Picocli on GraalVM: Blazingly Fast Command Line Apps
GraalVM
GraalVM allows you to compile your programs ahead-of-time into a native executable. The resulting program has faster startup time and 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.
Reflective Access
Picocli currently uses reflection to discover classes and methods annotated with @Command
, and fields, 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 3.7.0 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.
Generating 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 System.out
, so you will want to redirect it to a file:
java -cp \ picocli-3.7.0.jar:picocli-codegen-3.7.0-tests.jar:picocli-codegen-3.7.0.jar \ picocli.codegen.aot.graalvm.ReflectionConfigGenerator picocli.codegen.aot.graalvm.Example > reflect.json
The generated reflect.json
files looks 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"] } ] }, ... ]
Tip | If necessary, it is possible to exclude classes with system property picocli.codegen.excludes , which accepts a comma-separated list of regular expressions of the fully qualified class names that should not be included in the resulting JSON String. |
Compiling a Native Image
This assumes you have GraalVM installed, with prerequisites. From the site:
To build a native image of the program use the
native-image
utility located in thebin
directory of the GraalVM distribution. For compilationnative-image
depends on the local toolchain, so please make sure:glibc-devel
,zlib-devel
(header files for the C library andzlib
) andgcc
are available on your system.
I also needed the static packages glibc-static
and zlib-static
, other than the devel packages.
We compile the example class with the following command:
graalvm-ce-1.0.0-rc6/bin/native-image \ -cp picocli-3.7.0.jar:picocli-codegen-3.7.0-tests.jar \ -H:ReflectionConfigurationFiles=reflect.json -H:+ReportUnsupportedElementsAtRuntime \ --static --no-server picocli.codegen.aot.graalvm.Example
The reflect.json
is in the current directory, and I added -H:+ReportUnsupportedElementsAtRuntime
to get a useful error message in case something goes wrong.
Tip | native-image --expert-options shows a list of other compilation options not shown in the output of native-image --help . |
Running the Native Image
If 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.jar:picocli-codegen-3.7.0-tests.jar \ picocli.codegen.aot.graalvm.Example --version 3.7.0 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 3.7.0 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.
Conclusion
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 the projects!
Published on Java Code Geeks with permission by Remko Popma, partner at our JCG program. See the original article here: Picocli on GraalVM: Blazingly Fast Command Line Apps Opinions expressed by Java Code Geeks contributors are their own. |