<== Chapter 5 -- Chapter 7 ==>

Chapter 6 - JNI C/C++ Interface

We now need to create an interface layer that will take our native function() Java code and forward it to our native C/C++ implementation of the function. JNI Interface Code

• The first thing we want to do is add #include <jni.h> to get access to the JNI class
• To ensure that the names declared in that portion of code have C linkage, and thus C++ name mangling is not performed we add these macro to surround the class

#ifdef __cplusplus
extern "C" {
#endif

.... // our code

#ifdef __cplusplus
}
#endif

• We are going to make a class next chapter so now is a good time to just declare the new C++ class at the top

#include "Tango_NDK_Tutorial.h"

static Tango_NDK_Tutorial app;

• Here we add a function call for each of the JNI native calls such as

JNIEXPORT void JNICALL
Java_com_demo_tutorial_tango_tango_1ndk_1tutorial_TangoJniNative_onCreate(
        JNIEnv* env, jobject /*obj*/, jobject caller_activity) {
    app.OnCreate(env, caller_activity);
}

• The format of these function calls that bring from Java to the native level follows a strict naming convention
• Always start your function with the return type of JNIEXPORT < Return_Type > JNICALL
  • the < Return_Type > can be void, jint, jstring, etc
    • JNI Data Types
• The name of function is made up of 3 different parts
  1. Package_Name
  2. Java_Class_Name
  3. Function_Name
• We concat them with underscores into the function name
  • Example for the onCreate call: Java_com_demo_tutorial_tango_tango_1ndk_1tutorial_MainActivity_onCreate
  • Package_Name => com.demo.tutorial.tango.tango_ndk_tutorial
    • NOTE: Since the package name has underscores in it you need to replace it with _1 as seen in this example
  • Java_Class_Name => MainActivity
  • Function_Name => onCreate
• Every
  • JNIEnv* - Pointer to a structure storing all JNI function pointers. Provides most of the JNI functions. Your native functions all receive a JNIEnv as the first argument.
    • The JNIEnv is used for thread-local storage. For this reason, you cannot share a JNIEnv between threads.
  • jobject - This is a reference to an object of type MainActivity, almost like a this of the Java Activity class instance
• We also call the app class we declared as a type of our future C++ class and run its internal OnCreate method we will make soon.
  • Notice we try to help distinguish the Java and C++ functions by making the Java calls lowerCamelCase and the C++ UpperCamelCase as our naming convention
    • Java -> onCreate
    • C++ -> OnCreate
• For the Java_com_demo_tutorial_tango_tango_1ndk_1tutorial_TangoJniNative_valueFromJNI function we add a third parameter which is the Java int argument we sent the value 5. Here we take that value, add 1, and return it as a jint type

<== Chapter 5 -- Chapter 7 ==>