AndroidStudio 3.0 configures NDK, and writes C++ and Java interaction cases

NDK introduction :

The Android NDK is a set of tools that allow you to embed C or C++ ("native code") into Android applications.

Java Native Interface (JNI) :

JNI is the interface that Java and C++ components use to communicate with each other.

android studio configuration ndk

1. Download ndk in Studio :

In the studio, setting -->Android SDK -->SDK Tools, check the three tools of CMake, LLDB, and NDK to download. After the download is successful, as shown in the figure below:

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Personally, it is recommended to download offline ndk, there may be problems with the SDK Manager download.

2. Configure the ndk path in the environment variable :

Open the environment variable, create NDK_ROOT, enter ndk文件夹所在路径, and add the variable to Path, as shown in the following figure:
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3. Existing problems (if not encountered, skip this step) :

Run the created ndk project, studio 3.0 reports an error:

Error:No toolchains found in the NDK toolchains folder for ABI with prefix: mips64el-linux-android

Open the ndk directory downloaded through the SDK Manager, and find the toolchains folder, as shown in the following figure:
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To download the ndk offline package, select the corresponding platform, ndk download address . After the download is complete, copy the missing files or replace the entire NDK folder.

Android development first NDK case

Android Studio creates a project that supports c++, as shown below:

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After the creation is complete, you will find a few more things than pure Java projects.

The first is the main/cpp folder, which contains a c++ source file.

The second is CMakeLists.txt, which AndroidStudio uses to compile the c++ file code

In addition, the configuration of camke is also added in the project's build.gradle.

At this point, run the project, and the textview will display "Hello from C++". If you are only satisfied with this, it is still too simple, and you can't even get started.

Write a C++ file by hand, and interact with Java through the jni interface .

1. Java static loads so library and native defined functions

In Activity, load the so library through the static static code block, and the library is named test-lib

    static {
        //加载c++文件
        System.loadLibrary("test-lib");
    }
    

   //native定义函数
   public  native  String getContentFromJNI();
   
   public void showContent(){
         textView.setText(getContentFromJNI());
   }

2. Write C++ code :

Right-click under the main/cpp folder, create a C/C++ Source File, and enter the file name test-lib.

First use #includethe command to import header files, such as jni, string and other header files, and add the namespace std.
Write the code as follows:

#include <string>
#include <jni.h>
using  namespace std;

Write the functions defined by native in the java layer, and the corresponding c++ layer functions:

Define the return value first

//jstring是Java原生接口规定的数据类型，它是指向java字符串的指针
JNIEXPORT jstring

The function name corresponding to c++ is the Java function name and the path of the file where the function name is located

JNICALL
Java_com_xingen_myapplication_MainActivity_getContentFromJNI( JNIEnv* env,jobject){
    
}

JNIEnv* is a pointer to the virtual machine and jobject is a pointer to the implicit this object passed from the Java side.

Naming rules:

Prepend Java_ to the name.
Describes the file path relative to the top-level source directory.
Use underscores instead of forward slashes.
Remove the .java file extension.
After the last underscore, append the function name.

Write the c++ code for the return value in the function:

    string content="android通过jni与c++交互的结果";
    return  env->NewStringUTF(content.c_str());

The complete test-lib.cpp file code is as follows:

// Created by $HeXinGen on 2018/11/20.
//
#include <string>
#include <jni.h>
using  namespace std;

extern "C"
//jstring是Java原生接口规定的数据类型，它是指向java字符串的指针
//
JNIEXPORT jstring

JNICALL
Java_com_xingen_myapplication_MainActivity_getContentFromJNI( JNIEnv* env,jobject){
    string content="android通过jni与c++交互的结果";
    return  env->NewStringUTF(content.c_str());
};

3. Configure the written c++ file into CMakelist.txt :

The first step: Create a new add_library, add the c++ source file name and file path

#添加可执行文件所需要的库
add_library( # Sets the name of the library.
                          test-lib

                          # Sets the library as a shared library.
                          SHARED

                          # Provides a relative path to your source file(s).
                          src/main/cpp/test-lib.cpp )

Step 2: Add c++ source file name to target_link_libraries, here is test-lib

target_link_libraries( # Specifies the target library.
                       native-lib
                       test-lib

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

4. The running effect is as follows :

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Resource reference :

android NDK documentation : https://developer.android.google.cn/ndk/guides/
How to write CMakeList in Android Studio : https://www.jianshu.com/p/39ca6c51bc20