Liking cljdoc? Tell your friends :D

Clojars Project Slack community


Automatically detect and initialize Clojure classes during native-image compilation.

⬆️ Upgrading to from v0 to v1?
Add --features=clj_easy.graal_build_time.InitClojureClasses to your native-image command line.

We compile our Clojure sources to .class files so that GraalVM native-image can in turn compile them into a binary executable. Due to their nature, Clojure .class files typically must be initialized at build time by GraalVM native-image. If they are not initialized at build time, GraalVM will still create your binary executable, but when you run it, it probably won't work. You will likely see it fail with an error that includes: Could not locate clojure/core__init.class, clojure/core.clj or clojure/core.cljc on classpath

In the early days, our solution was to use native-image's --initialize-at-build-time to globally initialize all classes at build time. Because global initialization of all classes can be problematic, GraalVM deprecated this usage. Instead, you must be explicit and specify the packages of the classes you need to initialize at build time. For example: --initialize-at-build-time=clojure,my_library,etc... But, this can be tedious and error-prone.

Hence, graal-build-time. This library automatically detects .class files created by Clojure and asks native-image to initialize them at build time.


We assume you are using a current stable release of GraalVM. We don't support older releases.

For your native-image build:

  1. If you are using --initialize-at-build-time, remove it.
  2. Add --features=clj_easy.graal_build_time.InitClojureClasses
  3. Include the graal-build-time library on your classpath. This is typically done by adding this library to your project dependencies (see the clojars link above).

During the native-image build process, you will see a line of output from graal-build-time describing the packages it has detected:

[clj-easy/graal-build-time] Registering packages for build time initialization: clojure, clj_easy.graal_build_time

How it works

graal-build-time hooks into the GraalVM native-image build process via a GraalVM Feature class. It inspects the classpath. Each class file that ends with __init.class is assumed to have been created by Clojure. The packages of these classes are then added to the list of packages to be initialized at build time.

Overriding classes

If there are classes in packages that you would like to initialize at runtime, you can use the native-image argument.

For example, when using http-kit, the org.httpkit package will be included for build-time initialization. You will need to override one class via native-image argument:


Single segment namespaces

This library doesn't work with single segment namespaces. A single segment namespace is one without any . characters in it, for example: (ns digest).

A single segment namespace, becomes, in the eyes of the JVM, package-less and ends up in the JVM default package. Single segment namespaces are problematic in general in Clojure and, because they are package-less, will not be initialized by graal-build-time.

graal-build-time will emit a warning when it detects .class files generated from a single segment namespace, for example:

[clj-easy/graal-build-time] WARN: Single segment namespace found for class: digest__init.class. Because this class has no package, it cannot be registered for initialization at build time. 


Run bb tasks for all relevant project tasks.

Tasks attempt to avoid unnecessary work by comparing source and target file dates. If you want to skip this optimization, run bb clean before running your task.

Use bb native-image-test to run our integration tests.

  • This task builds native images for a hello world app, and then runs them.

  • The hello world Clojure sources are compiled to Java classes.

  • We use GraalVM's native-image with graal-build-time on the classpath to create 2 variants of the same app:

    • one built from the uberjar
    • the other built directly from the classes dir
  • Note that we omit --initialize-at-build-time when creating the native images. The work that this deprecated option carried out is now taken care of by graal-build-time.

  • During native image creation, you'll see output that looks like:

    [clj-easy/graal-build-time] WARN: Single segment namespace found for class: single_segment_example__init.class. Because this class has no package, it cannot be registered for initialization at build time.
    [clj-easy/graal-build-time] Registering packages for build time initialization: clojure, clj_easy.graal_build_time, gbt_test_org, hello, hello_world


    • [clj-easy/graal-build-time] Registering packages for build time initialization... will always appear
    • [clj-easy/graal-build-time] WARN: Single segment namespace found... will appear to warn you of the repercussions of a compiled Clojure single segment namespace found in your build.


Licensed under the MIT license, see LICENSE.

Copyright © 2021-2023 Michiel Borkent, Eric Dallo, Rahul Dé, Lee Read and contributors.

Can you improve this documentation? These fine people already did:
Michiel Borkent, Lee Read, Paweł Stroiński & Eric Dallo
Edit on GitHub

cljdoc is a website building & hosting documentation for Clojure/Script libraries

× close