Android JNI搭建与实现

May you return with a young heart after years of fighting.
愿你出走半生，归来仍是少年。​​​

1.创建项目与NDK配置

第一步:创建项目
项目名：JNIDemo
包名：com.pfj.jnidemo
第二步：配置NDK环境
下载ndk需要的工具：打开SDKManager 然后选择SDK Tools

需要下载LLDB、CMake、NDK如图。
第三步
下载完成以后打开 File – Project Structure – SDK Location，使用SDK Tools下载成功的NDK。

2.JNI开发

2.1创建一个 类HelloNDK.java

public class HelloNDK {
    // 动态导入 so 库
    static {
        System.loadLibrary("HelloNDK");
    }
    //创建一个 native 方法,交给C实现
    public native static String helloWorld();
}

2.2创建 C 语言文件，创建 so 库
在命令行中执行如下两个命令：
cd app/src/main/java
javah com.pfj.jnidemo.HelloNDK（即：javah HelloNDK全类名）
注意：先把HelloNDK的中文注释删掉，再执行命令，否则会报错。
生成了一个com_pfj_jnidemo_HelloNDK.h 文件
然后在main目录下创建cpp目录，并将上面生成的.h文件移动到该目录
com_pfj_jnidemo_HelloNDK.h代码如下：

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

#ifndef _Included_com_pfj_jnidemo_HelloNDK
#define _Included_com_pfj_jnidemo_HelloNDK
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     com_pfj_jnidemo_HelloNDK
 * Method:    helloWorld
 * Signature: ()Ljava/lang/String;
 */
JNIEXPORT jstring JNICALL Java_com_pfj_jnidemo_HelloNDK_helloWorld
  (JNIEnv *, jclass);

#ifdef __cplusplus
}
#endif
#endif

2.3编写 c++ 实现方法
在cpp目录下创建HelloNDK.h（可为空文件）和HelloNDK.cpp文件，并在HelloNDK.cpp编写如下代码：

#include "HelloNDK.h"
#include "com_pfj_jnidemo_HelloNDK.h"

JNIEXPORT jstring JNICALL Java_com_pfj_jnidemo_HelloNDK_helloWorld
(JNIEnv *env, jclass jclass1){

  return env->NewStringUTF("你得到了Native返回数据");
}

2.4创建一个 CMakeLists.txt 文件
在app目录下创建CMakeLists.txt，代码如下：

# 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.
           HelloNDK
           # Sets the library as a shared library.
           SHARED
           # Provides a relative path to your source file(s).
        src/main/cpp/HelloNDK.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.
                    HelloNDK
                     # Links the target library to the log library
                     # included in the NDK.
                     ${log-lib} )

2.5引用 CMakeLists.txt 文件
在app目录下的build.gradle中加上如下代码：

..
android {
    compileSdkVersion 28
    buildToolsVersion '28.0.3'
    defaultConfig {
        applicationId "com.pfj.jnidemo"
        minSdkVersion 19
        targetSdkVersion 29
        versionCode 1
        versionName "1.0"
        testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
        //jni配置
        externalNativeBuild{
            cmake{
                abiFilters 'armeabi-v7a'
            }
        }
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
        }
    }
    //jni配置
    externalNativeBuild{
        cmake{
            path 'CMakeLists.txt'
        }
    }
}
..

2.6生成so文件

2.7运行程序

得到了C文件返回数据。