• собираем классы

cd $PROJECT_ROOT
[rm ./build]
[mkdir ./build]
[mkdir ./build/classes]
javac -d ./build/classes ./src/main/java/ru/chsergeyg/jni/example/n/Calc.java \
                         ./src/main/java/ru/chsergeyg/jni/example/Main.java

• подготавливаем файл хедеров

cd $PROJECT_ROOT/build/classes
javah -d $PROJECT_ROOT/src/main/cpp ru.chsergeyg.jni.example.n.Calc
mv $PROJECT_ROOT/src/main/cpp/*.h $PROJECT_ROOT/src/main/cpp/Calc.h

• реализуем методы в .h

cp $PROJECT_ROOT/src/main/cpp/Calc.h $PROJECT_ROOT/src/main/cpp/Calc.cpp
vim $PROJECT_ROOT/src/main/cpp/Calc.cpp

• собираем в бинарник

g++ -I"$JAVA_HOME/include" -I"$JAVA_HOME/include/linux" -fPIC Calc.cpp -shared -o $PROJECT_ROOT/jniTest/bin/calc.so

• не забыть линкануть calc.so в Main.

 System.load("$PROJECT_ROOT/bin/calc.so");

• запуск

java -cp ./build/classes/ ru.chsergeyg.jni.example.Main