JNI example to call java method (example : System.out.println)

hello.cpp

#include "jni.h"

void println_java(JNIEnv *env, const char *msg);

extern "C" jint JNI_OnLoad(JavaVM *jvm, void *) {
  JNIEnv *env = nullptr;
  jvm->GetEnv((void **)&env, JNI_VERSION_1_6);
  println_java(env, "Hello from JNI!");
  return JNI_VERSION_1_6;
}

void println_java(JNIEnv *env, const char *msg) {
  // Get system class
  jclass syscls = env->FindClass("java/lang/System");
  // Lookup the "out" field
  jfieldID fid = env->GetStaticFieldID(syscls, "out", "Ljava/io/PrintStream;");
  jobject out = env->GetStaticObjectField(syscls, fid);
  // Get PrintStream class
  jclass pscls = env->FindClass("java/io/PrintStream");
  // Lookup printLn(String)
  jmethodID mid = env->GetMethodID(pscls, "println", "(Ljava/lang/String;)V");
  // Invoke the method
  jstring str = env->NewStringUTF(msg);
  env->CallVoidMethod(out, mid, str);
}

extern "C" JNIEXPORT jstring JNICALL Java_Test_stringFromJNI(JNIEnv *env,
                                                             jobject thiz) {
  return env->NewStringUTF("Hello String from JNI");
}

run.sh

export CPLUS_INCLUDE_PATH="$JAVA_HOME/include:$JAVA_HOME/include/linux"
clang  hello.cpp  -shared -o libhello.so
javac Test.java && java -Djava.library.path=.  Test

Test.java

class Test {
    static {
        System.loadLibrary("hello");
    }

    public static void main(String[] args) {
        System.out.println("Test OK");
        System.out.println(stringFromJNI());
    }

    public static String message() {
        return "Hello from Java";
    }

    public native static String stringFromJNI();

}

Output :
❯ javac Test.java && java -Djava.library.path=. Test
Hello from JNI!
Test OK
Hello String from JNI