Develop Reference

Android Studio JNI with C++ UnsatisfiedLinkError

I am using Android Studio 2.1.2 I am not using Experimental Plugin
Please Check the following files and Check the error I am getting.
I solved the issue. Edited file is as follows. They way I fixed it may not be the correct way as I am setting property to use deprecated way, but it works. Experimental plugin can still be unstable. I will try with experimental plugin soon.
build.gradle from Module
sourceSets.main {
jniLibs.srcDir 'src/main/libs'
/*jni.srcDirs = [] not using this, I commented this. Please check SO links which explained when to use this and when not to use this*/
}
following 4 files are in jni folder of main
Android.mk
LOCAL_PATH := $(call my-dir)
include $(CLEAR_VARS)
LOCAL_MODULE := mylib
LOCAL_SRC_FILES := HelloJni.cpp
include $(BUILD_SHARED_LIBRARY)
Application.mk
APP_ABI := all
HelloJni.cpp
#include <jni.h>
#include <Header.h>
JNIEXPORT jstring JNICALL Java_com_example_m1035325_ndksampleapp_MainActivity_getStringFromJni(JNIEnv *env,jobject thiz)
{
env-> NewStringUTF ( "Hellofrom JNI!");
}
Header.h
#include <jni.h>;
using namespace std;
#ifndef HEADER
#define HEADER
extern "C" {
JNIEXPORT jstring JNICALL Java_com_example_m1035325_ndksampleapp_MainActivity_getStringFromJni(JNIEnv *env, jobject obj);
}
#endif //NDKSAMPLEAPP_HEADER_H
MainActicity.java
static
{
System.loadLibrary("mylib");
}
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
TextView tvHello=(TextView)findViewById(R.id.tvHello);
tvHello.setText(getStringFromJni());
}
public native String getStringFromJni();
Here when I hover on method getStringFromJni it shows Can't resolve corresponding JNI function
I have set NDK path in Project Structure and in Path environment variable too.
I am getting following error
Process: com.example.m1035325.ndksampleapp, PID: 12831
java.lang.UnsatisfiedLinkError: No implementation found for java.lang.String com.example.m1035325.ndksampleapp.MainActivity.getStringFromJni() (tried Java_com_example_m1035325_ndksampleapp_MainActivity_getStringFromJni and Java_com_example_m1035325_ndksampleapp_MainActivity_getStringFromJni__)
at com.example.m1035325.ndksampleapp.MainActivity.getStringFromJni(Native Method)
I searched a lot on SO also but I am not getting what I missed?
No error now , above error is fixed. Please check my answer to this question.

I think the problem is in your Android.mk file:
LOCAL_SOURCE_FILE := HelloJni.cpp
AFAIK the Android build system doesn't use variable of that name. It should be:
LOCAL_SRC_FILES := HelloJni.cpp

One important change I made is in file gradle.properties, is as follows
android.useDeprecatedNdk=true
so its related to Android Studio version 2.1.2 , for this version experimental plugin is the default option to use and the approach I am using is deprecated.
I will be trying with experimental plugin soon.

Wrapp existing library (.so) in jni for android app

I wanna realize this idea. I spent several days searching for information, but could not find anything. All tutorials say how to write my own library with JNI, but how to wrap already existing? I need just simple tutorial step by step (and why? if it possible). So I wanna start create native android application.
What I have :
I create C++ library in QTCreator by tutorial from youtube: simple library on C++ (.so) with headers (.h) which do simple cout in console:
Not compiled code mylib.cpp:
#include "mylib.h"
MyLib::MyLib() { }
void MyLib::Test() {
qDebug() << "Hello from our DLL";
// .so
}
Header mylib.h:
#ifndef MYLIB_H
#define MYLIB_H
#include "mylib_global.h"
#include <QDebug>
class MYLIBSHARED_EXPORT MyLib
{
public:
MyLib();
void Test();
};
#endif // MYLIB_H
and mylib_global.h (I think it does't matter)
So after build I have myLib.so.
And now I need wrap it in my android app. So I don't understand what I need to do for it.
I'm develop in Android Studio. And what I know:
Create in java package LibWrappClass with native method - something like "simplePrint()":
public native void simplePrint();
I need to create in /src/main folder "jni". Create Android.mk, myLibWrapper.h and myLibWrapper.cpp. But I don't understand what I need to write in Android.mk for connect my myLib.so to "myLibWrapper.h", and where should I put my library with headers. Can anyone help?

After adding the native method in your java code, You simply build the project. Now you need to move to the location where the class files are written by your IDE. Since you use Android Studio, it must be somewhere your project folder with path
out/production/YourModuleName
Open the location in commandline and run the javah command to generate the header file for your native function
javah -d <your jni folder path> <com.YourPackage.YourClass>
The class YourClass should be where you have declared the native method. This command will create a header file with name something like com_YourPackage_YourClass.h with a function declaration looks like
JNIEXPORT void JNICALL Java_com_YourPackage_YourClass_simplePrint
(JNIEnv *, jobject);
Implement this function in a C/C++ file with whatever operations you have to perform on jni side.
Then, Define the Android.mk file, In this case it will be something like
LOCAL_PATH := $(call my-dir)
include $(CLEAR_VARS)
LOCAL_MODULE := lib
LOCAL_SRC_FILES := lib.c
include $(BUILD_SHARED_LIBRARY)
If you have multiple source files add them in LOCAL_SRC_FILES separated by space.
Next, go to your jni folder and build the project using command ndk-build. This will place the .so file inside the android folder libs/armeabi
Finally rebuild the Android project from Android studio and run.
For detailed instructions and complete source checkout this gist

How compile c++ project via Android NDK

I have small C++ library project with one class.
class Test
{
public:
Test(){};
~Test(){};
int SomeFunc() { return 5; }
}
I can build this class via Android NDK. (Question 1).
I can use .so file into Java application. How I can call SomeFunc method from Java code (Question 2).

