The Android Native Development Kit (NDK) - Beginning Android 3D Game Development - page 395

The Java Interface Pointer
Native code in C/C++ accesses the Java Virtual Machine through JNI functions that are accessed
through the Java Interface Pointer. The JNI Interface Pointer is a pointer to an array of pointers that
point to JNI functions. (See Figure 11-3 .)

JNI Interface Pointer
(JNIEnv* env)

Array of pointers
to JNF Functions

An Interface Function

NewStringUTF()

Pointer
Pointer

Pointer

GetObjectClass()

Pointer
...

GetMethodID()

Figure 11-3. The Java Interface Pointer

Loading and Linking Native C/C++ Methods
In order to use native classes in your Android Java code (see Listing 11-1), you will have to

Load the compiled library by using the System.loadLibrary() function. 1.

2. Declare the native class that is defined in the C/C++ source code as native in

the Android Java code by using the native keyword in a function declaration.

Listing 11-1. Loading and Linking Native C/C++ Methods from Android Java Code

package robs.gldemo.robsgl20tutorial;
class GLES20TriangleRenderer implements GLSurfaceView.Renderer
{
/* this is used to load the 'hello-jni' library on application
* startup. The library has already been unpacked into
* /data/data/com.example.hellojni/lib/libhello-jni.so at
* installation time by the package manager.
*/
static {
System.loadLibrary("hello-jni");
}
public native String RobsstringFromJNI();
}