Jni 常用工具函数

Jni编程会用到的工具函数，做积累

1. 打开文件路径，返回文件描述符给Java

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//打开路径文件，并抬FileDescriptor jobject  openFile(JNIEnv *env, jstring path, jint flags){     int fd;     jobject mFileDescriptor;       /* Opening device */     {         jboolean iscopy;         const char *path_utf = env->GetStringUTFChars(path, &iscopy);         fd = open(path_utf, O_RDWR | flags);         env->ReleaseStringUTFChars(path, path_utf);         if (fd == -1)         {             /* Throw an exception */             LOGE("Cannot open port");             return NULL;         }     }       /* Create a corresponding file descriptor */     {         jclass cFileDescriptor = env->FindClass("java/io/FileDescriptor");         jmethodID iFileDescriptor = env->GetMethodID(cFileDescriptor, "<init>", "()V");         jfieldID descriptorID = env->GetFieldID(cFileDescriptor, "descriptor", "I");         mFileDescriptor = env->NewObject(cFileDescriptor, iFileDescriptor);         env->SetIntField(mFileDescriptor, descriptorID, (jint)fd);     }       return mFileDescriptor; }

2. Char转换成String

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//char * to string jstring charToString(JNIEnv *env, const char *chr){     LOGI("charToString: %s\n", chr);     jclass strClass = env->FindClass("java/lang/String");     jmethodID strConstruct = env->GetMethodID(strClass, "<init>", "([BLjava/lang/String;)V");     jbyteArray bytes = env->NewByteArray(strlen(chr));     env->SetByteArrayRegion(bytes, 0, strlen(chr), (jbyte*)chr);     jstring encoding = env->NewStringUTF("utf-8");       return (jstring)env->NewObject(strClass, strConstruct, bytes, encoding); }

3. String转换为Char

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//jstring to char* char* jstringToChar(JNIEnv* env, jstring jstr) {     char* rtn = NULL;     jclass clsstring = env->FindClass("java/lang/String");     jstring strencode = env->NewStringUTF("utf-8");     jmethodID mid = env->GetMethodID(clsstring, "getBytes", "(Ljava/lang/String;)[B");     jbyteArray barr= (jbyteArray)env->CallObjectMethod(jstr, mid, strencode);     jsize alen = env->GetArrayLength(barr);     jbyte* ba = env->GetByteArrayElements(barr, JNI_FALSE);       if (alen > 0)     {         rtn = (char*)malloc(alen + 1);         memcpy(rtn, ba, alen);         rtn[alen] = 0;     }       env->ReleaseByteArrayElements(barr, ba, 0);     return rtn; }

4. 上层传一个ByteBuffer给Jni用

Java：

private ByteBuffer mImageBuffer;
mImageBuffer = ByteBuffer.allocateDirect(mImageBufferSize);
ret = setDirectBuffer(mImageBuffer, mImageBufferSize);

Jni：

jint Java_com_camera_simplewebcam_CameraPreview_setDirectBuffer(JNIEnv *env, jobject thiz, 
    jobject jbuf, jint jlen) {
 
    char *pImageBuf = NULL;
    //获取Buffer首地址
    pImageBuf = (char *)(*env)->GetDirectBufferAddress(env, jbuf);
    if (NULL == pImageBuf) {
        LOGE("Failed to get direct buffer");
        return -1;
    }
    return 0;
}

5. 色彩Buffer转换成Bitmap类。

在UVC中把一帧的buffer转换成上层需要显示的Bitmap。Android.mk里要加入库LOCAL_LDLIBS    := -ljnigraphics

void 
Java_com_camera_simplewebcam_CameraPreview_pixeltobmp( JNIEnv* env,jobject thiz,jobject bitmap){
 
	jboolean bo;
	
	AndroidBitmapInfo  info;
	void*              pixels;
	int                ret;
	int i;
	int *colors;
 
	int width=0;
	int height=0;
	
	//提取bmp信息
	if ((ret = AndroidBitmap_getInfo(env, bitmap, &info)) < 0) {
		LOGE("AndroidBitmap_getInfo() failed ! error=%d", ret);
		return;
	}
    
	width = info.width;
	height = info.height;
 
	if(!rgb || !ybuf) return;
 
	if (info.format != ANDROID_BITMAP_FORMAT_RGBA_8888) {
		LOGE("Bitmap format is not RGBA_8888 !");
		return;
	}
	
	//bitmap提取buf
	if ((ret = AndroidBitmap_lockPixels(env, bitmap, &pixels)) < 0) {
		LOGE("AndroidBitmap_lockPixels() failed ! error=%d", ret);
	}
 
	colors = (int*)pixels;
	int *lrgb =NULL;
	lrgb = &rgb[0];
	
	//把目录buf拷贝到bitmap的buffer里
	for(i=0 ; i<width*height ; i++){
		*colors++ = *lrgb++;
	}
 
	AndroidBitmap_unlockPixels(env, bitmap);
}