Using ClassGraph on GraalVM

This repo demonstrates how to build a native image of a java application that uses ClassGraph with GraalVM.

Build and test

Install GraalVM

$ mvn clean package

Execute the java image

$ java -jar target/classgraph-on-graalvm-1.0-SNAPSHOT.jar
Initializing class com.softinstigate.App
annotated classes [com.softinstigate.FindMe]
	annotated method executeMe
Executing targets [{ com.softinstigate.FindMe, executeMe }]
	class annotation name: findMe
	class annotation priority: 10
	method annotation name: executeMe
	method annotation priority: 10
Hello World!!

Execute the native image

$ ./target/cog
Executing targets [{ com.softinstigate.FindMe, executeMe }]
	class annotation name: findMe
	class annotation priority: 10
	method annotation name: executeMe
	method annotation priority: 10
Hello World!!

The issue

ClassGraph.scan() simply doesn't work at runtime on a native image. However it does work at build time (see Build-Time-Scanning on ClassGraph wiki).

Approach

The native-image.properties includes the following option:

--initialize-at-build-time=com.softinstigate.App,io.github.classgraph.,nonapi.io.github.classgraph.

com.softinstigate.App performs the class scanning at class initialization time defining:

public class App {
    static {
        // here executes ClassGraph.scan()
    }
}

Since the class com.softinstigate.App is initialized at build time by native-image, the scanning occurs.

The found classes are saved to a static field and used in the class. The example code looks for classes and methods with given annotations and them invoke the methods using reflection.

Since reflection is used, native-image must run properly configured. The following section provides instructions on how to automatically generate the configuration.

See Reflection Use in Native Images from GraalVM documentation.

Generate native-image configuration

Run the app with the native-image-agent

$ mvn clean package
$ java -agentlib:native-image-agent=config-merge-dir=src/main/resources/META-INF/native-image/com.softinstigate/classgraph-on-graalvm/ -jar target/classgraph-on-graalvm-1.0-SNAPSHOT.jar