【Android】AS添加JNI支持

JNI入坑

索引：

1. 旧 NDK 使用姿势(AS 2.2 以下 参考链接：Create hello-JNI with android studio)
2. 新 NDK 使用方法(AS 2.2及以上 参考链接：Add C and C++ Code to Your Project)

参考：
JNI Tips
(链接均需VPN)

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旧 NDK-build：

简单来说分这么几步：

1. 添加ndk库支持
2. 在 gradle.properties中添加：

android.useDeprecatedNdk=true

2. 在 app级build.gradle中添加：

  defaultConfig {
        ndk {
            moduleName "test-jni" // name the jni lib you want to add
        }
}

3. 创建load 静态块，添加待实现native api

public class JniTest {
    static {
        System.loadLibrary("test-jni");
    }

    public native String getMsgFromJni();
}

在 目标api所在行停留一会，会出现红色灯泡提示按钮，点击图标，会有Create Function Java_%full class name%_getMsgFromJni选项出现；点击之后会自动创建一个与java目录并列的cpp目录，并在目录下生成test-jni.c文件
具体代码如下：

#include <jni.h>

JNIEXPORT jstringJNICALL
Java_com_zealens_face_jnitest_JniTest_getMsgFromJni(JNIEnv
*env, jobject instance)
{

// TODO
return (*env)->NewStringUTF(env, returnValue);
}

说明：

1. 对于第3行代码：
   AS生成的代码有错误，需将JNICALL与前面的jstring隔开；
2. 将returnValue修改为常量值（eg："hello"）

至此，build之后调用API就能得到刚才填入的hello值。

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新CMake

AS现在默认的C++工具已经修改为CMake（需要在SDK-Tools中安装CMake、LLDB、NDK），旧的ndk-build库也可以手动导入

准备工作：

1. 如果有则移除：

// Remove this line
android.useDeprecatedNdk = true

1. install build tools

（略：创建新的C++support Application）

1. 向现有项目添加C/C++ 代码：
   a. 新建cpp目录；
   b. 添加cpp文件，代码如下：

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

extern "C"
JNIEXPORT jstring JNICALL
Java_%your test class full name%_stringFromJNI(
        JNIEnv* env,
        jobject /* this */) {
    std::string hello = "Hello from C++";
    return env->NewStringUTF(hello.c_str());
}

在欲链接cpp的根目录中（与build.gradle同级）创建文件：CMakeLists.txt，键入如下代码：（各API含义见参考链接末尾部分，或 CMake command）

# 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).
             src/main/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} )

之后，在同一模块目录上单击右键，选择link c++...，选中欲链接的cpp文件，build.gradle就会生成如下代码：（android模块中）

    externalNativeBuild {
        cmake {
            path "CMakeLists.txt"
        }
    }

此时再调用相应API就可以看到效果了：

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

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

pay attention to the lib name

高阶设置请参考手册，这里也只是介绍一个最小的CMake环境

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