Hybrid 3D Graphics with OpenGL and JNI - Pro Android Games - page 150

jmethod mSendStr = (*env)->GetStaticMethodID(env, jNativesCls, "OnMessage"
, "(Ljava/lang/String;)V");
(*env)->CallStaticVoidMethod(env, jNativesCls, mSendStr
, (*env)->NewStringUTF(env, text) );

Note that to call this method, you need to obtain its JNI method ID using its name ( OnMessage ) and
its signature (Ljava/lang/String;)V . The signature describes the method's arguments (a string in this
case) and the return type ( void ). With this information, you call the static void method sending the
corresponding arguments.

■

Note C strings must be converted into Java strings before invoking Java methods, using (*env)->
NewStringUTF(env, MyCString) .

Native Interface Call
The native interface function (see Listing 5-14) is the C implementation of the Java native method
opengl.jni.Natives.NativeRender() . This function performs the following tasks:

• It saves a reference to the Java VM, required by the Java callbacks of the previous

section.

• It initializes the scene.

• It renders one frame. This function is meant to be called multiple times within the

rendering thread (implemented by GLThread.java ).

Listing 5-14. Native Interface Function from cuberenderer.c

/*
* Class: opengl_jni_Natives
* Method: RenderTest
* Signature: ()V
*/
JNIEXPORT jint JNICALL Java_opengl_jni_Natives_NativeRender
(JNIEnv * env, jclass cls)
{

(*env)->GetJavaVM(env, &g_VM);
static int initialized = 0;

if ( ! initialized ) {
jni_printf("Native:RenderTest initscene");
init_scene();

initialized = 1;

}