Android JNI：Java 通过 JNI 调用 C++ 类对象

Question

Android Studio 项目： 我试图通过从java传递一个变量（整数，双精度...）来调用一个C++类对象，并使用JNI接口从C++类对象返回一个结果（双精度）。当我编译/构建项目时，没有错误或警告，它编译得很好。但是，当我运行应用程序时，它崩溃了“不幸的是，App_xxx 已停止”。我已经追踪到我调用对象的方式（在 java 和 NTI 文件中）的错误。

我是否从 MainActivity 和 JNI 接口中正确调用了该对象？谁能帮忙看看我的代码有什么问题？

这是 MainActivity java 的代码：

package com.abc.www.apps;

import ...

public class MainActivity extends AppCompatActivity {


   double ax, bx, cx;
   double totalA, totalB;

   // 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);

      //Call object in .cpp file (C++)
      ax = 2;
      bx = 3;
      cx = 4;
      totalA = getSum(ax, bx, cx);
      totalB = getDiff(ax, cx);

      //Display results
      TextView tv = (TextView) findViewById(R.id.id_dispTotA);
      tv.setText(Double.toString(totalA));

      TextView tv = (TextView) findViewById(R.id.id_dispTotB);
      tv.setText(Double.toString(totalB));
   }

   /**
    * A native method that is implemented by the 'native-lib' native library,
    * which is packaged with this application.
   */
   public native double getSum(double ax, double bx, double cx);
   public native double getDiff(double ax, double cx);
}

这是native-lib.cpp的代码：

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

extern "C"
JNIEXPORT jdouble JNICALL
Java_com_abc_www_apps_MainActivity_getSum(
    JNIEnv *env, jobject,
    jfloat a, jfloat b, jfloat c) {

    jdouble value;
    mathResult m;

    value = m.getSum(a, b, c);
    return value;
}

JNIEXPORT jdouble JNICALL
Java_com_abc_www_apps_MainActivity_getDiff(
    JNIEnv *env, jobject,
    jfloat a, jfloat c) {

    jdouble value;
    mathResult m;

    value = m.getDiff(a, c);
    return value;
}

这是 C++ 类对象 (.cpp) 的代码：

#include "mathResult.h"
#include "math.h"

//Sum
double mathResult::getSum(double a, double b, double c)
{
   Sum = a + b + c;
   return Sum;
}

//Subtraction
double mathResult::getDiff(double a, double c)
{
   Diff = c - a;
   return Diff;
}

任何帮助将不胜感激，谢谢。

Answer 1

MainActivity

 public class MainActivity extends AppCompatActivity {

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


        double ax, bx, cx;
        double totalA, totalB;

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


            ax = 2;
            bx = 3;
            cx = 4;
            totalA = getSum(ax, bx, cx);
            totalB = getDiff(ax, cx);


            // Example of a call to a native method
            TextView tv = (TextView) findViewById(R.id.sample_text);
            tv.setText(Double.toString(totalA) + " === " + Double.toString(totalB));
        }

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

        public native double getSum(double ax, double bx, double cx);

        public native double getDiff(double ax, double cx);
    }

Nativelib.cpp

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


extern "C"
JNIEXPORT jstring

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



extern "C"
JNIEXPORT jdouble JNICALL
Java_com_abc_www_apps_MainActivity_getSum(JNIEnv *env, jobject instance, jdouble ax,
                                                jdouble bx, jdouble cx) {

    return math().getSum(ax, bx, cx);

}extern "C"
JNIEXPORT jdouble JNICALL
Java_com_abc_www_apps_MainActivity_getDiff(JNIEnv *env, jobject instance, jdouble ax,
                                                 jdouble cx) {

    return math().getDiff(ax, cx);

}

ma​​th.h

#ifndef JNIDEMO_MATH_H
#define JNIDEMO_MATH_H


#include <jni.h>

class math {

public:
    jdouble  getSum(jdouble ax, jdouble bx, jdouble cx);

    jdouble  getDiff(jdouble ax,jdouble bx);
};


#endif //JNIDEMO_MATH_H

数学.cpp

#include <jni.h>
#include "math.h"




jdouble math::getSum(jdouble ax, jdouble bx, jdouble cx) {
    return ax+bx+cx;
}

jdouble math::getDiff(jdouble ax, jdouble bx) {
    return ax-bx;
}

注意： 例如，您必须在 CmakerLists.txt 中添加新添加的文件 ma​​th.cpp 更改cmakerLists.txt 代码，如下所示以添加文件

add_library( # Sets the name of the library.
             native-lib

             # Sets the library as a shared library.
             SHARED


             # Provides a relative path to your source file(s).
             src/main/cpp/native-lib.cpp
              src/main/cpp/math.cpp)

这里我用 add_library 添加了 src/main/cpp/math.cpp