Native call Java

NativeCallJavaTest.kt

package com.practice

class NativeCallJavaTest {

    external fun test()

    init {
        System.loadLibrary("native-call-java-lib")
    }
}

Call the member methods

Counter.kt

      package com.practice

      class Counter {

      private var count = 0


      fun add() {
          println("Counter#add(): old count is $count")
          count++
      }


      fun count(): Int {
          println("Counter#count(): $count")
          return count
      }

    }

native-call-java-lib.cpp

      #include <jni.h>
      #include <android/log.h>
      #include <iostream>

      #define TAG "counter"

      jint getCount(JNIEnv *env, jclass cls, jobject obj) {
          jmethodID methodId = env->GetMethodID(cls, "count", "()I");
          return env->CallIntMethod(obj, methodId);
      }

      void addCount(JNIEnv *env, jclass cls, jobject obj) {
          jmethodID methodId = env->GetMethodID(cls, "add", "()V");
          env->CallVoidMethod(obj, methodId);
      }

      extern "C" JNIEXPORT void JNICALL
      Java_com_practice_NativeCallJavaTest_test(JNIEnv *env, jobject thiz) {
          __android_log_print(ANDROID_LOG_DEBUG, TAG, "test()");

          // new object
          jclass cls = env->FindClass("com/practice/Counter");
          jmethodID constructId = env->GetMethodID(cls, "<init>", "()V");
          jobject obj = env->NewObject(cls, constructId);

          // get count
          __android_log_print(ANDROID_LOG_DEBUG, TAG, "count is %d", getCount(env, cls, obj));

          // add count
          addCount(env, cls, obj);
          __android_log_print(ANDROID_LOG_DEBUG, TAG, "count is %d", getCount(env, cls, obj));
      }

Call the static method of companion object

Counter.kt

class Counter {

  companion object {
    @JvmStatic
    fun getName(): String {
      return "Kotlin static method is called."
    }
  }

}

native-call-java-lib.cpp

jstring getName(JNIEnv *env) {
  jclass cls = env->FindClass("com/practice/Counter");
  if (cls == nullptr) {
    __android_log_print(ANDROID_LOG_DEBUG, TAG, "Counter/$Companion is not found");
  }
  jmethodID methodId = env->GetStaticMethodID(cls, "getName", "()Ljava/lang/String;");
  if (methodId == nullptr) {
    __android_log_print(ANDROID_LOG_DEBUG, TAG, "Counter.getName is not found");
  }

  return (jstring) env->CallStaticObjectMethod(cls, methodId);
}

extern "C" JNIEXPORT void JNICALL
Java_com_practice_NativeCallJavaTest_test(JNIEnv *env, jobject) {
  jstring jName = getName(env);

  const char *name = env->GetStringUTFChars(jName, 0);
  __android_log_print(ANDROID_LOG_DEBUG, TAG, "name is %s", name);
  env->ReleaseStringUTFChars(jName, name);
}

Call the static method from singleton

Singleton.kt

package com.practice

object Singleton {

    @JvmStatic
    fun getValue(input: Int): Int = input * 2

}

native-call-java-lib.cpp

jint getValue(JNIEnv *env, jint input) {
    jclass cls = env->FindClass("com/practice/Singleton");
    if (cls == nullptr) {
        __android_log_print(ANDROID_LOG_DEBUG, TAG, "Singleton class not found");
    }

    jmethodID methodId = env->GetStaticMethodID(cls, "getValue", "(I)I");
    if (methodId == nullptr) {
        __android_log_print(ANDROID_LOG_DEBUG, TAG, "Singleton.getValue(int) not found");
    }

    return env->CallStaticIntMethod(cls, methodId, input);
}

extern "C" JNIEXPORT void JNICALL Java_com_practice_NativeCallJavaTest_test(JNIEnv *env, jobject) {
    __android_log_print(ANDROID_LOG_DEBUG, TAG, "value from Singleton is %d", getValue(env, 3));
}

No such method error

When I remove the @JvmStatic annotation from Counter.getName(), I will get below error.

D/counter: Counter.getName is not found
A/art: art/runtime/java_vm_ext.cc:410] JNI DETECTED ERROR IN APPLICATION: JNI CallStaticObjectMethodV called with pending exception java.lang.NoSuchMethodError: no static method "Lcom/practice/Counter;.getName()Ljava/lang/String;"

Android NDK. Calling Kotlin from native code

Call the method of nested class (also named static nested class in Java)

Counter.kt

class Counter {
    class Comparator(val num: Int) {
        fun isSameAs(num: Int): Boolean {
            return num == this.num
        }
    }
}

native-call-java-lib.cpp

jboolean isSameAs(JNIEnv *env, jclass cls, jobject obj, jint num) {
    jmethodID methodId = env->GetMethodID(cls, "isSameAs", "(I)Z");
    return env->CallBooleanMethod(obj, methodId, num);
}

extern "C" JNIEXPORT void JNICALL Java_com_practice_NativeCallJavaTest_test(JNIEnv *env, jobject) {
    // new inner class
    jclass cls = env->FindClass("com/practice/Counter$Comparator");
    if (cls == nullptr) {
        __android_log_print(ANDROID_LOG_DEBUG, TAG, "can not find inner class.");
    }
    jmethodID constructId = env->GetMethodID(cls, "<init>", "(I)V");
    if (constructId == nullptr) {
        __android_log_print(ANDROID_LOG_DEBUG, TAG, "can not get construct of inner class.");
    }
    jobject obj = env->NewObject(cls, constructId, 3);
    if (obj == nullptr) {
        __android_log_print(ANDROID_LOG_DEBUG, TAG, "can not create instance of inner class.");
    }

