3D Shooters for Doom - Pro Android Games - page 160

For improved performance, static references to the Java native interface class
( doom.jni.Natives ) are kept in jNativesCls . References to the Java methods to send
the video image ( jSendImageMethod ) and sound file ( jStartSoundMethod ) are also kept.
The reason is that these methods are invoked multiple times, and looking up these
names every time can slow things.

static jclass jNativesCls;
static jmethodID jSendImageMethod;
static jmethodID jStartSoundMethod;

Also, because you may send a pixel buffer (image) multiple times per second, you should
keep a reference to the Java array and its size, as in the following fragment:

static jintArray jImage;
static int iSize;
extern int doom_main(int argc, char **argv);

The line extern int doom_main defines the main engine function and tells the compiler it is
defined somewhere else in the library. The header jni_doom.h included up front defines the
constants and method signatures required to invoke the C to Java callbacks. For example,
the following fragment of the header defines constants for the Java native interface class
( doom/jni/Natives ) and the method names and signatures for the callbacks OnImageUpdate
and OnStartSound (see the “C to Java Callbacks” section for more details):

#define CB_CLASS "doom/jni/Natives"
#define CB_CLASS_IU_CB "OnImageUpdate"
#define CB_CLASS_IU_SIG "([I)V"

#define CB_CLASS_SS_CB "OnStartSound"
#define CB_CLASS_SS_SIG "([BI)V"

Let's take a look at the actual implementations. They are divided into the following three
groups:
 Native game loop : This invokes the engine loop doom_main .
 Key and motion events : These post key and motion events to
the engine.

 C to Java callbacks : These callbacks are critical for the Java code to
receive information from the Doom engine.

Native Game Loop
The native game loop's job is to extract the arguments sent as a jobjectArray into a C char
** array and invoke the main Doom engine function ( doom_main ). This function performs the
following additional steps:

Obtains a reference to the JVM using (*env)->GetJavaVM(env,
&g_VM) . This reference will be used by the C to Java callbacks.

1.