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Spring AOT and native support
Support for compiling Spring applications to native executables via Ahead-Of-Time transformations and GraalVM native image compiler will be shipped as part of Spring Framework 6+ and is documented in this page.
In order to obtain an optimized image size and memory footprint, GraalVM native image compiler performs a static analysis of the code of applications from the provided entry points (main + reflection entries configured) in order to identify reachable code paths which are then compiled to native code.
By default GraalVM, like the JVM, initializes class static fields and blocks at runtime, but also provides the capability to initialize classes at build-time in order to allow more code removal during the compilation and reduce the amount of reflection required at runtime.
This mechanism has a significant drawback : build-time class initialization is a viral behavior that can leads to unsolvable incompatibilities. All fields and all classes transitively used at build-time are initialized at build-time as well, which can be an issue for various use cases including thread pool creation, field controller by a runtime system property or a logger that will transitively use a library like Jackson.
That’s why in Spring Framework 6+ and related Spring portfolio projects, the goal is to reduce as much as possible build-time class initialization which should be limited to classes that do not compile yet to native image (often due to GraalVM bugs like this one). If and only if there is no other way, it is possible to add a META-INF/native-image/<groupId>/<artifactIdId>/native-image.properties resource file to specify classes to initialize at build-time. Examples of such classes in Spring Framework can be found listed in spring-core or spring-web.
For the frequent use case of classpath checks via static boolean fields like in WebMvcConfigurationSupport where there is a possibility to perform build-time code removal, Spring Framework provides a GraalVM feature that initializes those boolean static fields at build-time without having to use build-time class initialization. The following patterns are used to detect those fields:
org.springframework.core.NativeDetector#imageCodeorg.springframework.*#*Presentorg.springframework.*#*PRESENTreactor.*#*Available
When building a project based on Spring Framework 6+, the fields initialized at build-time are logged during the native image build, for example:
1 user-provided feature(s)
- org.springframework.aot.graalvm.ConstantFieldFeature
Field org.springframework.core.NativeDetector#imageCode set to true at build time
Field org.springframework.core.KotlinDetector#kotlinReflectPresent set to false at build time
Field org.springframework.core.KotlinDetector#kotlinPresent set to false at build time
Field org.springframework.web.context.support.StandardServletEnvironment#jndiPresent set to true at build time
Field org.springframework.format.support.DefaultFormattingConversionService#jsr354Present set to false at build time
Field org.springframework.web.servlet.config.annotation.WebMvcConfigurationSupport#romePresent set to false at build time
Field org.springframework.web.servlet.config.annotation.WebMvcConfigurationSupport#jaxb2Present set to false at build time
Field org.springframework.web.servlet.config.annotation.WebMvcConfigurationSupport#jackson2Present set to true at build time
Field org.springframework.web.servlet.config.annotation.WebMvcConfigurationSupport#jackson2XmlPresent set to false at build time
Field org.springframework.web.servlet.config.annotation.WebMvcConfigurationSupport#jackson2SmilePresent set to false at build time
Field org.springframework.web.servlet.config.annotation.WebMvcConfigurationSupport#jackson2CborPresent set to false at build time
Field org.springframework.web.servlet.config.annotation.WebMvcConfigurationSupport#gsonPresent set to false at build time
Field org.springframework.web.servlet.config.annotation.WebMvcConfigurationSupport#jsonbPresent set to false at build time
Field org.springframework.web.servlet.config.annotation.WebMvcConfigurationSupport#kotlinSerializationJsonPresent set to false at build time
...
As a result, code like if (jackson2Present) { converters.add(new MappingJackson2HttpMessageConverter()); } will be evaluated as if (false) { converters.add(new MappingJackson2HttpMessageConverter()); } at build-time allowing to remove MappingJackson2HttpMessageConverter from the native image.
While limited to a subset of use cases, this feature should be preferred as it is safe to use and do not imply the same compatibility side-effects than class build-time initialization.