TL;DR
JDK 21 introduces significant performance boosts, security enhancements, and new language features, making it an attractive upgrade for developers.
However, its standout feature — virtual threads — comes with synchronization challenges that may lead to bottlenecks in high-concurrency scenarios.
While a fix is planned for JDK 24, with full stability expected in JDK 25, developers should strategically evaluate the timing of their upgrade to balance the benefits and risks.
Performance Improvements
JDK 21 introduces several performance optimizations over its predecessors. Benchmark analyses indicate modest yet consistent improvements in both micro and real-world scenarios.
For instance, tests conducted by Timefold on their solver application revealed small performance gains across various benchmarks when transitioning from JDK 17 to JDK 21. These enhancements suggest that applications may experience better throughput and efficiency upon upgrading.
Source: https://timefold.ai/blog/java-21-performance
The Promise and Challenges of Virtual Threads
One of the most exciting additions in JDK 21 is virtual threads, which promise to revolutionize concurrency by enabling faster and more efficient multitasking.
However, this feature is not without its challenges. Current synchronization issues can create bottlenecks, potentially offsetting some of the performance gains.
Detailed explanation provided by Netflix team: https://netflixtechblog.com/java-21-virtual-threads-dude-wheres-my-lock-3052540e231d
Proposed Solution and Recommendations
To address the pinning issue, the Patricio Chilano Mateo & Alan Bateman proposed JEP 491, titled “Synchronize Virtual Threads without Pinning.” This proposal aims to modify the synchronization mechanism to allow virtual threads to release their platform threads even when blocked in synchronized methods or statements, thereby improving scalability. However, as of now, this enhancement is scheduled for inclusion in JDK 24.
Given the current limitations, it is advisable for developers to exercise caution when adopting virtual threads in production environments. For applications heavily reliant on synchronized methods or blocks, waiting until the next LTS release- JDK 25, where the pinning issue is addressed, would be prudent.
This approach ensures that the benefits of virtual threads can be fully realized without encountering the current synchronization challenges.
Solution JEP: https://openjdk.org/jeps/491
Conclusion
JDK 21 represents a significant step forward for Java developers, delivering performance, security, and language enhancements that can drive productivity and innovation.
While the introduction of virtual threads is a game-changer, its current limitations highlight the importance of careful evaluation and strategic planning.
By aligning upgrades with the evolving stability of these features — such as the fixes expected in JDK 24 and JDK 25 — developers can confidently embrace the benefits of JDK 21 while mitigating potential risks.
JDK 21: Maximize Gains, Minimize Risks was originally published in ableneo tech & transformation on Medium, where people are continuing the conversation by highlighting and responding to this story.