Try
javah -jni com.example.JavaHowTo
where com.example
is your package.
You also need to run javah from the directory containing com/example/JavaHowTo.class
e.g. if your structure is
/home/user/com/example/JavaHowTo.class
run javah from
/home/user
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…