jni入门级别教程

前提条件

笔者假想着：能看到这篇博客的读者 NDK环境 已经配置好了哈

开发步骤

第一步：新建工程

工程建完之后在真机或模拟器上运行一遍，确保工程建的没问题

第二步：配置NDK

第三步：在Java中添加 native 函数 并 调用

我们这里就借助 MainActivity 来写一个 native 函数并调用

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package com.wust.jnitest3;

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 {
        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(jniTest());  //我们在这里调用一下
    }

    /**
     * A native method that is implemented by the 'native-lib' native library,
     * which is packaged with this application.
     */
    public native String stringFromJNI();
    public native String jniTest();   //这里是我们自己定义的 原生函数
}

第四步：在cpp 中创建c/c++ 源文件

第五步：C++代码的编写

//
// Created by yiqi on 2021/4/1.
//
// jni 开发必须的 头文件
#include "../../../../../../sfDownload/sdk/ndk/21.0.6113669/toolchains/llvm/prebuilt/windows-x86_64/sysroot/usr/include/jni.h" 

JNICALL extern "C" JNIEXPORT // 不加好像会报错
        jstring // native 方法的返回值
// c/c++ native 方法命名规则。  Java_包名_native方法所在的类名_native的方法名
// 后面那两个参数是必须要有的。  *env。 是java 和 c/c++ 互调常用的方法   thiz。  是nation 方法所在类的对象
        Java_com_wust_jnitest3_MainActivity_jniTest(JNIEnv* env,jobject thiz){
    char * str = "我来自C++";
    jstring text = (*env).NewStringUTF(str);
    return text;
}

第六步：添加编译配置

# 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.
             # 设置生成.so 的文件名，也是你在 java 代码里调用的名字，填一个就好了，记住！！！
             native-lib

             # Sets the library as a shared library.
             #设置库的类型  一种静态文件  STATIC .a   一种动态文件  SHARED .so
             SHARED

             # Provides a relative path to your source file(s).
             # 需要编译的c/c++ 文件，这里填相对路径
             native-lib.cpp
             myFirstJni.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.
                       # 指定链接的目标库
                       native-lib

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

第七步：运行