Skip to content

Reference: Clojure CLI JVM Optionsλ︎

JDK_JAVA_OPTIONS Environment Variable

JDK_JAVA_OPTIONS is the official Environment Variable for setting options when calling java, javac and other Java commands to start running a Java Virtual Machine (Java version 9 onward).

Java Virtual Machine options can be passed using the Clojure CLI, either via the -J command line flag or :jvm-opts in a deps.edn alias.

Java Virtual Machine configuration and reporting

Java Virtual Machine section covers commonly used options, reporting JVM metrics and optimisation of the JVM process.

Clojure CLI command line optionsλ︎

Clojure CLI -J flag passes configuration options to the JVM. When there are multiple, each must be prefixed with -J.

clojure -J-XX:+UnlockDiagnosticVMOptions -J‑XX:NativeMemoryTracking=summary -J‑XX:+PrintNMTStatistics

Clojure CLI deps.edn configurationλ︎

:jvm-opts key in an alias adds JVM options to Clojure CLI deps.edn configuration. The :jvm-opts key has a value that is a collection of string JVM options ["-Xms2048m" "-Xmx4096"]

Alias to set a large heap size

:jvm/heap-max-2g {:jvm-opts ["-Xmx2G"]}

Report a full breakdown of the HotSpot JVM’s memory usage upon exit using the following option combination:

:jvm/report {:jvm-opts ["-XX:+UnlockDiagnosticVMOptions"

Add a Java module

:jvm/xml-bind {:jvm-opts ["–add-modules java.xml.bind"]}

Ignoring unrecognised options

:jvm-opts ["-XX:+IgnoreUnrecognizedVMOptions"]

The aliases can be used with the Clojure CLI execution options: -A (for built-in REPL invocation), -X and -T (for clojure.exec function execution), or -M (for clojure.main execution).

-J JVM options specified on the command line are concatenated after the alias options

Calling A Clojure Uberjarλ︎

JVM options must be specified when calling an uberjar with the java command, :jvm-opts in the project deps.edn are not used with the java command

java -jar project-uberjar.jar -J...

Use JDK_JAVA_OPTIONS to define JVM options

JDK_JAVA_OPTIONS environment variable is used to define options that are used whenever the java command is called, greatly simplifying java commands.

The JDK_JAVA_OPTIONS environment variable can be used with deployment systems and passed to container environments to simplify adjustment of resources used by the JVM process.

Specify options or system properties to set up the Clojure service

-Dclojure.compiler.disable-locals-clearing=true - make more info available to debuggers - print stack traces to standard error instead of saving to file, useful if process failing on startup

-Dclojure.spec.skip-macros=false - skip spec checks against macro forms

Memory Managementλ︎

-XX:CompressedClassSpaceSize=3G - prevent a specific type of OOMs

-XX:MaxJavaStackTraceDepth=1000000 - prevents trivial Stack Overflow errors

-Xmx24G - set high maximum heap, preventing certain types of Out Of Memory errors (ideally high memory usage should be profiled if cause not known)

-Xss6144k - increase stack size x6 to prevent Stack Overflow errors

The current default can be found with java -XX:+PrintFlagsFinal -version 2>/dev/null | grep "intx ThreadStackSize"

-Xms6G - Set minimum memory that is equal or greater than memory used by a running REPL, to improve performance

-Xmx1G - limit maximum heap allocation so a process can never use more memory, useful for environments with limited memory resources

:jvm/mem-max1g {:jvm-opts ["-Xmx1G"]}

Container Memory Managementλ︎

JDK_JAVA_OPTIONS environment variable should be used for setting JVM options within a container or in the provisioning service (e.g. Kubernettes / Argo CD) that deploys containers.

Use JVM options that optimise running in a container

  • -XshowSettings:system to output the resources the JVM believes it has access too, a very simple diagnostic tool to include

  • -XX:+UseContainerSupport instruct the JVM that it is running in a container environment, disabling the checks the JVM would otherwise carry out to determine if it was running in a container. Can save a very small amount of start up time, though mainly used to ensure the JVM knows its in a container.

  • -XX:MaxRAMPercentage=90 to set a relative maximum percentage of heap to use, based on the memory available from the host, e.g. -XX:MaxRAMPercentage=80 will use a heap size of 80% of the available host memory

Dockerfile example with JDK_JAVA_OPTIONS environment variableλ︎

In this Dockerfile excerpt the JDK_JAVA_OPTIONS environment variable is used to print out the resources the JVM believes it has access to at startup. The JVM is instructed that it is running in a container environment and should use a maximum 90% heap size of the hosts memory resource.

ENV JDK_JAVA_OPTIONS "-XshowSettings:system -XX:+UseContainerSupport -XX:MaxRAMPercentage=90"
CMD ["java", "-jar", "/opt/practicalli-service.jar"]

Low latency systemsλ︎

For systems that require very low latency, use the Z Garbage collector

"-XX:+UnlockExperimentalVMOptions -XX:+UseZGC"

Stack tracesλ︎

-XX:+TieredCompilation - enable tiered compilation to support accurate bench-marking (increases startup time)

-XX:-OmitStackTraceInFastThrow - don't elide stack traces

Startup optionsλ︎

-Xverify:none option reduces startup time of the JVM by skipping verification process


The verification process is a valuable check, especially for code that has not been run before. So the code should be run through the verification process before deploying to production.

Benchmark optionsλ︎

Enable various optimizations, for guaranteeing accurate benchmarking (at the cost of slower startup):


-Djava.awt.headless=true - disable all UI features for disabling the clipboard for personal security:

-Dapple.awt.UIElement=true - remove icon from the MacOSX Dock - ?

Garbage Collectionλ︎

Setup GC with short STW pauses which can be relevant for very high web server workloads

{:jvm-opts ["-XX:+UseG1GC"

View JVM options of a running JVM processλ︎

Use a JMX client, e.g. VisualVM

jcmd pid VM.system_properties or jcmd pid VM.flags using jcmd -l to get the pid of the JVM process

On Linux ps -ef | grep java which includes the options to run the JVM process, ps -auxww to show long arguments

Getting the parameters of a running JVM


JVM Options cheatsheet - JRebel