OpenJDK Advances: Generational Shenandoah, Project Leyden, and a Six-Month Release Cycle

Serving on the JCP Expert Group

The Java Community Process (JCP) Expert Group is adapting to a faster pace of development, driven by OpenJDK’s six-month release cycle. Deputy CTO at Azul, Simon Ritter, who has served on the JCP Expert Group since JDK 9, discussed how this shift allows for more frequent feature delivery and quicker iteration on preview features, even enabling the removal of features like String Templates when necessary.

Why This Matters

Traditionally, Java releases were large, infrequent events, increasing the risk of delivering features that didn’t meet evolving needs. The six-month cadence mitigates this risk, but also introduces complexity in managing a continuous stream of updates and ensuring stability. The cost of delaying features for years due to infrequent releases was substantial, hindering the platform’s responsiveness to developer demands.

Key Insights

• Six-month release cycle, 2014: Introduced to accelerate Java innovation and reduce the risk associated with lengthy release cycles.
• JEPs (JDK Enhancement Proposals): Smaller, focused proposals that drive OpenJDK development, replacing the older JSR process.
• Generational Shenandoah: A garbage collector implementation that was temporarily pulled from JDK 21 due to timing constraints, demonstrating the flexibility of the new release cycle.
• Project Leyden (JDK 24/25): A series of JEPs focused on reducing startup time and improving performance through Ahead-of-Time (AOT) compilation techniques.
• Compact Object Headers: A new feature in JDK 25 that reduces heap usage and CPU utilization, demonstrating performance gains without API changes.

Working Example

// Example demonstrating Java 25's Compact Object Headers (performance benefit, no code change needed)
public class Example {
    public static void main(String[] args) {
        // Create a large number of objects
        List<Object> objects = new ArrayList<>();
        for (int i = 0; i < 1000000; i++) {
            objects.add(new Object());
        }
        // Perform some operations on the objects
        // (The performance will be improved by Compact Object Headers)
    }
}

Practical Applications

• Azul/BellSoft: Integrating Coordinated Restore at Checkpoint (CRaC) into their OpenJDK distributions for faster application recovery.
• Microservices Architectures: Leveraging AOT compilation (Project Leyden) to reduce startup times for containerized Java applications.

References:

• https://www.infoq.com/podcasts/latest-open-java-development-kit/
• https://www.infoq.com/news/2024/11/java-24-reduce-object-header-size
• https://openjdk.org/jeps/483
• https://openjdk.org/jeps/514
• https://openjdk.org/jeps/515
• https://openjdk.org/jeps/516
• https://openjdk.org/jeps/529
• https://openjdk.org/jeps/530