Header Files

Observe

Compare the java files and generated header file.

• Java

        public class Header{
            public static native void a();
            public native void b();
            public static native int c();
            public native void d(int para);
            public native void e(String para);
        }
  

• Header file

    JNIEXPORT void JNICALL Java_Header_a
      (JNIEnv *, jclass);
  
    JNIEXPORT void JNICALL Java_Header_b
      (JNIEnv *, jobject);
  
    JNIEXPORT jint JNICALL Java_Header_c
      (JNIEnv *, jclass);
  
    JNIEXPORT void JNICALL Java_Header_d
      (JNIEnv *, jobject, jint);
  
    JNIEXPORT void JNICALL Java_Header_e
     (JNIEnv *, jobject, jstring);
  

• We find all kind of fuctions in Java is mapped to the JNIEnv * argument in header file.

• The second argument depends on whether the function is static or member function.
• Primary type is mapped to jxxx type.
• Reference type is mapped to opaque references, C pointer types that refer to internal data structures in the JVM.

Conclusion

• JNIEnv *:

  a pointer that points to another pointer pointing to a function table (array of pointer). Each entry in this function table points to a JNI function. These are the functions we are going to use for type conversion

• jclass -> static function : refering to the class in which the method is define.
• jobject -> member function : refering to the object on which the method is invoked.