Java & AI: What Developers Need to Know

Why Your CRaC Restores Fail in Production: Debugging Resource Leaks with JDK 26 JFR Events

With scale-to-zero Java microservices dominating cloud-native architectures in 2026, Project CRaC (Coordinated Restore at Checkpoint) has become the de facto standard for sub-millisecond startup times. But blindly snapshotting running JVMs without auditing open files, sockets, and threads is a recipe for silent restore failures and catastrophic production outages.

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Why Most Developers Get This Wrong

• Treating CRaC like GC: Assuming the JVM will magically clean up active TCP connections or file handles during a checkpoint.
• Leaking Sockets: Leaving database connection pools active, which causes the CRaC runtime to throw a hard CheckpointException and abort the freeze.
• Silent Restore Failures: Failing to write idempotent recovery hooks, resulting in deadlocks and broken state when the JVM wakes up in a cloned environment.

The Right Way

You must design your resource lifecycle hooks to be strictly idempotent, utilizing JDK 26's new native JFR events to audit and verify clean state transitions before freeze.

• Implement the org.crac.Resource interface defensively, ensuring beforeCheckpoint and afterRestore can run multiple times without throwing errors.
• Leverage the newly introduced JDK 26 jdk.Checkpoint and jdk.Restore JFR events to profile freeze duration and pinpoint exactly which class blocked the operation.
• Use a centralized ResourceReleaser coordinator to gracefully drain active HTTP requests and close open file descriptors before the checkpoint engine kicks in.

Show Me The Code

Here is how to implement a clean, idempotent database pool resource that safely registers with the CRaC context:

public class IdempotentDbPool implements org.crac.Resource {
    private HikariDataSource ds;

    @Override
    public void beforeCheckpoint(Context<? extends Resource> context) {
        if (ds != null && !ds.isClosed()) {
            ds.close(); // Prevent socket leaks during checkpoint
        }
    }

    @Override
    public void afterRestore(Context<? extends Resource> context) {
        if (ds == null || ds.isClosed()) {
            this.ds = initHikariPool(); // Idempotent re-initialization
        }
    }
}

Key Takeaways

• Zero-Socket Policy: Every network socket and file descriptor must be closed before checkpointing, or the JVM will refuse to freeze.
• JDK 26 JFR is Mandatory: Use the new native flight recorder events to trace exactly which thread or resource blocked a restore operation.
• Idempotency Over Everything: Assume afterRestore can be called in a cloned environment and design your initialization hooks accordingly.

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