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5. 安卓模拟器安装和第一个c++安卓程序编译运行~1

发布时间：2021-05-25 14:51:31编辑：雪饮阅读（）

 在前面我們新建的Native C++項目成功后

自動產生MainActivity.java(下面這個是自己手動添加了注釋的):
package com.example.ndktest;
import androidx.appcompat.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.TextView;
public class MainActivity extends AppCompatActivity {
    // Used to load the 'native-lib' library on application startup.
    static {
        //在运行期间，APP 的 MainActivity 会调用 System.loadLibrary() 方法，加载 native library
        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 = findViewById(R.id.sample_text);
        tv.setText(stringFromJNI());
    }
    /**
     * A native method that is implemented by the 'native-lib' native library,
     * which is packaged with this application.
     */
    //一个native方法就是一个Java调用非Java代码的接口。一个native方法是指该方法的实现由非Java语言实现，比如用C或C++实现。
    public native String stringFromJNI();
}
這裏主要是用java去調用非java實現的stringFromJNI方法
同時新項目中cpp目錄下CMakeLists.txt（這裏也加了自己的一些簡潔注釋）：

# 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 3.10.2)

# Declares and names the project.

project("ndktest")

# 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).
             # CMake 会根据构建脚本的指令去编译一个 C++ 源文件，也就是 native-lib.cpp
             native-lib.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} )

這個文件主要用來實現编译一个 C++ 源文件，也就是 native-lib.cpp，并将编译后的产物扔进共享对象库中，并将其命名为 libnative-lib.so，然后 Gradle 将其打包到 APK 中。

然後同樣的新項目中cpp文件夾中的native-lib.cpp：
#include <jni.h>
#include <string>
extern "C" JNIEXPORT jstring JNICALL
Java_com_example_ndktest_MainActivity_stringFromJNI(
        JNIEnv* env,
        jobject /* this */) {
    std::string hello = "Hello from C++";
    return env->NewStringUTF(hello.c_str());
}

這是一個c++源代碼，為上面MainActivity.java中所調用的stringFromJNI方法的一個實現。

最後編譯並運行到安卓設備中如：