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App/Dynamic Class Data Sharing (CDS) in HotSpot JVM

The demo is based on spring-petclinic project

Make sure the OpenJDK HotSpot Runtime is properly configured

Build the application sources

cd spring-petclinic/

AppCDS

Note: I will refer to the AppCDS archive as a static archive

Build the application sources

./mvnw package -DskipTests

Create the AppCDS class list

java -Xshare:off -XX:DumpLoadedClassList=static-cds.lst -jar target/*.jar

Create the AppCDS archive based on the previously created class list

java -Xshare:dump -XX:SharedClassListFile=static-cds.lst -XX:SharedArchiveFile=static-cds.jsa -jar target/*.jar

Start the application and specify the name of the AppCDS archive as argument

java -XX:SharedArchiveFile=static-cds.jsa -jar -Dserver.port=8080 target/*.jar

Enable CDS and class load logging for further analysis

Optionally, you can also start the application with CDS and class load logging enabled and redirect the output to separate files for further analysis

java -Xlog:cds:file=static-cds.log -Xlog:class+load:file=static-cds-class-load.log -XX:SharedArchiveFile=static-cds.jsa -jar -Dserver.port=8080 target/*.jar

Dynamic CDS

Create the dynamic CDS archive

Build the application sources

./mvnw package -DskipTests

Create the dynamic CDS archive

java -XX:ArchiveClassesAtExit=dynamic-cds.jsa -jar -Dserver.port=8080 target/*.jar

Once the application properly starts, call the existing endpoints at localhost:8080 and press CTRL+C

Note: Since only loaded classes will be added to the archive (i.e., classloading happens lazily) you must invoke the functionality of your application to cause all the relevant classes to be loaded

Start the application and specify the name of the dynamic CDS archive as argument

java -Xlog:cds -XX:SharedArchiveFile=dynamic-cds.jsa -jar -Dserver.port=8080 target/*.jar

Enable CDS and class load logging for further analysis

Optionally, you can also start the application with CDS and class load logging enabled and redirect the output to separate files for further analysis

java -Xlog:cds:file=dynamic-cds.log -Xlog:class+load:file=dynamic-cds-class-load.log -XX:SharedArchiveFile=dynamic-cds.jsa -jar -Dserver.port=8080 target/*.jar

Once the application properly starts press CTRL+C

Check the CDS logs

cat dynamic-cds.log

Count the total number of shared classes

cat dynamic-cds-class-load.log | grep "shared objects file" | wc -l

Count the total number of Spring framework shared classes

cat dynamic-cds-class-load.log | grep "shared objects file" | grep "org.springframework" | wc -l

Enabling the "thin jar" deployment use case using spring-boot-thin-layout plugin and repeating all previous steps more classes will be included in the shared archive

Compare the start-up times between static base CDS (i.e., default) and dynamic CDS archives

Make sure there is no other instance running

pkill -f 'petclinic'

Start a few instances with static base CDS

java -jar -Dserver.port=8080 target/*.jar 1>/dev/null & ./time-to-first-response.sh 8080

java -jar -Dserver.port=8081 target/*.jar 1>/dev/null & ./time-to-first-response.sh 8081

Start a few instances with dynamic CDS

java -XX:SharedArchiveFile=dynamic-cds.jsa -jar -Dserver.port=8082 target/*.jar 1>/dev/null & ./time-to-first-response.sh 8082

java -XX:SharedArchiveFile=dynamic-cds.jsa -jar -Dserver.port=8083 target/*.jar 1>/dev/null & ./time-to-first-response.sh 8083

Two layers of CDS archives

HotSpot JVM can use up to two archives

Option 1: create the dynamic CDS archive based on an AppCDS archive (instead of the default static CDS)

java -XX:SharedArchiveFile=static-cds.jsa -XX:ArchiveClassesAtExit=dynamic-cds.jsa -jar -Dserver.port=8080 target/*.jar

Then, start the application using the previously created archive

java -XX:SharedArchiveFile=dynamic-cds.jsa -jar -Dserver.port=8080 target/*.jar

Option 2: chain the AppCDS archive and the dynamic CDS archive in the same command line

java -XX:SharedArchiveFile=static-cds.jsa:dynamic-cds.jsa -jar -Dserver.port=8080 target/*.jar

Note: the separator on Windows is ; (backslash semicolon) instead of : (colon)