This example demonstrates the ability of native-image to run parts of your application at the image build time.

In both examples we use the Jackson framework to parse a JSON file to determine which Handler should be used by the application (CurrentTimeHandler or HelloWorldHandler) at runtime.

• In configure-at-runtime-example the JSON parsing happens at image run time and thus contributes to the image startup time. In addition, all methods and static fields originated from the Jackson framework that are needed become part of a native executable.

• In contrast, configure-at-buildtime-example performs the JSON parsing as part of building the native executable. In this case the executable does not contain any parts of the Jackson framework (can be verified easily by adding -H:+PrintAnalysisCallTree to the <buildArgs> in pom.xml). When this image gets executed, it can run the handler right away since it was already determined at build time which hander should be used.

1. Download and install the latest GraalVM JDK with Native Image using the GraalVM JDK Downloader:

   bash <(curl -sL https://get.graalvm.org/jdk) 

2. Download or clone the repository and navigate into the native-image-configure-examples directory:

   git clone https://github.com/graalvm/graalvm-demos

   cd graalvm-demos/native-image-configure-examples

1. Change to one of the demo subdirectories, for example, configure-at-runtime-example:

   cd graalvm-demos/native-image-configure-examples/configure-at-runtime-example

2. Build the project:

   mvn package

3. Once the Maven build succeeds, a native executable called "example" will be generated in the configure-at-runtime-example/target/ directory. Run it:

   ./target/example
   Tue Mar 23 22:17:33 EET 2021

4. Repeat the same steps for the other sub-demo:

   cd ..
   cd configure-at-buildtime-example

   mvn package

   ./target/example

   You will see the next output:

   Hello, world!
   

5. Finally, you may shutdown the native-image server:

   $JAVA_HOME/bin/native-image --server-shutdown

Loading application configuration at executable build time can speed up application startup. See Build-Time Initialization to learn more.

To read more about the topic of class initialization, see Initialize Once, Start Fast: Application Initialization at Build Time.