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</head>
<body class="article">
<div id="header">
<h1>Picocli on GraalVM: Blazingly Fast Command Line Apps</h1>
<div class="details">
<span id="revnumber">version 4.6.1</span>
</div>
</div>
<div id="content">
<div id="preamble">
<div class="sectionbody">
<div class="imageblock">
<div class="content">
<img src="https://picocli.info/images/picocli-on-graalvm.png" alt="picocli on graalvm">
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_graalvm">GraalVM</h2>
<div class="sectionbody">
<div class="paragraph">
<p><a href="https://www.graalvm.org/">GraalVM</a> 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.</p>
</div>
<div class="paragraph">
<p>GraalVM has limited support for Java reflection and it needs to know ahead of time the reflectively accessed program elements.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_reflective_access">Reflective Access</h2>
<div class="sectionbody">
<div class="paragraph">
<p><a href="https://github.com/remkop/picocli">Picocli</a> currently uses reflection to discover
<a href="https://picocli.info/#CheckSum-application">classes</a> and <a href="https://picocli.info/#command-methods">methods</a>
annotated with <code>@Command</code>, and <a href="https://picocli.info/#_options_and_parameters">fields</a>,
<a href="https://picocli.info/#option-parameters-methods">methods</a> or
<a href="https://picocli.info/#command-methods">method parameters</a> annotated with <code>@Option</code> and <code>@Parameters</code> 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.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_reflectionconfiggenerator_tool">ReflectionConfigGenerator Tool</h2>
<div class="sectionbody">
<div class="paragraph">
<p>Picocli 3.7.0 includes a <a href="https://github.com/remkop/picocli/tree/master/picocli-codegen"><code>picocli-codegen</code> module</a>, with a tool that generates a GraalVM configuration file.</p>
</div>
<div class="paragraph">
<p><code>ReflectionConfigGenerator</code> 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.</p>
</div>
<div class="paragraph">
<p>The output of <code>ReflectionConfigGenerator</code> is intended to be passed to the <code>-H:ReflectionConfigurationFiles=/path/to/reflectconfig</code> option of the <code>native-image</code> GraalVM utility. This allows picocli-based applications to be compiled to a native image.</p>
</div>
<div class="sect2">
<h3 id="_example_usage">Example Usage</h3>
<div class="paragraph">
<p>We will use the <code>picocli.codegen.aot.graalvm.Example</code> class that is in the tests for the <code>picocli-codegen</code> module as an example. First, we will generate a <code>reflect.json</code> configuration file with the <code>ReflectionConfigGenerator</code> tool. Next, we will compile the <code>Example</code> 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.</p>
</div>
</div>
<div class="sect2">
<h3 id="_generating_the_configuration_file">Generating the Configuration File</h3>
<div class="paragraph">
<p>Run the <code>ReflectionConfigGenerator</code> tool and specify one or more fully qualified class names of the <code>@Command</code>-annotated classes. The output is printed to <code>System.out</code>, so you will want to redirect it to a file:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlightjs highlight"><code class="language-bash hljs" data-lang="bash">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</code></pre>
</div>
</div>
<div class="paragraph">
<p>The generated <code>reflect.json</code> files looks something like this:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlightjs highlight"><code class="language-json hljs" data-lang="json">[
{
"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"] }
]
},
...
]</code></pre>
</div>
</div>
<div class="admonitionblock tip">
<table>
<tr>
<td class="icon">
<div class="title">Tip</div>
</td>
<td class="content">
If necessary, it is possible to exclude classes with system property <code>picocli.codegen.excludes</code>, 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.
</td>
</tr>
</table>
</div>
</div>
<div class="sect2">
<h3 id="_compiling_a_native_image">Compiling a Native Image</h3>
<div class="paragraph">
<p>This assumes you have GraalVM installed, with prerequisites. From <a href="https://www.graalvm.org/docs/reference-manual/aot-compilation/">the site</a>:</p>
</div>
<div class="quoteblock">
<blockquote>
<div class="paragraph">
<p>To build a native image of the program use the <code>native-image</code> utility located in the <code>bin</code> directory of the GraalVM distribution. For compilation <code>native-image</code> depends on the local toolchain, so please make sure: <code>glibc-devel</code>, <code>zlib-devel</code> (header files for the C library and <code>zlib</code>) and <code>gcc</code> are available on your system.</p>
</div>
</blockquote>
</div>
<div class="paragraph">
<p>I also needed the static packages <code>glibc-static</code> and <code>zlib-static</code>, other than the devel packages.</p>
</div>
<div class="paragraph">
<p>We compile the example class with the following command:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlightjs highlight"><code class="language-bash hljs" data-lang="bash">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</code></pre>
</div>
</div>
<div class="paragraph">
<p>The <code>reflect.json</code> is in the current directory, and I added <code>-H:+ReportUnsupportedElementsAtRuntime</code> to get a useful error message in case something goes wrong.</p>
</div>
<div class="admonitionblock tip">
<table>
<tr>
<td class="icon">
<div class="title">Tip</div>
</td>
<td class="content">
<code>native-image --expert-options</code> shows a list of other compilation options not shown in the output of <code>native-image --help</code>.
</td>
</tr>
</table>
</div>
</div>
<div class="sect2">
<h3 id="_running_the_native_image">Running the Native Image</h3>
<div class="paragraph">
<p>If compilation went well, we now have a native executable <code>picocli.codegen.aot.graalvm.example</code> in the current directory:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlightjs highlight"><code class="language-bash hljs" data-lang="bash">$ ls -alh picocli*
-rwxrwxr-x 1 remko remko 15M Oct 4 21:35 picocli.codegen.aot.graalvm.example</code></pre>
</div>
</div>
<div class="paragraph">
<p>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 <code>native-image [options] -jar jarfile</code> to build an image for the jar file.</p>
</div>
<div class="paragraph">
<p>Let’s first run the application in Java, and time it to see how long it takes to start up.</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlightjs highlight"><code class="language-none hljs">$ 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</code></pre>
</div>
</div>
<div class="paragraph">
<p>On Java Hotspot, it takes about half a second to run. Now, we run the native image:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlightjs highlight"><code class="language-none hljs">$ time ./picocli.codegen.aot.graalvm.example --version
3.7.0
real 0m0.003s
user 0m0.000s
sys 0m0.004s</code></pre>
</div>
</div>
<div class="paragraph">
<p>The startup time is now down to 3 milliseconds!</p>
</div>
<div class="paragraph">
<p>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.</p>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_conclusion">Conclusion</h2>
<div class="sectionbody">
<div class="paragraph">
<p>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.</p>
</div>
<div class="paragraph">
<p>The <code>ReflectionConfigGenerator</code> tool included in the <code>picocli-codegen</code> module allows picocli-based applications to be compiled to native executables with extremely fast startup times.</p>
</div>
<div class="paragraph">
<p>Please star ☆ <a href="https://github.com/oracle/graal">GraalVM</a> and <a href="https://github.com/remkop/picocli">picocli on GitHub</a> if you like the projects!</p>
</div>
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