Android NDK get object from Java

Question

I want to know how to get a custom object from Java to c++?

I need to implement a method in c++ for get performance. I already have the method working in java but I want to port to c++.

On Java a I call the method like this:

private native boolean P(Mat Previous, String Name);

On CPP file I need to get the mat object. Getting string is easy! But how can I get the custom mat object similar to c++(cv::Mat)? I need to get java Mat into a cv::Mat. Here the cpp file:

JNIEXPORT bool JNICALL Java_br_raphael_detector_SimpsonDetector_P
                      (JNIEnv* env,jobject thiz, jobject Previous, jstring Name){
jboolean sim = false;

const char* N = env->GetStringUTFChars(Name,0);
std::string Nome = N;


//Release
env->ReleaseStringUTFChars(Name,N);

//Then Return
return sim;

}

Answer 1

A java Mat object is a totally different thing from a native cv::Mat , you can't get one directly from the other.

That said, if you know what fields are inside Mat , and you know the corresponding fields in cv::Mat , you can write a conversion function that copies the contents of the fields one-by-one.

// First get the Mat class
jclass Mat = (*env)->GetObjectClass(env, Previous);

// To get a field
jfieldId field = (*env)->GetFieldID(env, Mat, "fieldName", field type);

// To get a method
jmethodId method = (*env)->GetMethodID(env, Mat, "methodName", method signature);

from there you can read the values of fields, or call methods

// getting a field
(*env)->GetObjectField(env, Previous, field);

// calling a method
(*env)->CallObjectMethod(env, Previous, method, parameters);

refer to http://docs.oracle.com/javase/1.5.0/docs/guide/jni/spec/functions.html for details

Answer 2

I'm one year late to the party, but the way you pass a Mat to C is with a jlong with the address of the Java's Mat. You can do something like this:

private static native boolean P(long matAddress, String name);

In C:

JNIEXPORT jboolean JNICALL Java_br_raphael_detector_SimpsonDetector_P
                  (JNIEnv* env,jobject thiz, jlong matAddress, jstring Name)
{  
    cv::Mat* image = (cv::Mat*)matAddress;
    // Do stuff with image.
}

Then you would call the method in Java like this:

P(myMat.getNativeObjAddr(), name);

OpenCV exposes this method just for cases like this. (I would link to the documentation's page but the website is not loading here, sorry.)