Android NDK C++ JNI (no implementation found for native…)

问题:

I'm trying to use the NDK with C++ and can't seem to get the method naming convention correct. my native method is as follows:

extern "C" { JNIEXPORT void JNICALL Java_com_test_jnitest_SurfaceRenderer_drawFromJni (JNIEnv* env, jclass c) {    // } } 

with a header wrapped in extern "C" {} aslo.

Everything compiles fine, creates a .so file and copies to the libs folder under my project, but when I debug and run in Eclipse I keep getting a log cat message that of "no implementation found for native...". Is there something i'm missing as all the NDK examples are in C?

Thanks.

回答1:

There are a couple of things that can lead to "no implementation found". One is getting the function prototype name wrong, another is failing to load the .so at all. Are you sure that System.loadLibrary() is being called before the method is used?

If you don't have a JNI_OnLoad function defined, you may want to create one and have it spit out a log message just to verify that the lib is getting pulled in successfully.

You already dodged the most common problem -- forgetting to use extern "C" -- so it's either the above or some slight misspelling. What does the Java declaration look like?

回答2:

An additional cause for this error: your undecorated native method name must not contain an underscore!

For example, I wanted to export a C function named AudioCapture_Ping(). Here is my export declaration in C:

JNI_EXPORT int Java_com_obsidian_mobilehashhost_MainActivity_AudioCapture_Ping(JNIEnv *pJniEnv, jobject object);  //Notice the underscore before Ping 

Here was my Java class importing the function:

package com.obsidian.mobileaudiohashhost; ... public class MainActivity extends Activity {     private native int AudioCapture_Ping();  // FAILS     ... 

I could not get Android to dynamically link to my native method until I removed the underscore:

JNI_EXPORT int Java_com_obsidian_mobilehashhost_MainActivity_AudioCapturePing(JNIEnv *pJniEnv, jobject object);   package com.obsidian.mobileaudiohashhost; ... public class MainActivity extends Activity {     private native int AudioCapturePing();  // THIS WORKS!     ... 

回答3:

I had the same problem, but to me the error was in the file Android.mk. I had it:

LOCAL_SRC_FILES := A.cpp LOCAL_SRC_FILES := B.cpp  

but should have this:

LOCAL_SRC_FILES := A.cpp LOCAL_SRC_FILES += B.cpp  

note the detail += instead :=

I hope that helps.

回答4:

An additional reason: Use LOCAL_WHOLE_STATIC_LIBRARIES instead of LOCAL_STATIC_LIBRARIES in android.mk. This stops the library from optimizing out unused API calls because the NDK cannot detect the use of the native bindings from java code.

回答5:

There is a cpp example under apps in ndk: https://android.googlesource.com/platform/development/+/marshmallow-release/ndk/platforms/android-5/samples/hello-gl2/jni/gl_code.cpp

回答6:

Called extern "C" as provided in the automatically-generated Studio example, but forgot to wrap the entire rest of the file, including following functions, in {} brackets. Only the first function worked.

回答7:

Use javah (part of Java SDK). Its the tool exactly for this (generates .h header from .class file).

回答8:

I try all above solutions, but no one can solved my build error(jni java.lang.UnsatisfiedLinkError: No implementation found for...), at last I found that I forget to add my verify.cpp source file to CMakeList.txt add_library segement(verify.cpp is auto generate by Ctrl + Enter short key, maybe other file name), hope my response can help some one.

my build environment: Gradle + CMake

回答9:

I Faced the same problem, and in my case the reason was that I had underscore in package name "RFID_Test" I renamed the Package and it worked. Thanks user1222021

回答10:

If your package name includes _ character, you should write 1(one) after _ character as shown below:

MainActivity.java

package com.example.testcpp_2; 

native-lib.cpp

JNICALL Java_com_example_testcpp_12_MainActivity_stringFromJNI( 

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