Calling a java method from c++ in Android

Question

I'm trying to get a simple Java method call from C++ while Java calls native method. Here's the Java code:

public class MainActivity extends Activity {     private static String LIB_NAME = "name";      static {         System.loadLibrary(LIB_NAME);     }      /** Called when the activity is first created. */     @Override     public void onCreate(Bundle savedInstanceState) {         super.onCreate(savedInstanceState);         setContentView(R.layout.main);         TextView tv = (TextView) findViewById(R.id.textview);         tv.setText(this.getJniString());     }      public void messageMe(String text) {         System.out.println(text);     }      public native String getJniString(); } 

I'm trying to call messageMe method from native code in the process of getJniString* method call from Java to native.

native.cpp:

#include <string.h> #include <stdio.h> #include <jni.h>  jstring Java_the_package_MainActivity_getJniString( JNIEnv* env, jobject obj, jint depth ){  //    JavaVM *vm; //    JNIEnv *env; //    JavaVMInitArgs vm_args; //    vm_args.version = JNI_VERSION_1_2; //    vm_args.nOptions = 0; //    vm_args.ignoreUnrecognized = 1; // //    // Construct a VM //    jint res = JNI_CreateJavaVM(&vm, (void **)&env, &vm_args);      // Construct a String     jstring jstr = env->NewStringUTF("This string comes from JNI");     // First get the class that contains the method you need to call     jclass clazz = env->FindClass("the/package/MainActivity");     // Get the method that you want to call     jmethodID messageMe = env->GetMethodID(clazz, "messageMe", "(Ljava/lang/String;)V");     // Call the method on the object     jobject result = env->CallObjectMethod(jstr, messageMe);     // Get a C-style string     const char* str = env->GetStringUTFChars((jstring) result, NULL);     printf("%s\n", str);         // Clean up     env->ReleaseStringUTFChars(jstr, str);  //    // Shutdown the VM. //    vm->DestroyJavaVM();      return env->NewStringUTF("Hello from JNI!"); } 

After clean compilation app stops with next message:

ERROR/AndroidRuntime(742): FATAL EXCEPTION: main         java.lang.NoSuchMethodError: messageMe         at *.android.t3d.MainActivity.getJniString(Native Method)         at *.android.t3d.MainActivity.onCreate(MainActivity.java:22) 

Apparently it means that method name is wrong, but it looks OK to me.

Answer 1

If it's an object method, you need to pass the object to CallObjectMethod:

jobject result = env->CallObjectMethod(obj, messageMe, jstr); 

What you were doing was the equivalent of jstr.messageMe().

Since your is a void method, you should call:

env->CallVoidMethod(obj, messageMe, jstr);

If you want to return a result, you need to change your JNI signature (the ()V means a method of void return type) and also the return type in your Java code.