accessing java targetCompatibility

build.gradle

import org.gradle.api.JavaVersion;
import org.gradle.api.Plugin;
import org.gradle.api.Project;
import org.gradle.api.plugins.JavaPlugin;
import org.gradle.api.plugins.JavaPluginConvention;
import org.gradle.api.provider.Property;

import java.util.concurrent.Callable;

public class AcmePlugin implements Plugin<Project> {
    private Property<JavaVersion> targetCompatibility;

    @Override
    public void apply(Project project) {

        // this expects the java-base plugin is already applied. alternatively you can do
        // project.getPlugins().apply(JavaBasePlugin.class) explicitly
        targetCompatibility = project.getObjects().property(JavaVersion.class);

        project.getPluginManager().apply(JavaPlugin.class);
        JavaPluginConvention convention = (JavaPluginConvention)project.getConvention().getPlugins().get("java");
        // you could access it directly via:

        JavaVersion initialTargetCompatibiltiy = convention.getTargetCompatibility();
        // but that only provides you with the value have access to targetCompatibility at the time your plugin was applied
        // To take user changes to that value into account made after your plugin was applied you have to use
        // the project.provider / property api

        this.targetCompatibility.set(project.provider(new Callable<JavaVersion>() {
            @Override
            public JavaVersion call() throws Exception {
                return convention.getTargetCompatibility();
            }
        }));

        // now targetCompatibility is a handle to the value. you should pass it down to your task and
        // access the javaversion value during execution time.

    }
}