Micronaut AOT

Build time optimizations for Micronaut


1 Introduction

Micronaut AOT is a framework which implements ahead-of-time (AOT) optimizations for Micronaut application and libraries. Those optimizations consist of computing at build time things which would normally be done at runtime. This includes, but is not limited to:

  • pre-parsing configuration files (yaml, properties, …​)

  • pre-computing bean requirements in order to reduce startup time

  • performing substitution of classes with "optimized" versions for a particular environment

Micronaut AOT can generate specific optimizations for different environments, in particular, it can make a difference between optimizations needed in JIT mode (traditional JVM applications) and native mode (application compiled with GraalVM native-image).

Unlike other Micronaut modules, Micronaut AOT is not a dependency which needs to be added to your application: it’s a framework which requires integration with your build process: it is typically aimed at being integrated into build tool plugins. Currently, integration with Micronaut AOT is only implemented by the Gradle plugin.

2 Release History

For this project, you can find a list of releases (with release notes) here:

3 Quick Start


Micronaut AOT is an experimental project. Use at your own risk.

The goal of Micronaut AOT is to create an optimized "binary" for a particular deployment environment. It is not to make development experience faster: because build time optimizations require deep analysis of the application, it will actually make the local development slower if you use optimized binaries.

You should consider Micronaut AOT similarly to what GraalVM’s native-image tool is: a "compiler" which generates a different application, aimed at a particular runtime. That is, depending on the optimizations which are enabled, a binary optimized by AOT may, or may not, work in a specific deployment environment.

Micronaut AOT projects

The Micronaut AOT project consists of 4 main modules:

  • micronaut-aot-core provides the APIs for implementing "AOT optimizers", or code generators.

  • micronaut-aot-api exposes the public API for interacting with the AOT compiler. It mostly consists of the MicronautAotOptimizer class which is responsible for loading the different AOT modules via service loading, then driving the AOT process.

  • micronaut-aot-std-optimizers implements a number of standard Micronaut AOT optimizers.

  • micronaut-cli is the command line tool responsible for calling the AOT compiler. It is recommended to integrate with Micronaut AOT via the CLI tool, so that the process is properly isolated.

How it works

Optimization process

Micronaut AOT is a post-processing tool. By that, we mean that it takes the output of regular Micronaut application compilation, then performs an analysis of this and generates new classes, resources, etc. which are then used to create a new binary (jar, native binary, …​).

In a nutshell, the inputs of Micronaut AOT are:

  • the application runtime classpath

  • the Micronaut AOT runtime (including the AOT optimizers)

  • the AOT optimizer configuration (including the target runtime, e.g JIT vs native)

And its outputs are:

  • generated of source files and their compiled (.class) versions

  • generated resource files

  • a list of resources which should be removed from the final binary (for example, if a YAML file is replaced with Java configuration, a class would be generated, but then we know we don’t need the YAML file anymore in the final binary)

  • log files (to diagnose what happened during the AOT process)

The MicronautAotOptimizer class is a special case of code generator which integrates the dynamically loaded AOT optimizers, and generates an ApplicationContextConfigurer which will initialize the optimizations.

It’s then the responsibility of integrators to take those outputs to generate different binaries.

User-code loading

In order to perform optimizations, so called optimizers (or AOT modules) are used. Those modules need access to the application context, so that they can, for example, determine if a bean is going to be needed in a particular deployment environment. Or, they may need access to the configuration which is dynamically loaded from an external source (think of distributed configuration) to generate static configuration instead.

Therefore, the AOT compiler needs to be executed in the same classloader as the application code itself. This is why there are 2 different classpaths for the AOT compiler:

  • the application classpath corresponds to the application runtime classpath. It is, technically speaking, the result of previous compilation plus all transitive dependencies needed by the application (or library).

  • the AOT classpath, which is the classpath of the AOT compiler and the AOT optimizers.

The role of the application context configurers

Given the application classpath, the AOT compiler will instantiate an ApplicationContext, which will have all the context configurers (classes implementing the ApplicationContextConfigurer interface and annotated with @ContextConfigurer) applied automatically. This is how the AOT compiler will "know" about particular application context customizations which can be done by a user.

Therefore, it is critical to understand that the AOT compiler cannot figure out arbitrary application context customizations. For example, in the following code:

class Application {
    public static void main(String...args) {

there’s a deduceEnvironment() call which is opaque to the AOT compiler: it cannot know that the application is configured that way (for this it would actually have to start the application and perform runtime interception which would be too expensive or impossible).

Therefore, all customizations need to be done using a different pattern:

class Application {
    public static class MyConfigurer implements ApplicationContextConfigurer {
        public void configure(ApplicationContextBuilder context) {

    public static void main(String... args) {
        Micronaut.run(Application.class, args);

Because @ContextConfigurer makes sure that any application context created will see the customizer applied, the application context created by the AOT compiler for its internal use will see the customizations.

Implementing an AOT optimizer

Current capabilities

Now that we understand how the AOT optimization environment is bootstrapped, we can start implementing an AOT optimizer.

An optimizer can do one or more of the following:

  • generate static initializers which will automatically be loaded thanks to the ApplicationContextConfigurer mechanism

  • generate new source files

  • generate new resource files

  • perform substitutions of one class with another

  • filter out resources

New capabilities will be included as part of AOT development.

Code generators

At the core of AOT optimizations is a code generator. A code generator needs to implement the AOTCodeGenerator interface and be annotated with @AOTModule.

The AOTModule annotation is responsible for giving metadata about the code generators, including:

  • an id is used to identify the code generator, and enable/disable it via configuration

  • a number of options (@Option) which are used to describe the parameters that the code generator takes (those are provided via configuration)

  • possibly dependencies to other code generators (for example, some code generators may only work properly if they execute after another one)

  • the target runtimes it applies to

Code generators contribute code via the AOTContext interface, which allows:

  • getting the name of the package of generated classes

  • registering generated code (source files, …​)

  • getting access to the ApplicationContext

  • sharing state

  • getting access to target runtime

For example, a simple code generator which generates a resource file may be declared as:

    id = MyResourceGenerator.ID,
    options = {
        @Option(name = "greeter.message", sampleValue = "Hello, world!", description = "The message to write")
public class MyResourceGenerator implements AOTCodeGenerator {
    public static final String ID = "my.resource.generator";

    public void generate(AOTContext context) {
        context.registerResource("/hello.txt", file -> {
            try (PrintWriter writer = new PrintWriter(file)) {
                String message = context.getConfiguration()

Then in a configuration file, the code generator would be configured this way:

greeter.message=Hello, world!
Different code generators may share the same option values: it is legal, but often simply required (for example if there’s a different implementation of a specific optimization based on the target runtime).

4 Repository

You can find the source code of this project in this repository: