Imagine if every time you opened your computer, you had to reboot it. Wait for the OS, open programs & files one by one, position windows just so. If it were me, I'd never close my laptop.

And yet, this is precisely what we do with our Java apps.

For every start and every restart, we load all classes and warm up the app until we achieve peak performance, again. We have no choice.

Or rather, we had no choice, until now.

Introducing CRaC:

Coordinated Restore at Checkpoint—or after some getting-used-to, CRaC—is a feature developed by Azul Systems that enables the JVM to start up from a warmed-up memory snapshot.

The checkpoint is a serialized memory representation of your app, in a safe state, of course, with files, sockets and threads closed/stopped.

At restore, the application will have the same level of "warmness" as the checkpoint snapshot.

Persisting CRaC:

The checkpoint is stored to disk. Local, Docker volume, Kubernetes volume. But perhaps the most interesting is embedding the snapshot as a layer into the app's Docker image itself. The app and the snapshot travel together, inseparably, all the way to production.

Scaling CRaC (to Zero!):

Imagine: one slow start to create a snapshot, then every start thereafter is instantaneous with immediate peak performance. If starting is that efficient, why not scale to zero and start as needed? Ka-ching!

CRaC with Spring:

With Spring Boot 3.2+, CRaC is mapped to the Spring Lifecycle. Checkpoints can be captured automatically (just before bean startup), or on-demand (after warmup).

CRaC with Spring (by example):

# Example of automatic CRaC checkpoint capture with Spring

java -Dspring.context.checkpoint=onRefresh \

-XX:CRaCCheckpointTo=$CRAC_FILES_DIR \

-jar target/app.jar

# Restore

java -XX:CRaCCheckpointFrom=$CRAC_FILES_DIR

Caveats:

- Only works with Linux.

- Checkpoint after warmup depends on adoption of CRaC by 3rd party libraries.

- Beware of storing in-memory secrets in snapshot. Use Spring's @RefreshScope to load different secrets on restore.

Get CraC-ing!

In short, CRaC provides some serious motivation to upgrade to Spring Boot 3.2 and Java 21! Faster startups requiring less memory to start, instantly achieve optimal performance, portable snapshots, and ability to scale to zero. What are you waiting for?