lein-native-image

A Leiningen plugin for generating GraalVM native images from your project.

The lein native-image command compiles your project then uses GraalVM's native-image to build a native image.

^{For deps.edn projects, try clj.native-image.}

Prerequisites

This plugin depends on GraalVM to build native images.

NOTE: As of GraalVM 19.0.0, native-image is no longer included by default:

Native Image was extracted from the base GraalVM distribution. Currently it is available as an early adopter plugin. To install it, run: gu install native-image. After this additional step, the native-image executable will be in the bin directory, as for the previous releases.
```
➜ $GRAALVM_HOME/bin/gu install native-image
Downloading: Component catalog from www.graalvm.org
Processing component archive: Native Image
Downloading: Component native-image: Native Image  from github.com
Installing new component: Native Image licence files (org.graalvm.native-image, version 19.0.0)
```
Your project.clj must set a :main namespace w/entrypoint and support AOT compilation:
```
:main ^:skip-aot my-app.core
```

Examples

See the examples directory for projects that can be compiled to native images with GraalVM:

jdnsmith - CLI command to read JSON from stdin and write EDN to stdout.
http-api - Basic HTTP server using Ring, Compojure, http-kit.
nlp - CLI command to analyze sentiment of text using StanfordNLP. Includes examples of reflection hints and delaying class initialization.
clojurl - cURL-like tool using clojure.spec, HTTPS, hiccup.

Usage

Configure your project with a custom image name, path to GraalVM's home directory or native-image path, or native-image CLI options:

(defproject my-app "0.1.0"
  :plugins [[io.taylorwood/lein-native-image "0.3.1"]]    ;; or in ~/.lein/profiles.clj

  :native-image {:name "my-app"                 ;; name of output image, optional
                 :graal-bin "/path/to/graalvm/" ;; path to GraalVM home, optional
                 :opts ["--verbose"]}           ;; pass-thru args to GraalVM native-image, optional

  ;; optionally set profile-specific :native-image overrides
  :profiles {:test    ;; e.g. lein with-profile +test native-image
             {:native-image {:opts ["--report-unsupported-elements-at-runtime"
                                    "--initialize-at-build-time"
                                    "--verbose"]}}

             :native-image ;; used by default
             {:jvm-opts ["-Dclojure.compiler.direct-linking=true"]}})

:native-image config keys:

:name is an optional name for the generated native image.
The :graal-bin path can be specified as a string or resolved from an environment variable using a keyword e.g. :env/GRAALVM_HOME. If :graal-bin is unspecified, the GRAALVM_HOME environment variable will be used by default.
:opts is an optional vector of arguments to native-image; see its documentation for more.

Note: task-specific :native-image profile will be merged in by default, or the :uberjar profile if that doesn't exist.

You can also specify these in your Leiningen user profile ~/.lein/profiles.clj:

{:user {:plugins [[io.taylorwood/lein-native-image "0.3.1"]]
        :native-image {:graal-bin "/path/to/graalvm-ce-19.0.0/Contents/Home/bin"}}}

Build a native image from your project:

➜ lein native-image
Compiling my-app.core
Build on Server(pid: 36212, port: 26681)
   classlist:     332.89 ms
       (cap):   1,289.90 ms
   8<----------------------
       write:     932.61 ms
     [total]:  11,789.08 ms
Created native image /path/to/my-app/target/my-app

Execute the native image:

➜ ./target/my-app with optional args
Hello, World!

Caveats

The primary benefits to using a GraalVM native image are faster startup, lower memory requirements, and smaller distribution footprint (no JDK/JRE required). This doesn't necessarily mean the same code will run faster than it would on the JVM. GraalVM Community Edition and Enterprise Edition also have different performance characteristics.

GraalVM's native image capabilities have evolved across many release candidates. Several AOT issues have been fixed since 1.0.0-RC1. GraalVM and Substrate VM's support for AOT compilation and native images has limitations, and there are unsupported features. This release and its example projects were tested with GraalVM 19.0.0 Community Edition.

GraalVM 19.0.0 (first non-RC release) changes the default class-initialization behavior of native-image. Now you must specify --initialize-at-build-time explicitly in your native-image options.

There is a known issue where usages of clojure.core/locking macro will fail compilation. Clojure 1.10 depends on a version of clojure.spec that uses locking. See this commit for an example workaround.

When the --report-unsupported-elements-at-runtime flag is set, some native-image AOT compilation issues will be deferred as runtime exceptions. You can try specifying this flag if native-image compilation fails. To avoid unexpected errors at runtime, don't use this flag for "production" builds.

Set --enable-url-protocols=http to use HTTP libraries. HTTPS is available as of 1.0.0-RC7 (e.g. --enable-url-protocols=http,https) but requires additional configuration.

Specifying :jvm-opts ["-Dclojure.compiler.direct-linking=true"] might allow for better compile-time optimizations.

This plugin doesn't shutdown GraalVM native-image build servers after builds, so that subsequent builds are slightly faster. You can set :opts ["--no-server"] to not spawn a build server at all, or use GraalVM's native-image command directly to manage build server(s).