Boost Java Startup Speed with Class Data Sharing (CDS): A Step-by-Step Guide

1. What is CDS?

Class Data Sharing (CDS) is a JVM feature that reduces Java application startup time and memory usage. Since JDK 12, the default CDS archive is packaged with the Oracle JDK binaries. The author tested with OpenJDK 64‑Bit Server VM Zulu21.34+19‑CA (build 21.0.3+9‑LTS), which also supports CDS.

2. How to use

2.1 Training

First run a training execution of the application in “extract” mode:

$ java -Djarmode=tools -jar my-app.jar extract --destination application
$ cd application
$ java -XX:ArchiveClassesAtExit=application.jsa -Dspring.context.exit=onRefresh -jar my-app.jar

This creates an application.jsa file in the application directory, which can be reused as long as the application is not updated.

2.2 Running with the archive

When starting the application, add the -XX:SharedArchiveFile option:

$ java -XX:SharedArchiveFile=application.jsa -jar my-app.jar

3. Results

The author generated a Spring Boot demo project and trained CDS. The directory structure after training is shown, followed by two execution scenarios.

3.1 Direct run

$ java -jar demo-0.0.1-SNAPSHOT.jar
... (log output) ...
Started DemoApplication in 1.702 seconds

3.2 Run with CDS

$ java -XX:SharedArchiveFile=application.jsa -jar demo-0.0.1-SNAPSHOT.jar
... (log output) ...
Started DemoApplication in 0.722 seconds

Using CDS reduces startup time by about one second compared with the direct run.

4. Summary

CDS, CRaC, and GraalVM all help improve Java startup speed, but they differ in use cases. CDS speeds up startup by sharing class metadata, CRaC optimizes runtime, and GraalVM provides AOT compilation. Developers can choose the appropriate technique based on their requirements.