A hello-world program using Java Native Interface (JNI). There’s nothing particularly difficult; the option to generate the header and the command to compile the share library might not be obvious though.

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class JNIHello {
    static native void hello();

    static { System.loadLibrary("JNIHello"); }

    public static void main(String[] args) {
        hello();
    }
}
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#include <stdio.h>
#include "JNIHello.h" // generated

JNIEXPORT void JNICALL Java_JNIHello_hello(JNIEnv *, jclass) {
    puts("Hello world");
}
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default : run-jni-hello

# using oneshell so that we can define my_jdk_home variable across lines
.ONESHELL:
SHELL = /usr/bin/zsh
.SHELLFLAGS = -i -ec

.PHONY: run-jni-hello
run-jni-hello
	my_jdk_home=...
	# echo $${my_jdk_home}
	javac -h . JNIHello.java
	clang++ -shared -fPIC -o libJNIHello.so \
		JNIHello.cpp \
		-I$${my_jdk_home}/include/ \
		-I$${my_jdk_home}/include/linux
	$${my_jdk_home}/bin/java -Djava.library.path=. JNIHello.java