public class MyJNI {
  private static final String TAG=MyJNI.class.getName();
  @Test
  public void test(){
    JNITest jniTest=new JNITest();
    Log.d(TAG,jniTest.nativeCalculate(2)+"");
  }
}

public class JNITest {

  private int number = 0;

  public int javaCalculate(int num){
    number=num+10;
    return number;
  }
  public native int nativeCalculate(int num);

  static {
    System.loadLibrary("jni_test");
  }
}

/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class com_example_xujiajia_sx_jnitest_JNITest */

#ifndef _Included_com_example_xujiajia_sx_jnitest_JNITest
#define _Included_com_example_xujiajia_sx_jnitest_JNITest
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:   com_example_xujiajia_sx_jnitest_JNITest
 * Method:  nativeCalculate
 * Signature: (I)I
 */
JNIEXPORT jint JNICALL Java_com_example_xujiajia_1sx_jnitest_JNITest_nativeCalculate
 (JNIEnv *, jobject, jint);

#ifdef __cplusplus
}
#endif
#endif

#include "jni_test.h"

JNIEXPORT jint JNICALL
Java_com_example_xujiajia_1sx_jnitest_JNITest_nativeCalculate(JNIEnv *env, jobject obj,jint num) {
  //獲取obj中對象的class對象
  jclass clazz = env->GetObjectClass(obj);
  //獲取clazz中的number字段的id
  jfieldID id_number = env->GetFieldID(clazz, "number", "I");
  jmethodID id_java_calculate=env->GetMethodID(clazz, "javaCalculate", "(I)I");
  //次獲取java中number的值
  jint number = env->GetIntField(obj, id_number);
  jint result=env->CallIntMethod(obj,id_java_calculate,number);

  env->SetIntField(obj,id_number,result+num);
  //再次獲取java中number的值并返回
  return env->GetIntField(obj, id_number);
}

android {
  ...
  defaultConfig {
    ...
    externalNativeBuild {
      cmake {
        cppFlags ""
        //生成多個版本的so文件
        abiFilters 'armeabi','armeabi-v7a','x86'
      }
    }
  }
  buildTypes {
    ...
  }
  externalNativeBuild {
    cmake {
      path "CMakeLists.txt"
    }
  }
}

#CMakeLists.txt
cmake_minimum_required(VERSION 3.4.1)

add_library( # Sets the name of the library.
    jni_test

    # Sets the library as a shared library.
    SHARED

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

include_directories(src/main/jni/)

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.
      # 制定目標庫.
      jni_test

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