JVM - Class Data Sharing

Topic: JVM Internals                                                                                               Level: Advanced

Problem Statement

When we run a Java application all the related classes and their dependencies associated are loaded at the runtime, adding to the startup time of the application as well as to the memory footprint.

Every time when we launch the application, the process would be the same and starting up multiple instances of the application would add up significant latency, and also becoming memory intensive, as the same class-related data need to be replicated and loaded.

What is CDS?

AppCDS (Application Class Data Sharing) improves the startup performance of the application by creating an archive of the library classes/selected classes from the application classpath such that when the JVM loads, the shared archive is consulted which is memory-mapped to allow sharing of read-only JVM metadata for the classes among multiple JVM processes. 

Because accessing the shared archive is faster than loading the classes, startup time is reduced and no additional memory allocation is required when starting up a new instance.

Image source: https://www.happycoders.eu/java/java-10-features/

How?

Available from JDK12, a default CDS will be created at the JDK build time by running, -Xshare:dump, using G1GC and 128M Java heap, containing default selective core library classes in the archive.

The CDS resides in the directory,

• /lib/[arch]/server/classes.jsa - for Linux and macOS platforms
• /bin/server/classes.jsa - for Windows platform

The default CDS archive is enabled at the runtime by default, and it could be turned off with the command -Xshare:off.

AppCDS allows the class loaders, garbage collectors, and custom loaders to load the archived pre-processed data. On the application launch, the shared CDS archive is visited and leveraged for bootstrapping, if not found then on application exit the archive will be created with the list of classes that were used to load the application.

The archived classes hold all the loaded application classes, and library classes that are not available as part of the default CDS archive, making it dynamically evolve on each application launch. To enable dynamic CDS archiving, -XX:+RecordDynamicDumpInfo; XX:ArchiveClassesAtExit which updates the shared CDS archive with the new list of classes on exiting the application.

Additionally, when there is a Java version mismatch in the shared CDS archive then on the application exit a new archive will be created overriding the redundant archive.

References

1. https://docs.oracle.com/en/java/javase/19/docs/specs/man/java.html#application-class-data-sharing
2. https://docs.oracle.com/en/java/javase/19/vm/class-data-sharing.html#GUID-2942983A-E83C-4DA3-A60C-60411D731D5A
3. https://inside.java/2022/09/26/sip067/
4. https://www.happycoders.eu/java/java-10-features/

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