Elevating Java Performance: Unpacking Generational ZGC in Java 21

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Garbage collection is a cornerstone feature in the Java Language. It automates memory management by reclaiming memory occupied by objects that are no longer in use, freeing you the developer up from manual memory allocation and deallocation tasks. This feature is vital for preventing memory leaks and ensuring that applications run efficiently over time.

Java's automated memory management through garbage collection is one of the factors that make it a preferred language for many developers. It abstracts away the complexities of manual memory management, allowing you to focus more on building functionality and less on mundane and error-prone tasks of memory management. Over the years, various types of garbage collectors have been introduced in the language to cater to different performance and system requirements. Some of these include:

Serial Garbage Collector: Suitable for single-threaded environments and smaller applications. Simplest of the garbage collectors that stops all application threads while it collects object garbage.

Parallel Garbage Collector: A multi-threaded collector that can use multiple threads to perform garbage collection. This can improve the performance of garbage collection on multi-core machines.

Concurrent Mark Sweep (CMS) Garbage Collector: A concurrent collector that can perform garbage collection while application threads are running. This makes CMS a good choice for applications that cannot tolerate long pause times. Not suitable for applications with large heaps.

Garbage First (G1) Garbage Collector: A garbage collector that is designed to provide high performance with large heap sizes, collecting garbage from regions of the heap likely to contain garbage instead of the whole heap.

With Java 21, a new garbage collector named Generational Z Garbage Collector (Generational ZGC) or GenZGC was introduced, ushering in a new era of garbage collection with its own set of advantages.

Multi-Generational Garbage Collection

Generational ZGC or GenZGC categorises objects into two logical regions in the heap: a Young Region and an Old Region. This categorization is based on the Weak-Generational Hypothesis, which states that most objects die shortly after their creation. Initially, new objects are placed in the Young Region, and if they survive enough garbage collection cycles, they are promoted to the Old Region. The Old Region is scanned less frequently compared to the Young Region, thus making garbage collection more efficient with CPU resources as it focuses on areas of the heap more likely to contain dead objects​​.

Advantages of Generational ZGC

  • Frequent Collection of Young Objects: By categorizing objects into generations, Generational ZGC can perform more frequent collections on young objects, thus reducing the memory footprint and improving application performance​​.
  • Reduced Garbage Collection Overheads: Generational ZGC reduces allocation stalls, lowers heap memory overhead, and decreases garbage collection CPU overload. These improvements lead to better performance and reduced latency in Java applications​​.
  • Improved Efficiency: The new garbage collector enhances the efficiency of cleaning up unused objects, thus ensuring that applications run smoothly and respond quickly, especially in scenarios with high rates of object allocation and deallocation​​.

Enabling Generational ZGC

To enable Generational ZGC in Java 21, you'll need to pass the following two VM arguments:


The -XX:+UseZGC argument enables the Z Garbage Collector (ZGC). The -XX:+ZGenerational argument enables the generational mode of ZGC.

GenZGC Use Cases

Real-Time and Responsive Applications

In the modern digital era, real-time and responsive applications have become a much-expected part of the user experience. Users demand instantaneous feedback and seamless interactions while navigating your applications. The cornerstone of such high-performance applications is efficient memory management which is where Generational ZGC comes in. Designed with low-latency and high-scalability memory management in mind, GenZGC drastically minimizes application pause times typically associated with garbage collection. By efficiently reclaiming memory, GenZGC ensures that your real-time and responsive Java applications continue to deliver stellar performance with minimal interruptions, meeting the high expectations of today's users.

Applications with High Allocation Rates

Applications that experience high rates of object allocation and deallocation are often plagued with performance issues stemming from traditional garbage collection mechanisms. The frequent collection of young objects, a hallmark of Generational ZGC, addresses this challenge. By focusing on the Young Region of the heap where new objects are allocated, GenZGC is able to significantly reduce the memory footprint and improve the performance of such applications. This targeted approach to garbage collection ensures that applications with high allocation rates remain responsive and efficient, regardless of the demanding memory management tasks they encounter.


The introduction of Generational ZGC in Java 21 is a testament to the continuous efforts aimed at optimising the performance and efficiency of Java applications. By catering to the specific needs of real-time, responsive applications, and those with high allocation rates, GenZGC not only alleviates common performance bottlenecks but also paves the way for the development of more robust and efficient Java applications in the future. Again, never a better time being a Java developer!

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