Jni的一个异常

　　UnsatisfiedLinkError:No implementation found for java.lang.String com.skymaster.hs.test4.MainActivity.getstringFromJNI()；

就是桌这个路径下的方法stringFromJNI();没有被实现，看看全部代码

public class MainActivity extends AppCompatActivity {

    // Used to load the 'native-lib' library on application startup.
    static {
        System.loadLibrary("native-lib");
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        // Example of a call to a native method
        TextView tv = (TextView) findViewById(R.id.sample_text);
        tv.setText(stringFromJNI());
        tv.append(getStringFromJNI());
    }

    /**
     * A native method that is implemented by the 'native-lib' native library,
     * which is packaged with this application.
     */
    public native String stringFromJNI();
    public native String getStringFromJNI();
}

下面是cpp目录中native-lib.cpp的代码

#include <jni.h>
#include <string>
using namespace std;
extern "C"

    jstring
    Java_com_skymaster_hs_test4_MainActivity_stringFromJNI(
            JNIEnv *env,
            jobject /* this */) {
        std::string hello = "Hello from C++";
        return env->NewStringUTF(hello.c_str());
    }

    jstring
    Java_com_skymaster_hs_test4_MainActivity_getStringFromJNI(
            JNIEnv *env,
            jobject /* this */) {
        string hello = "    Hello Android";
        return env->NewStringUTF(hello.c_str());
    }

这里虽然是c++的源文件，但是两个函数都是c的函数，所以需要用extern "C"(大写的C)来告诉c++编译器需要按照c的的方式来编译与链接。问题出在
extern "C"的作用范围，如果不用大括号包括2个函数就会出问题。修改如下就好了

#include <jni.h>
#include <string>
using namespace std;
extern "C" {

    jstring
    Java_com_skymaster_hs_test4_MainActivity_stringFromJNI(
            JNIEnv *env,
            jobject /* this */) {
        std::string hello = "Hello from C++";
        return env->NewStringUTF(hello.c_str());
    }

    jstring
    Java_com_skymaster_hs_test4_MainActivity_getStringFromJNI(
            JNIEnv *env,
            jobject /* this */) {
        string hello = "    Hello Android";
        return env->NewStringUTF(hello.c_str());
    }
}