此處的代碼可能會有幫助。一旦你瞭解了這些宏的用途，以及如何區分java和可可關注點，這並不像想象的那麼困難。
你需要記住的是，JNF_COCOA_ENTER和JNF_COCOA_EXIT爲您做兩兩件事：
他們建立一個本地範圍內（所以裏面定義的變量不可外 - 這是幫你不要做「啞巴」有變量的東西你不應該觸及），並且設置一個自動釋放池，所以當範圍消失時，在該範圍內的「自動釋放」的可可對象將會消失。 （這是本地範圍爲什麼有用/有用的原因之一）他們還會執行一些異常分析，以便您可以捕獲Java中的Cocoa異常。
也就是說，下面的代碼是LEGAL，你只需要非常小心地管理內存訪問和數據所有權。如果可能，避免混合使用Java所有權和Objective-C所有權，否則讓對象管理所有權，並在java對象被GCed時清理。
jbyteArray bytes;
JNF_COCOA_ENTER(env);
// Assign and full the bytes array here, doing any requisite transformations.
// Remember to COPY any data out of COCOA objects, as references will be dead soon!
JNF_COCOA_EXIT(env);
return bytes;
從C使用Java對象很複雜，但不是不可行。儘管如此，方法調用的數量並不是微不足道的，而跳回和第四是耗時的，所以如果您經常調用此方法，或者時間緊迫，請儘可能使用基本類型。
如果您需要將代碼中的數據從Cocoa推送到Java，事情會稍微複雜一些，但不是不可行。以下是我管理的一個名爲QTCubed的項目的一部分，完全是這樣。 didOutputVideoFrame是委託方法，必須使用目標Java Object Reference和JNI調用的方法來初始化此對象。一旦初始化，它就會被設置爲委託對象，並從相機接收更新。
@implementation QTKitCaptureDecompressedVideoOutput
- (QTKitCaptureDecompressedVideoOutput *)initWithEnv:(JNIEnv *) env javaObject:(jobject) javaObjectRef {
[super init];
// Save a reference to the VM
(*env)->GetJavaVM(env,&g_vm);
// Create a global reference to this object so we can access it later
objectRef = (*env)->NewGlobalRef(env,javaObjectRef);
return self;
}
- (void)captureOutput:(QTCaptureOutput *)captureOutput didOutputVideoFrame:(CVImageBufferRef)videoFrame withSampleBuffer:(QTSampleBuffer *)sampleBuffer fromConnection:(QTCaptureConnection *)connection {
// Move into Java to deliver the data
JNIEnv *env;
(*g_vm)->AttachCurrentThread (g_vm, (void **) &env, NULL);
void * rawData = [sampleBuffer bytesForAllSamples];
int length = [sampleBuffer lengthForAllSamples];
QTFormatDescription * formatDescription = [sampleBuffer formatDescription];
QTTime duration = [sampleBuffer duration];
float frameDuration = duration.timeValue/duration.timeScale;
float fps = 1/frameDuration;
jint format = [formatDescription formatType];
NSValue * pixelSize = [formatDescription attributeForKey:QTFormatDescriptionVideoEncodedPixelsSizeAttribute];
NSSize size = [pixelSize sizeValue];
jint width = size.width;
jint height = size.height;
//NSLog(@"Outputting frame sized %d x %d of length %d with format: %#x",width,height,length,format);
switch (format) {
// 8 bit codecs
case kCVPixelFormatType_1Monochrome:
case kCVPixelFormatType_2Indexed:
case kCVPixelFormatType_4Indexed:
case kCVPixelFormatType_8Indexed:
case kCVPixelFormatType_1IndexedGray_WhiteIsZero:
case kCVPixelFormatType_2IndexedGray_WhiteIsZero:
case kCVPixelFormatType_4IndexedGray_WhiteIsZero:
case kCVPixelFormatType_8IndexedGray_WhiteIsZero:
case kCVPixelFormatType_422YpCbCr8:
case kCVPixelFormatType_4444YpCbCrA8:
case kCVPixelFormatType_4444YpCbCrA8R:
case kCVPixelFormatType_444YpCbCr8:
case kCVPixelFormatType_420YpCbCr8Planar:
case kCVPixelFormatType_422YpCbCr_4A_8BiPlanar:
case kCVPixelFormatType_24RGB:
case kCVPixelFormatType_24BGR:
default:
{
// Re-use the existing array if possible
if (byteFrameData == nil || (*env)->GetArrayLength(env,byteFrameData) < length) {
// Clean up the previously allocated global reference
if (byteFrameData != nil) {
(*env)->DeleteGlobalRef(env,byteFrameData);
byteFrameData = nil;
}
// Create an appropriately sized byte array to hold the data
byteFrameData = (*env)->NewGlobalRef(env,(*env)->NewByteArray(env,length));
}
if (byteFrameData) {
// Copy the raw data into the byteArray
(*env)->SetByteArrayRegion(env,byteFrameData,0,length,rawData);
// Get the class reference for our object
jclass classRef = (*env)->GetObjectClass(env,objectRef);
// Get the pushFrame methodId
jmethodID methodId = (*env)->GetMethodID(env,classRef,"pushFrame","([BIIIF)V");
