In the previous article, I introduced how to build a new project with ndk. What if I add jni to the old project?

1. Create a new CPP folder under the app/main folder and a new. cpp file in the CPP folder:

2. Create a new tool class:

public class JniUtil {
    static {
        System.loadLibrary("jni_lib");
    }

    public native String getStringFromJni();
}

3. Create a new CMakeLists.txt file under the app directory:

cmake_minimum_required(VERSION 3.4.1)

add_library( # Sets the name of the library.
             jni_lib

             # Sets the library as a shared library.
             SHARED

             # Provides a relative path to your source file(s).
             src/main/cpp/native_lib.cpp )

find_library( # Sets the name of the path variable.
              log-lib

              # Specifies the name of the NDK library that
              # you want CMake to locate.
              log )

target_link_libraries( # Specifies the target library.
                       jni_lib

                       # Links the target library to the log library
                       # included in the NDK.
                       ${log-lib} )

(Note: CMakeLists.txt file names are case-sensitive; jni_lib in the red box is the System.loadLibrary("jni_lib") in the second step of the new tool class, and the path of the new cpp file.)

4. Add configuration in build.gradle:

externalNativeBuild {
        cmake {
            path "CMakeLists.txt"
        }
    }

5. Finally, write the. cpp file. In fact, after the last step, you can press the Alt+Enter key under the native method to create the jni method. The code is as follows:

#include <jni.h>
#include <string>

extern "C"
JNIEXPORT jstring JNICALL
Java_com_sonic_jnidemo1_JniUtil_getStringFromJni(JNIEnv *env, jobject instance) {
    // TODO
    std::string hello = "Welcome to C The world!";
    return env->NewStringUTF(hello.c_str());
}

Finally, it's running. The screenshot above: