Pass Java double array to C++ in Android Studio project

Question

I'm working in an Android Studio project in Java that requires C++ for some signal processing functions. I need to be able to pass an array type Double from Java to C++. I've tried to lean heavily on examples, such as this (Get java array from c++ via JNI), with the MainActivity.java file already linked as a jobject in my native-lib.cpp file, I think I am making this too complicated. Is there is an easier way to pass variables back and forth? (In my very rudimentary example below, I created a method called 'getDouble' just to test this out)

C++ code:

#include <jni.h>
#include <string>

extern "C" JNIEXPORT jstring JNICALL
Java_com_example_covid19_MainActivity_stringFromJNI(
        JNIEnv* env,
        jobject /* this */) {
    std::string hello = "Test string from C to java";
    return env->NewStringUTF(hello.c_str());

    "()[java/lang/Double;" describes a method expecting no arguments and returning a double array.
            jmethodID methodID = env->GetMethodID(MainActivity, "getDouble", "()[java/lang/Double;");
}
jobjectarray doubles = env->CallObjectMethod(MainActivity, methodID);

int index = 0;
jdouble doubleArray = env->GetObjectArrayElement(double, index);

Java code:

public class MainActivity extends AppCompatActivity {

    // Used to load the 'native-lib' library on application startup.
    static {
        System.loadLibrary("native-lib");
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        // Example of a call to a native method
        TextView tv = findViewById(R.id.sample_text);
        tv.setText(stringFromJNI());
    }

    public double[] getDouble(){
        double[] j = {0, 4, 5, 6, 7};

        return j;
    }

    /**
     * A native method that is implemented by the 'native-lib' native library,
     * which is packaged with this application.
     */
    public native String stringFromJNI();
}

I think passing variables/inputs back and forth is fine but it shouldn't be too much complex (model classes), because then it would arise complexity on native side parsing and generating it. Even though I would suggest to follow this link as reference just in case: docs.oracle.com/javase/8/docs/technotes/guides/jni/spec/… — Jeel Vankhede
– Jeel Vankhede, Commented Sep 2, 2020 at 6:51

Botje · Accepted Answer · 2020-09-02 07:15:35Z

1

No, your approach is pretty much correct. The implementation is a bit off, however:

You should get a reference to the MainActivity class by using env->GetObjectClass(obj), where obj is the second argument of your function.
The signature of getDouble is ()[D. Use javap -s to see correct signatures instead of guessing.

If the array is large you might want to consider GetDoubleArrayRegion to get a double* you can read directly, or GetDoubleArrayElements/ReleaseDoubleArrayElements to manipulate it.

answered Sep 2, 2020 at 7:15

Botje

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1 Comment

wnjl Over a year ago

Thank you very much for your insight. Where does the signature ()[D go? Where I currently have ()[java/lang/Double ? Sorry...not terribly familiar with C++ so I am struggling my way through this conversion process.

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