    // isSameAs 3
    __android_log_print(ANDROID_LOG_DEBUG, TAG, "Is 3 same as the number in inner class? %d", isSameAs(env, cls, obj, 3));
}

Call the method of inner class (non-static nested class in Java)

Inner class can access the member of outer class.

Outer.kt

package com.practice

class Outer {
    private val bar: Int = 1

    inner class Inner {
        fun getBar() = bar
    }
}

native-call-java-lib.cpp

jobject createOuterInstance(JNIEnv *env) {
    jclass cls = env->FindClass("com/practice/Outer");
    if (cls == nullptr) {
        __android_log_print(ANDROID_LOG_DEBUG, TAG, "can not find outer class.");
    }

    jmethodID constructId = env->GetMethodID(cls, "<init>", "()V");
    if (constructId == nullptr) {
        __android_log_print(ANDROID_LOG_DEBUG, TAG, "can not get construct of outer class.");
    }

    return env->NewObject(cls, constructId);
}

jobject createInnerInstance(JNIEnv *env, jclass innerCls, jobject outerObj) {
    jmethodID innerConstructId = env->GetMethodID(innerCls, "<init>", "(Lcom/practice/Outer;)V");
    if (innerConstructId == nullptr) {
        __android_log_print(ANDROID_LOG_DEBUG, TAG, "can not get construct of inner class.");
    }

    return env->NewObject(innerCls, innerConstructId, outerObj);
}

jint getBar(JNIEnv *env, jclass cls, jobject obj) {
    jmethodID methodId = env->GetMethodID(cls, "getBar", "()I");
    return env->CallIntMethod(obj, methodId);
}

extern "C" JNIEXPORT void JNICALL Java_com_practice_NativeCallJavaTest_test(JNIEnv *env, jobject) {
    __android_log_print(ANDROID_LOG_DEBUG, TAG, "test()");

    // New Outer class
    jobject obj = createOuterInstance(env);
    if (obj == nullptr) {
        __android_log_print(ANDROID_LOG_DEBUG, TAG, "can not create instance of outer class.");
    }

    // New Inner class
    jclass innerCls = env->FindClass("com/practice/Outer$Inner");
    if (innerCls == nullptr) {
        __android_log_print(ANDROID_LOG_DEBUG, TAG, "can not find inner class.");
    }

    jobject innerObj = createInnerInstance(env, innerCls, obj);
    if (innerObj == nullptr) {
        __android_log_print(ANDROID_LOG_DEBUG, TAG, "can not create instance of inner class.");
    }

    __android_log_print(ANDROID_LOG_DEBUG, TAG, "value in inner class is %d", getBar(env, innerCls, innerObj));
}

No such method error

D/native: can not get construct of inner class.
A/zygote: java_vm_ext.cc:504] JNI DETECTED ERROR IN APPLICATION: JNI NewObjectV called with pending exception java.lang.NoSuchMethodError: no non-static method "Lcom/practice/Outer$Inner;.<init>()V"

native-call-java-lib.cpp

jobject createInnerInstance(JNIEnv *env, jclass innerCls, jobject outerObj) {
    jmethodID innerConstructId = env->GetMethodID(innerCls, "<init>", "()V"); // wrong here!
    if (innerConstructId == nullptr) {
        __android_log_print(ANDROID_LOG_DEBUG, TAG, "can not get construct of inner class.");
    }

    return env->NewObject(innerCls, innerConstructId); // also wrong here!
}

I can use javap to check the signature of inner class constructor.

$ javap -s -p com/practice/Outer\$Inner.class 
Compiled from "Outer.kt"
public final class com.practice.Outer$Inner {
  final com.practice.Outer this$0;
    descriptor: Lcom/practice/Outer;
  public final int getBar();
    descriptor: ()I

  public com.practice.Outer$Inner();
    descriptor: (Lcom/practice/Outer;)V // That's it!
}

It still has the second wrong.

E/zygote: JNI ERROR (app bug): accessed stale Global 0xd67b6e72  (index 224900839 in a table of size 507)
A/zygote: java_vm_ext.cc:504] JNI DETECTED ERROR IN APPLICATION: use of deleted global reference 0xd67b6e72

Inner object will hold the reference of outer object, so I have to pass the outer instance to the constructor of inner class even it has no parameter.

Nested Classes of The Java™ Tutorials @Oracle

Static inner class in Kotlin @StackOverflow

Nested and inner class @Kotlin Official Side

native call Java

Native call Java

Call the member methods

Call the static method of companion object

Call the static method from singleton

No such method error

Call the method of nested class (also named static nested class in Java)

Call the method of inner class (non-static nested class in Java)

No such method error

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