// Call pushFrame with the byte array
(*env)->CallVoidMethod(env,objectRef,methodId,byteFrameData,format,width,height,fps);
}
break;
}
// 16 bit (short) storage of values
case kCVPixelFormatType_16BE555:
case kCVPixelFormatType_16LE555:
case kCVPixelFormatType_16LE5551:
case kCVPixelFormatType_16BE565:
case kCVPixelFormatType_16LE565:
case kCVPixelFormatType_16Gray:
case kCVPixelFormatType_422YpCbCr16:
case kCVPixelFormatType_422YpCbCr10:
case kCVPixelFormatType_444YpCbCr10:
{
// Re-use the existing array if possible
if (shortFrameData == nil || (*env)->GetArrayLength(env,shortFrameData) < length/2) {
// Clean up the previously allocated global reference
if (shortFrameData != nil) {
(*env)->DeleteGlobalRef(env,shortFrameData);
shortFrameData = nil;
}
// Create an appropriately sized byte array to hold the data
shortFrameData = (*env)->NewGlobalRef(env,(*env)->NewShortArray(env,length/2));
}
if (shortFrameData) {
// Copy the raw data into the byteArray
(*env)->SetShortArrayRegion(env,shortFrameData,0,length/2,rawData);
// Get the class reference for our object
jclass classRef = (*env)->GetObjectClass(env,objectRef);
// Get the pushFrame methodId
jmethodID methodId = (*env)->GetMethodID(env,classRef,"pushFrame","([SIIIF)V");
// Call pushFrame with the short array
(*env)->CallVoidMethod(env,objectRef,methodId,shortFrameData,format,width,height,fps);
}
break;
}
// 32 bit (int) storage of values
case kCVPixelFormatType_32ABGR:
case kCVPixelFormatType_32AlphaGray:
case kCVPixelFormatType_32ARGB:
case kCVPixelFormatType_32BGRA:
case kCVPixelFormatType_32RGBA:
{
// Re-use the existing array if possible
if (intFrameData == nil || (*env)->GetArrayLength(env,intFrameData) < length/4) {
// Clean up the previously allocated global reference
if (intFrameData != nil) {
(*env)->DeleteGlobalRef(env,intFrameData);
intFrameData = nil;
}
// Create an appropriately sized byte array to hold the data
intFrameData = (*env)->NewGlobalRef(env,(*env)->NewIntArray(env,length/4));
}
if (intFrameData) {
// Copy the raw data into the byteArray
(*env)->SetByteArrayRegion(env,intFrameData,0,length/4,rawData);
// Get the class reference for our object
jclass classRef = (*env)->GetObjectClass(env,objectRef);
// Get the pushFrame methodId
jmethodID methodId = (*env)->GetMethodID(env,classRef,"pushFrame","([IIIIF)V");
// Call pushFrame with the int array
(*env)->CallVoidMethod(env,objectRef,methodId,intFrameData,format,width,height,fps);
}
break;
}
}
// Detatch from Java
(*g_vm)->DetachCurrentThread (g_vm);
}
/* Clean up java references so they can be properly GCed in java */
- (void) dealloc {
// Attach to java so we can release references
JNIEnv *env;
(*g_vm)->AttachCurrentThread (g_vm, (void **) &env, NULL);
// release the references we hold
if (objectRef != nil) {
(*env)->DeleteGlobalRef(env,objectRef);
objectRef = nil;
}
if (byteFrameData != nil) {
(*env)->DeleteGlobalRef(env,byteFrameData);
byteFrameData = nil;
}
if (shortFrameData != nil) {
(*env)->DeleteGlobalRef(env,shortFrameData);
shortFrameData = nil;
}
if (intFrameData != nil) {
(*env)->DeleteGlobalRef(env,intFrameData);
intFrameData = nil;
}
// Detatch from Java
(*g_vm)->DetachCurrentThread (g_vm);
g_vm = nil;
[super dealloc];
}
@end
這裏有傳遞進來的java對象引用Java端的方法簽名：
protected void pushFrame(byte[] frameData, int formatInt, int width, int height, float frameRate);
protected void pushFrame(short[] frameData, int formatInt, int width, int height, float frameRate);
protected void pushFrame(int[] frameData, int formatInt, int width, int height, float frameRate);
一旦你有一個正在運行的應用程序，使用的儀器，以確保你參考處置PROPERLY！