CODEWITHSUNDEEP
javaandroidandroid-ndkjava-native-interface
herolover
herolover

Reputation: 803

Java JNI GetMethodID return NULL

I have this class in Java:

public class CameraActivity extends Activity {

  ...      

  public class RecognitionResult {
    public String text;
    public int x;
    public int y;
    public int width;
    public int height;

    public RecognitionResult(String text, int x, int y, int width, int height) {
      this.text = text;
      this.x = x;
      this.y = y;
      this.width = width;
      this.height = height;
    }
  }
}

And this is my JNI code:

extern "C"
jobject Java_com_example_parkingcontrolsystem_CameraActivity_recognizeNumberPlate(JNIEnv *env,
                                                                                  jobject object,
                                                                                  jstring filename)
{
  jclass cls = env->FindClass("com/example/parkingcontrolsystem/CameraActivity$RecognitionResult");
  std::cout << "Class: " << cls << std::endl;

  jmethodID constructor = env->GetMethodID(cls, "<init>", "(Ljava/lang/String;IIII)V");
  std::cout << "Method: " << constructor << std::endl;

  recognition_result = env->NewObject(cls, constructor, "Hello from JNI", 0, 0, 50, 100);
  std::cout << "Object: " << recognition_result << std::endl;

  return recognition_result;
}

FindClass returns not a NULL, OK. But GetMethodID always returns zero. I tried to make the default constructor without args, and than use the "()V" signature, but GetMethodID returns NULL.

What is wrong?

Upvotes: 5

Views: 16930

Answers (1)

fyr
fyr

Reputation: 20859

If instantiating a inner class using JNI the parent must be passed also in the signature used with GetMethodIdin order to receive a valid jmethodID.

Assuming Bis a inner class of A and has a constructor like: public B(int someint) the following code would work(jobj is pointing to a instance of A):

jclass jclazz = env->FindClass("org/somepackage/A$B");
assert(jclazz != nullptr);
jmethodID mid = env->GetMethodID(jclazz, "<init>", "(Lorg/somepackage/A;I)V");
assert(mid != nullptr);
env->NewObject(jclazz, mid, jobj, 1);

A solution to the ops question is:

 jclass cls = env->FindClass("com/example/parkingcontrolsystem/CameraActivity$RecognitionResult");
 std::cout << "Class: " << cls << std::endl;
 jmethodID constructor = env->GetMethodID(cls, "<init>","Lcom/example/parkingcontrolsystem/CameraActivity;Ljava/lang/String;IIII)V");
 std::cout << "Method: " << constructor << std::endl;
 recognition_result = env->NewObject(cls, constructor, object, "Hello from JNI", 0, 0, 50, 100);
 std::cout << "Object: " << recognition_result << std::endl;

Note: Creds to Selven who posted also the same answer in the comments but didn`t post an answer.

Upvotes: 4

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