Here are the steps:
1.) Create Android.mk in the project's "jni" folder:
LOCAL_PATH:= $(call my-dir)
include $(CLEAR_VARS)
LOCAL_MODULE := main_jni
LOCAL_CFLAGS :=
LOCAL_SRC_FILES := main.cpp
LOCAL_LDLIBS :=
include $(BUILD_SHARED_LIBRARY)
2.) Create main.cpp in the "jni" folder:
#include <jni.h>
using namespace std;
#ifdef __cplusplus
extern "C" {
#endif
class Test {
public:
Test(){};
~Test(){};
int SomeFunc() { return 5; }
};
jint Java_com_example_activity_MainActivity_SomeFunc(JNIEnv *env, jobject thiz)
{
Test *test = new Test();
return test->SomeFunc();
}
#ifdef __cplusplus
}
#endif
3.) Add a call to load the library in your calling activity (MainActivity.java in this example):
static {
System.loadLibrary("main_jni");
}
4.) Define the native function in the calling activity:
native int SomeFunc();
5.) Call it from the activity:
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
TextView text = (TextView) this.findViewById(R.id.text);
text.setText(String.valueOf(SomeFunc()));
}
6.) Run the "ndk-build" command from the project's root folder (Note: refresh the project in Eclipse after this step)
7.) Re-build and run the application

I'm not sure I understand the questions correctly but this link could be useful to you.
I'm personally do not know much of C except for the very basics, but I look forward to getting to more C with the NDK.

Whatever code you write in c you need to define it like java_urpackagename_class_methodname then before using this code u need to first create a native method in java class like public native int ABC(); telling it that you are going to use this method.To use it do create an Android.mk or Application.mk as per your need. .so will help your java class know ,what your c code wants to do.

Project containing jni file

I am trying to import an android project to my Eclipse.
When i run this project, i got an error, "library is null".
I figured out that this project was different, it contain a file named "jni". My library is in that file.
I guess the library in this file was never compiled (i don't know why)
I have seen in some topic that i need to use NDK?
Did any one have a clear answer about this?

Refer the below links. link to download ndk sample 1 sample 2
Step1: First create a project then create a folder named jni in your project directory
Step2: Create addition.c file in jni folder and add the below lines.
#include "com_ndkadd_Addition.h"
JNIEXPORT jint JNICALL Java_com_ndkadd_Addition_messageFromNativeCode
(JNIEnv * env, jobject jObj,jint value1, jint value2)
{
return (value1 + value2);
}
Step3: Create Android.mk file in jni folder and the below code
LOCAL_PATH := $(call my-dir)
include $(CLEAR_VARS)
LOCAL_MODULE := Addition
LOCAL_SRC_FILES := addition.c
include $(BUILD_SHARED_LIBRARY)
Step4: navigate to bin folder of your project from command prompt. type the below line and press enter.
javah -jni com.ndkadd.Addition
Step5: Move the created com_ndkadd_Addition.h file to jni folder.
Step6: Now Compiling the Native Code as below from command prompt.
location_of_ndk\project_name> location_of_ndk\ndk-build
Step7: below the code code for your activity and usage of created .so file in your libs folder.
public class Addition extends Activity {
TextView txtHello;
static
{
System.loadLibrary("Addition");
}
public native int messageFromNativeCode(int v1,int v2);
/** Called when the activity is first created. */
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
TextView txtHello = new TextView(this);
txtHello.setText(""+messageFromNativeCode(5,5));
setContentView(txtHello);
}
}
Note:: Better to have NDK in the place where you have SDK and the project containing JNI in android-ndk-r8 folder.

Android std and stl support

I am playing with android ndk. I am using Window Vista with cygwin (latest version). I compiled and launched the hello world jni sample on my phone. It is working. The code is (is a .cpp file):
#include <string.h>
#include <jni.h>
extern "C" {
JNIEXPORT jstring JNICALL Java_org_android_helloworld_HelloworldActivity_invokeNativeFunction(JNIEnv* env, jobject javaThis);
};
jstring Java_org_android_helloworld_HelloworldActivity_invokeNativeFunction(JNIEnv* env, jobject javaThis)
{
return env->NewStringUTF("Hello from native code!");
}
I wanted to add some modifications, just to play with it a bit:
#include <algorithm>
and then, in the function above, i added:
int a;
a=std::min<int>(10, 5);
but the compiler says that it cannot find the file 'algorithm' and that min() is not part of std.
After a bit of searching, i have found that the android ndk has a gnu-libstdc++ directory with all the std files needed. Reading the NDK docs, i have learned that usint std::* should work without any modification to the code (if one include the proper header files). But it seems that gcc on cygwin is not able to find the needed files.
What are the steps to do in order to be able to use std and stl within a .cpp file in an android ndk app?

From NDK r5's docs/CPLUSPLUS-SUPPORT.html:
By default, the headers and libraries for the minimal C++ runtime system
library (/system/lib/libstdc++.so) are used when building C++ sources.
You can however select a different implementation by setting the variable
APP_STL to something else in your Application.mk, for example:
APP_STL := stlport_static
To select the static STLport implementation provided with this NDK.
Value APP_STL values are the following:
system -> Use the default minimal C++ runtime library.
stlport_static -> Use STLport built as a static library.
stlport_shared -> Use STLport built as a shared library.
gnustl_static -> Use GNU libstdc++ as a static library.
Which NDK are you using? Have you tried compiling one of the sample applications that utilize the STL such as test-libstdc++?