从JNI访问Java类对象方法

Question

我有一个Android应用程序，我在其中用C++代码实现了与WebSocket的连接。

现在，我想通过带有JNI的C++代码调用Java类中初始化的对象的方法。

有可能做到吗？

这是我的活动：

public class MainActivity extends AppCompatActivity{
private MyCustomObject object; //This object is initialized in the life cycle of the activty
}

我想做的是从JNI调用object.myCustomMethod()。

Answer 1

我试着为您的用例放入部分代码。

1. 在onCreate期间将自定义对象传递给JNI

   //MainActivity.java
   public class MainActivity extends AppCompatActivity {
   // Used to load the 'native-lib' library on application startup.
   static {
   System.loadLibrary("native-lib");
   }
   private MyCustomObject object;
   protected void onCreate(Bundle savedInstanceState) {
   object = new MyCustomObject();
   //object is passed tthrough JNI call
   intJNI(object);
   }
   public class MyCustomObject{
   public void myCustomMethod(){
   }
   }
   
   /**
   * A native method that is implemented by the 'native-lib' native library,
   * which is packaged with this application.
   */
   public native void intJNI(MyCustomObject obj);
   }
   

2. 在本机端，您保留对象的引用，并在适当的时间调用

   //JNI 
   static jobject globlaRefMyCustomObject;
   static JavaVM *jvm;
   extern  "C" JNIEXPORT void JNICALL
   Java_test_com_myapplication_MainActivity_intJNI(
   JNIEnv* env,
   jobject callingObject,
   jobject myCustomObject) {
   jint rs = env->GetJavaVM(&jvm);
   assert (rs == JNI_OK);
   //take the global reference of the object
   globlaRefMyCustomObject =     env->NewGlobalRef(myCustomObject);
   }
   //this is done in any background thread in JNI 
   void callJavaCallbackFucntion(){
   JNIEnv *env;
   jint rs = jvm->AttachCurrentThread(&env, NULL);
   assert (rs == JNI_OK);
   jclass MyCustomObjectClass = env->GetObjectClass(globlaRefMyCustomObject);
   jmethodID midMyCustomMethod = env->GetMethodID(MyCustomObjectClass, "myCustomMethod", "()V");
   env->CallVoidMethod(globlaRefMyCustomObject,midMyCustomMethod);
   /* end useful code */
   jvm->DetachCurrentThread();
   }
   
   //Release the Global refence at appropriate time
   void JNI_OnUnload(JavaVM *vm, void *reserved){
   JNIEnv* env;
   if (vm->GetEnv(reinterpret_cast<void**>(&env), JNI_VERSION_1_6) != JNI_OK) {
   return JNI_ERR;
   }
   env->DeleteGlobalRef(globlaRefMyCustomObject);
   }