JVM team made this design choice on purpose. Shenandoah focused strongly on low pause times and concurrent execution. This focus sometimes meant skipping traditional GC optimizations. This original approach explains both Shenandoah’s strengths and the reasons that later led to generational support.

2.1. Single-Generation Design in Earlier Shenandoah

In earlier versions, Shenandoah did not divide the heap into young and old generations. The JVM placed every object into the same heap space. It ran garbage collection across the entire heap.

Shenandoah performed key GC phases such as marking, evacuation, and compaction concurrently. Most of this work happened while application threads continued to run. The JVM used short pauses only when required for correctness. Because Shenandoah did not use generations, it avoided tracking object age or handling promotions.

This simple heap model helped Shenandoah achieve its primary goal. It kept pause times very small, even on large heaps.

2.2. Strengths of Pre-Generational Shenandoah

Pre-generational Shenandoah delivered predictable low pause times. The garbage collector ran most of its work concurrently. Because of this, application threads rarely stopped for long periods.

Another major strength was concurrent compaction. Shenandoah moved objects and reduced heap fragmentation while the application continued to run. Older collectors often required long stop-the-world pauses to perform the same work.

Because of these features, Shenandoah worked well for latency-sensitive applications. Examples include large services and real-time systems.

2.3. Limitations Without Generational Support

The single-generation design helped reduce pause times. However, it also introduced inefficiencies. In real-world applications, most objects live for a short time. Shenandoah still processed them in the same way as long-lived objects.

As a result, the garbage collector scanned and managed the entire heap. This happened even when most garbage appeared in newly created objects. This increased GC work and reduced efficiency, especially in allocation-heavy workloads.

Without generational support, Shenandoah could not fully use the fact that most objects die young. As modern Java applications began creating more short-lived objects, this limitation became harder to ignore. This gap ultimately led to the introduction of generational Shenandoah.

3. JEP 404: Experimental Generational Shenandoah

Generational support did not arrive in Shenandoah as a finished feature. It was first introduced as an experimental change under JEP 404. This allowed the JVM team to explore how generations could work with Shenandoah’s concurrent design without making long-term promises about behavior or performance.

JEP 404 played a critical role in reducing risk. It gave the JVM team real-world feedback and confidence before turning generational Shenandoah into a supported feature in Java 25.

3.1. Why Generational Shenandoah Started as Experimental

Adding generations to Shenandoah was not a small change. Shenandoah was designed from the beginning to minimize pause times by running most GC work concurrently. Introducing young and old generations adds complexity, especially around object movement, promotion, and write barriers.

If implemented incorrectly, generational support could have increased pause times or broken Shenandoah’s low-latency guarantees. Because of this risk, the JVM team chose to introduce generational Shenandoah as an experimental feature first. This allowed them to test the idea safely and refine the design based on real usage.

3.2. What JEP 404 Introduced

JEP 404 introduced the basic idea of young and old generations into Shenandoah. New objects were allocated in the young generation, while objects that survived multiple collections were moved to the old generation.

The experimental implementation focused on keeping Shenandoah’s concurrent behavior intact. Young generation collections were designed to be frequent and lightweight, while old generation collections were less frequent and handled larger, long-lived objects.

At this stage, the goal was not full optimization, but correctness and stability. The JVM team wanted to ensure that generational behavior could work reliably with Shenandoah’s existing mechanisms.

3.3. Enabling Generational Shenandoah in JEP 404

Because the feature was experimental, it was disabled by default. Developers had to explicitly enable it using JVM options.

A typical command looked like this:

java -XX:+UseShenandoahGC -XX:+UnlockExperimentalVMOptions -XX:+UseGenerationalShenandoahGC -jar app.jar

This made it clear that generational Shenandoah was still under evaluation and not meant for general production use at that stage.

3.4. Learnings from the Experimental Phase

The experimental phase helped validate that generational Shenandoah could deliver real benefits without sacrificing low pause times. It showed that focusing GC work on the young generation reduced unnecessary scanning of long-lived objects.

It also helped identify edge cases and tuning challenges that only appear in real applications. These learnings allowed the JVM team to improve the design, simplify behavior, and make generational Shenandoah safe for general use.

By the time JEP 521 was proposed, the core ideas introduced in JEP 404 had been tested and refined. This made it possible to move from an experimental feature to a fully supported and production-ready implementation in Java 25.

