I never tried JNI(Java Native Interface) before. jni is an api for to call native C code from inside a java code. for a principle I dont like this kind of mixed things. calling java inside c# or calling C# code from C++ is always makes hard codes to understand. anyway I was wondering about the performance of printf between java and jni.

basically there are 2 codes. one is written in java one is written in C. here is our C code

/* ctest.c */
#include <jni.h>
#include <stdio.h>

JNIEXPORT void JNICALL Java_HelloWorld_helloFromC
  (JNIEnv * env, jobject jobj)
{
    printf("Hello from C!\n");
}

and here is our java code

public class HelloWorld {
   native void helloFromC(); /* (1) */
   static {
       System.loadLibrary("ctest"); /* (2) */
   }
   static public void main(String argv[]) {
       HelloWorld helloWorld = new HelloWorld();
       helloWorld.helloFromC(); /* (3) */
   }
}

you can compile C code like this

gcc -o libctest.so -shared -I/usr/lib/jvm/java-8-oracle/include/ ctest.c -lc -fPIC

and you can compile java code like this

javac HelloWorld.java

after you compile both. you can run the code like this

java -Djava.library.path=. HelloWorld

what it does actually you call Java_HelloWorld_helloFromC function of C from java. and it prints hello world string on console. in my ubuntu it took a while to compile that C code. I needed to install a lot of libraries like this

apt-get install libc6-dev-i386 g++-multilib
apt-get install libc6-dev-i386
apt-get install --reinstall libc6-dev

because gcc was givin this error

mascix@mascix-HP-Pavilion-dv7-Notebook-PC:~/tmp/jniexample$ g++ -o libctest.so -shared -I/usr/lib/jvm/java-8-oracle/include/ ctest.c -lc
In file included from /usr/include/stdio.h:27:0,
                 from /usr/lib/jvm/java-8-oracle/include/jni.h:39,
                 from ctest.c:1:
/usr/include/features.h:374:25: fatal error: sys/cdefs.h: No such file or directory
 #  include <sys/cdefs.h>
                         ^

after it compiled I run the java and jni and see the output but my intentions was only to investigate to see how fast is jni is compared to C. so what I did is actually run commands in linux like this and see the output

mascix@mascix-HP-Pavilion-dv7-Notebook-PC:~/tmp/jniexample$ time java -Djava.library.path=. HelloWorld 
Hello from C!

real	0m0.130s
user	0m0.122s
sys	0m0.021s

mascix@mascix-HP-Pavilion-dv7-Notebook-PC:~/tmp/jniexample$ time ./a.out 
Hello World
real	0m0.002s
user	0m0.001s
sys	0m0.002s

compared to JNI, C is very fast. I was not expecting this difference.65 times faster in C.