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Picocli on GraalVM: Blazingly Fast Command Line Apps

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Picocli on GraalVM: Blazingly Fast Command Line Apps

Want to learn more about using the Picocli framework for creating Java command line applications? Check out this post on using GraalVM to get you started!

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GraalVM allows you to compile your programs ahead of time into a native executable. The resulting program has faster startup time and a lower runtime memory overhead compared to a Java VM. This is especially useful for command line utilities, which are often short-lived.

GraalVM has limited support for Java reflection and it needs to know ahead of time the reflectively accessed program elements.

Picocli: Reflective Access

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Picocli is a one-file framework for creating Java command line applications with almost zero code. It currently uses reflection to discover classes and methods annotated with @Commandfields, methods, or method parameters annotated with @Option and @Parameters and other Picocli annotations. A future Picocli release may include an annotation processor to do this work at compile time, but as it stands, it uses reflection.

ReflectionConfigGenerator Tool

Picocli 3.7 includes a picocli-codegen module, with a tool that generates a GraalVM configuration file.

ReflectionConfigGenerator generates a JSON String with the program elements that will be accessed reflectively in a Picocli-based application in order to compile this application ahead of time into a native executable with GraalVM.

The output of ReflectionConfigGenerator is intended to be passed to the -H:ReflectionConfigurationFiles=/path/to/reflectconfig option of the native-image GraalVM utility. This allows Picocli-based applications to be compiled to a native image.

Example Usage

We will use the picocli.codegen.aot.graalvm.Example class that is in the tests for the picocli-codegen module as an example. First, we will generate a reflect.json configuration file with the ReflectionConfigGenerator tool. Next, we will compile the Example class to a native application, and finally, we will run this application and see what the difference is in startup time between the native application and running on Hotspot.

Creating the Configuration File

Run the ReflectionConfigGenerator tool and specify one or more fully qualified class names of the @Command-annotated classes. The output is printed to the System.out, so you will want to redirect it to a file:

java -cp \
  picocli-3.7.0-SNAPSHOT.jar:picocli-codegen-3.7.0-SNAPSHOT-tests.jar:picocli-codegen-3.7.0-SNAPSHOT.jar \
  picocli.codegen.aot.graalvm.ReflectionConfigGenerator \
  picocli.codegen.aot.graalvm.Example > reflect.json

The generated reflect.json files look something like this:

    "name" : "picocli.codegen.aot.graalvm.Example",
    "allDeclaredConstructors" : true,
    "allPublicConstructors" : true,
    "allDeclaredMethods" : true,
    "allPublicMethods" : true,
    "fields" : [
        { "name" : "spec" },
        { "name" : "unmatched" },
        { "name" : "timeUnit" },
        { "name" : "file" }
    "methods" : [
        { "name" : "setMinimum", "parameterTypes" : ["int"] },
        { "name" : "setOtherFiles", "parameterTypes" : ["[Ljava.io.File;"] },
        { "name" : "multiply", "parameterTypes" : ["int", "int"] }

Compiling a Native Image

This assumes you have GraalVM installed, with some extra prerequisites. From the site:

To build a native image of the program, use the native-image utility located in the bin directory of the GraalVM distribution. For compilation, native-image depends on the local toolchain, so please make sure: glibc-devel , zlib-devel (header files for the C library and zlib ), and gcc are available on your system.

In my case, I also needed the static packages glibc-static and zlib-static , other than the devel packages.

The example class can be compiled with the following command:

graalvm-ce-1.0.0-rc6/bin/native-image \
    -cp picocli-3.7.0-SNAPSHOT.jar:picocli-codegen-3.7.0-SNAPSHOT-tests.jar \
    -H:ReflectionConfigurationFiles=reflect.json \
    -H:+ReportUnsupportedElementsAtRuntime \
    --static --no-server picocli.codegen.aot.graalvm.Example

This command expects the reflect.json file to exist in the current directory. I added -H:+ReportUnsupportedElementsAtRuntime to get a useful error message in case something goes wrong.

Tip: try executing native-image --expert-options. This shows a list of other compilation options not shown in the output of the native-image --help command.

Running the Native Image

Assuming compilation went well, we now have a native executable picocli.codegen.aot.graalvm.example in the current directory:

$ ls -alh picocli*
-rwxrwxr-x 1 remko remko 15M Oct  4 21:35 picocli.codegen.aot.graalvm.example

The name of the executable is derived from the main class name. If the jar is an executable jar (with the Main-Class specified in the manifest), we could have run native-image [options] -jar jarfile to build an image for the jar file.

Let’s first run the application in Java and time it to see how long it takes to start up.

$ time java -cp  picocli-3.7.0-SNAPSHOT.jar:picocli-codegen-3.7.0-SNAPSHOT-tests.jar \
    picocli.codegen.aot.graalvm.Example --version

real    0m0.492s
user    0m0.847s
sys     0m0.070s

On Java Hotspot, it takes about half a second to run. Now, we run the native image:

$ time ./picocli.codegen.aot.graalvm.example --version

real    0m0.003s
user    0m0.000s
sys     0m0.004s

The startup time is now down to 3 milliseconds!

All command line parsing functionality works as expected, with type conversion, validation, and help with ANSI colors. This is exciting news when you want to write command line applications and services in Java and have them run instantaneously.


GraalVM is an exciting new technology that allows Java programs to run as native code. This gives reduced memory usage and startup time, which is especially useful for short-running programs like command line utilities.

The ReflectionConfigGenerator tool, included in the picocli-codegen module, allows Picocli-based applications to be compiled to native executables with extremely fast startup times.

Please star GraalVM and Picocli on GitHub if you like these projects!

graalvm ,picocli ,performance ,command line apps ,command line tools ,java ,native ,tutorial

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