4. What JEP 521 Finalizes in Generational Shenandoah

JEP 521 marks the point where generational Shenandoah moves from an experimental concept to a stable and supported feature. The JVM team takes the ideas tested under JEP 404 and integrates them into the standard Shenandoah garbage collector in Java 25. This change makes generational behavior reliable and predictable. It also makes it suitable for production use.

This section explains what becomes final in JEP 521 and why this transition matters.

4.1. From Experimental to Supported Feature

Under JEP 404, the JVM treated generational Shenandoah as an experimental feature. Developers had to unlock experimental options explicitly. The JVM did not provide long-term guarantees around tuning or behavior.

JEP 521 removes this uncertainty. Java 25 now supports generational Shenandoah as part of the standard JVM. The JVM team maintains it alongside other production-ready garbage collectors. This change shows confidence in the design, correctness, and performance. Real-world testing during the experimental phase validated these aspects.

4.2. Stabilization of Young and Old Generations

With JEP 521, young and old generations become a core part of Shenandoah’s design. The JVM applies clear and stable rules for object allocation, promotion, and collection across generations.

The garbage collector focuses most of its work on the young generation. Short-lived objects appear frequently there. It collects the old generation less often because most objects there remain alive. This clear separation reduces unnecessary scanning. It also allows Shenandoah to work more efficiently across different workloads.

4.3. Preserving Shenandoah’s Low-Pause Guarantees

Adding generations raised an important concern: longer pause times. Shenandoah has always aimed to keep pauses short and predictable, even on large heaps.

JEP 521 keeps this goal intact. The JVM integrates generational support and continues to perform marking, evacuation, and compaction concurrently. Most GC work still runs alongside application threads. Pauses remain small.

As a result, JEP 521 improves efficiency without changing Shenandoah’s core behavior. Developers gain the benefits of generational garbage collection. They also retain the low-latency characteristics that define Shenandoah.

5. How Generational Shenandoah Works in Java 25

Generational Shenandoah in Java 25 follows a practical idea: the garbage collector should spend most of its time where garbage actually exists. Instead of scanning the entire heap repeatedly, Shenandoah now focuses on object lifetime and adjusts its work accordingly.

Modern Java applications continuously allocate objects. Many of these objects are short-lived and tied to a single operation or request. JEP 521 allows Shenandoah to clean up such objects early, while long-lived objects are handled separately and less often.

5.1. Object Allocation and the Young Generation

In Java 25, new objects are allocated in the young generation. This is where most allocation activity happens and where garbage accumulates quickly.

In simple terms, think of a typical backend service handling requests:

• Every request creates temporary objects such as DTOs, strings, maps, and buffers
• Most of these objects are used briefly and then become unreachable
• The JVM runs young collections frequently to clean them up early

Because the young generation fills quickly, collecting it often is both cheap and effective. The garbage collector can reclaim a large amount of memory without scanning long-lived objects. This keeps allocation fast and avoids unnecessary work across the rest of the heap.

5.2. Promotion to the Old Generation

Some objects survive longer because they are meant to be reused. Examples include caches, shared state, configuration objects, or in-memory data structures.

When such objects survive multiple young collections, Shenandoah promotes them to the old generation. Promotion separates stable objects from temporary ones and prevents them from being scanned again and again during young collections.

In simple terms:

• Objects that live longer than expected are treated as long-lived
• These objects are moved out of the young generation
• The young generation remains focused on short-lived garbage

The old generation is therefore more stable and grows slowly. It is collected less frequently, which reduces overall GC overhead while keeping young collections lightweight.

5.3. Concurrent Collection Across Generations

Even with the introduction of generations, Shenandoah continues to follow its core principle: do most garbage collection work concurrently.

Both young and old generation collections rely on concurrent marking and evacuation. The JVM spreads GC work over time, allowing application threads to continue running with minimal interruption. Short pauses are still required, but they are kept small and predictable.

In simple terms:

• Young collections clean up short-lived objects quickly
• Old collections handle long-lived objects more gradually
• Most of the heavy GC work happens while the application is running

This design allows Shenandoah to gain the efficiency benefits of generational garbage collection without giving up its low-pause guarantees.

6. Performance and Practical Impact of JEP 521

The main goal of JEP 521 is not to introduce a new garbage collector. Instead, it helps Shenandoah work smarter. By adding generations, the JVM reduces unnecessary GC work. It also improves throughput and keeps latency predictable under real production load.

These benefits become clear in modern Java applications. This is especially true for allocation-heavy workloads.

