Example of building Xamarin.Android java sources with gradle

build.gradle

// I put this in obj\Debug\android\build.gradle
// Ran `gradlew assemble` -> 'build\libs\classes.jar`
// Unfortunately this took 2s with gradle, compared to 569ms with just javac
plugins {
    id 'java'
}

sourceSets {
    main {
        java {
            srcDirs = ['src']
        }
    }
}

dependencies {
    implementation files(
		'C:/src/xamarin-android/bin/Debug/lib/xamarin.android/xbuild-frameworks/MonoAndroid/v9.0/mono.android.jar',
		'C:/src/xamarin-android/bin/Debug/lib/xamarin.android/xbuild/Xamarin/Android/java_runtime.jar',
	)
	compileOnly files(
		'C:/Users/jopepper/android-toolchain/sdk/platforms/android-28/android.jar',
	)
}

jar {
    archiveName 'classes.jar'
}