java - 无法访问 java 中的 c++ 对象并在 android studio 中跨 JNI 层从 long objectptr 获取错误结果 c++ 函数
问题描述
我试图通过创建 c++ 类的指针并将其存储在 java 中的 long 值中作为 java 类中的数据成员来调用 c++ 方法,并尝试从通过传递存储的 long 值并将其传递给本机方法而创建的对象访问调用 c++ 方法我想打电话。我正在将其转换为本机方法中的 Numbers 类,即我想要访问其方法的类。但是给出错误的输出。我猜是一些垃圾值。我对这个概念完全陌生。任何人都可以告诉我确切的问题或纠正错误。
现在,我正在尝试创建 Numbers 类的非常简单的示例,该类支持对两个数字、add、mul、sub 和一个构造函数来初始化数字 - a 和 b 的简单操作。请找到各种类的代码:
MainActivity.java
package com.example.maths;
import androidx.appcompat.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.TextView;
public class MainActivity extends AppCompatActivity {
private long numberptr = 0; //c++ object reference
// 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);
//Creating Number object
numberptr = createNumber();
//Performing add operation
int result = nativeAdd(numberptr);
// Example of a call to a native method
TextView tv = findViewById(R.id.sample_text );
tv.setText(stringFromJNI() + "result is " + result);
}
/**
* A native method that is implemented by the 'native-lib' native library,
* which is packaged with this application.
*/
public native String stringFromJNI();
//Native method to create instance of Number (c++ class) and store it in java in numberptr long object
public native long createNumber();
public native int nativeAdd(long numberptr);
}
本机-lib.cpp
#include <jni.h>
#include <string>
#include "Numbers.h"
extern "C" JNIEXPORT jstring JNICALL
Java_com_example_maths_MainActivity_stringFromJNI(
JNIEnv* env,
jobject /* this */) {
std::string hello = "Hello from C++";
return env->NewStringUTF(hello.c_str());
}
extern "C"
JNIEXPORT jlong JNICALL
Java_com_example_maths_MainActivity_createNumber(JNIEnv *env, jobject thiz) {
// TODO: implement createNumber()
return reinterpret_cast<jlong>(new Numbers(3, 4));
}
extern "C"
JNIEXPORT jint JNICALL
Java_com_example_maths_MainActivity_nativeAdd(JNIEnv *env, jobject thiz, jlong numberptr) {
// TODO: implement nativeAdd()
Numbers* num = reinterpret_cast<Numbers *>(numberptr);
return num->add();
}
数字.h
#ifndef MATHS_NUMBERS_H
#define MATHS_NUMBERS_H
class Numbers {
int a, b;
public:
Numbers(int,int);
int add();
int mul();
int sub();
};
#endif //MATHS_NUMBERS_H
数字.cpp
#include "Numbers.h"
Numbers::Numbers(int a, int b) {
a = a;
b = b;
}
int Numbers::add() {
return a+b;
}
int Numbers::mul() {
return a*b;
}
int Numbers::sub() {
return a-b;
}
CmakeLists.txt
# For more information about using CMake with Android Studio, read the
# documentation: https://d.android.com/studio/projects/add-native-code.html
# Sets the minimum version of CMake required to build the native library.
cmake_minimum_required(VERSION 3.4.1)
# Creates and names a library, sets it as either STATIC
# or SHARED, and provides the relative paths to its source code.
# You can define multiple libraries, and CMake builds them for you.
# Gradle automatically packages shared libraries with your APK.
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).
native-lib.cpp
Numbers.cpp
)
# Searches for a specified prebuilt library and stores the path as a
# variable. Because CMake includes system libraries in the search path by
# default, you only need to specify the name of the public NDK library
# you want to add. CMake verifies that the library exists before
# completing its build.
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 )
# Specifies libraries CMake should link to your target library. You
# can link multiple libraries, such as libraries you define in this
# build script, prebuilt third-party libraries, or system libraries.
target_link_libraries( # Specifies the target library.
native-lib
# Links the target library to the log library
# included in the NDK.
${log-lib} )
请告诉我问题是什么或应该如何准确处理。提前致谢。
解决方案
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