6.1. Reduced GC Work for Short-Lived Objects

Most garbage in Java applications comes from short-lived objects. Before generational support, Shenandoah processed short-lived and long-lived objects together. This increased GC work.

With JEP 521, the JVM handles this differently. Young collections now focus only on the young generation. Most garbage exists there. The collector no longer scans long-lived objects repeatedly when they are unlikely to be garbage.

In simple terms:

• Applications create many short-lived objects
• Most of them die quickly
• Young collections clean them up early

This focused approach reduces GC effort. It also frees memory more efficiently, especially when allocation rates are high.

6.2. Improved Throughput for Allocation-Heavy Applications

Many modern Java applications allocate objects continuously. This pattern appears often in microservices, REST APIs, stream processing, and message-driven systems.

A typical microservice request creates temporary objects. These include request objects, response objects, and intermediate collections. Most of these objects live only for the duration of the request.

With generational Shenandoah, the JVM reclaims this memory quickly through frequent young collections. The application does not slow down during this process.

As a result:

• Allocation stays fast
• GC work spreads more evenly
• Application threads do more useful work

This improves throughput, especially under sustained load.

6.3. More Predictable Latency in Production

In production systems, latency consistency often matters more than raw speed. Shenandoah already aims to keep pause times short. JEP 521 strengthens this goal.

By separating short-lived and long-lived objects, the JVM avoids unnecessary GC work during critical paths. Young collections stay small and efficient. Old generation collections run less often and mostly work concurrently.

This behavior matters for systems such as:

• Fintech and payment platforms
• Trading and risk engines
• Low-latency APIs and real-time services

In these environments, even small latency spikes cause problems. JEP 521 helps keep pause times short and predictable. This makes Shenandoah a safer choice for latency-sensitive workloads.

7. Why JEP 521 Matters for Java’s Garbage Collection Evolution

JEP 521 is an important step in the steady evolution of Java’s garbage collection system. Rather than introducing a completely new collector, it improves an existing low-latency collector in a way that better matches how modern Java applications behave. This makes Java’s GC ecosystem more mature and better suited for a wide range of production workloads.

7.1. Strengthening Java’s Low-Latency GC Options

Java already offers multiple garbage collectors designed for different goals. Shenandoah has always focused on keeping pause times short, even on large heaps. With JEP 521, it now also benefits from generational behavior, which has long been a proven optimization in garbage collection.

Generational Shenandoah fits naturally into Java’s low-latency GC landscape. It combines two important ideas: focusing on short-lived objects for efficiency, and using concurrent techniques to keep pause times small. This gives developers another strong option when building systems that need both good throughput and predictable latency.

Instead of forcing a trade-off between speed and pause times, JEP 521 helps Shenandoah deliver a better balance of both.

7.2. What This Means for Java 25 and Beyond

For Java 25, JEP 521 makes Shenandoah a more complete and practical garbage collector. It is no longer limited by a single-generation design and can now adapt better to allocation-heavy workloads that dominate modern Java applications.

Looking ahead, JEP 521 does not promise dramatic or risky changes. Its value lies in refinement and stability. By turning experimental ideas into supported features, Java continues its approach of careful, incremental improvement. This helps developers adopt new JVM capabilities with confidence, knowing they are built on proven designs rather than untested ideas.

In this way, JEP 521 strengthens Java’s foundation for future releases, without changing the principles that have made its garbage collection reliable and predictable over time.

8. Conclusion

JEP 521 completes an important chapter in the evolution of the Shenandoah garbage collector. By adding generational support, Shenandoah now takes advantage of a common pattern in Java applications. Most objects live for a very short time. This allows the JVM to focus garbage collection work where it matters most. It also keeps pause times low.

JEP 521 stands out not just because it adds generations. It also stands out because of how carefully the JVM team integrates them. The feature builds on the experimental work from JEP 404. It turns that work into a stable and supported design in Java 25. Young and old generations now form a natural part of Shenandoah. They work smoothly with its concurrent marking and evacuation model.

For developers running modern Java workloads, this change brings clear benefits. Applications gain better efficiency and higher throughput under allocation-heavy load. They also see more predictable latency in production. JEP 521 avoids risky behavior and large design shifts. Instead, it refines an existing low-latency collector. It aligns Shenandoah better with real-world application patterns.

Overall, JEP 521 makes Shenandoah a more complete and practical garbage collector for Java 25. It also reinforces Java’s approach of careful evolution. At the same time, Java continues to meet the demands of modern systems.