What is JNI

Java Native Interface (JNI) It allows certain methods of Java classes to be implemented natively while letting them be called and used as ordinary Java methods.

Declaration of Native Methods

The method declaration of stringFromJNI contains the native keyword to inform the Java compiler that the implementation of this method is provided in another language.

public native String stringFromJNI();

Loading the Shared Libraries

Native methods are compiled into a shared library. This shared library needs to be loaded first for the virtual machine to find the native method implementations. The java.lang.System class provides two static methods, load and loadLibrary, for loading shared libraries during runtime. The caveat here is that loadLibrary will load the shared library as soon as it finds a library with the same name while going through the Java library path.

static {
        System.loadLibrary("native-lib");
    }

Implementing the Native Methods

Java_com_pm_nativeapp01_MainActivity_stringFromJNI() This explicit function naming allows the virtual machine to automatically find native functions in loaded shared libraries

Method Declarations

JNIEXPORT jstring JNICALL Java_com_example_hellojni_HelloJni_stringFromJNI
(JNIEnv *, jobject);

JNIEnv, is an interface pointer that points to a function table of available JNI jobject, is a Java object reference to the native-lib.cpp class instance.

Depending on whether the native code is a C or C++ source file, the syntax for calling JNI functions differs.

• In C code, JNIEnv is a pointer to JNINativeInterface structure.

• Since the JNI functions in C code do not know the current JNI environment, the JNIEnv instance should be passed as the first argument to each JNI function call

  return (*env)->NewStringUTF(env, "Hello from JNI !");
  

• In C++ code, JNIEnv is actually a C++ class instance.

• Since JNI methods in C++ code have access to the current JNI environment, the JNI method calls do not require the JNIEnv instance as an argument.

   return env->NewStringUTF("Hello from JNI !");  
  

Instance vs. Static Methods

• Instance methods are associated with a class instance, and they can only be called on a class instance.

  JNIEXPORT jstring JNICALL Java_com_example_hellojni_HelloJni_stringFromJNI
  (JNIEnv * env, jobject thiz);
  

• Static methods are not associated with a class instance, and they can be called directly from a static context.

  JNIEXPORT jstring JNICALL Java_com_example_hellojni_HelloJni_stringFromJNI
  (JNIEnv * env, jclass clazz);