Using CMake for JNI development notes in Android project development

I wanted to make a Demo for JNI practice. Since I was using a previously created project and C++ was not included when creating the project, I reviewed the article "Using CMake for JNI Development (Android Studio)" in Android Notes . Following the steps in this article, it was possible before, but a configure Error occurred during compilation. Finally, I found the Android studio synchronization project failed: External Native Build Issues: Error configuring , which said it was a problem with the gradle plug-in version, so I changed the gradle The plug-in version solves the problem.

The current environment and implementation steps are re-recorded here for comparison and update in the future.

environment:

Android Studio 3.1.4
gradle plugin is 3.2.1 , which can be viewed in build.gradle in the root directory of the project

buildscript {
    
    repositories {
        google()
        jcenter()
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:3.2.1'  // gradle插件版本
        

        // NOTE: Do not place your application dependencies here; they belong
        // in the individual module build.gradle files
    }
}

allprojects {
    repositories {
        google()
        jcenter()
    }
}

task clean(type: Delete) {
    delete rootProject.buildDir
}

Add steps

1. Add CMakeLists.txt
2. Module configuration CMakeLists.txt
3. Write C/CPP file
4.java call

It is assumed here that the C++ file JniLib.cpp has been written under src/main/cpp

1. Create a new CMakeLists.txt under the project module.
Here we take the app module as an example, as shown below:

The content of CMakeLists.txt is as follows:

# 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.
			#库名,可修改
             JniLib 

             # Sets the library as a shared library.
             SHARED

             # Provides a relative path to your source file(s).
             # c++源代码文件路径
             src/main/cpp/JniLib.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.

# 不是必须添加的
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.

# 不是必须添加的
target_link_libraries( # Specifies the target library.
                       JniLib

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

2. The configuration module supports jni calling.
Right-click on the project module, select Link C++ Project with Gradle, and

select the newly configured CMakeLists.txt file in the pop-up prompt box:

Wait for the compilation to complete, and the following content will be generated in the android node of the module.

android {
    ...
    externalNativeBuild {
        cmake {
            path 'CMakeLists.txt'
        }
    }
}

Note: For gradle plugins below 3.2.0, the externalNativeBuild node is added, and a configure error will be reported during compilation.

4. Write the C/CPP file
JniLIb.cpp

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

extern "C" JNIEXPORT jstring // 指定方法的返回类型，不设置的话会报错
JNICALL
Java_com_sharedream_demo_activity_JNIDemoActivity_getMessage(
      JNIEnv *env,
      jobject /* this */) {
    
    
  std::string hello = "Hello from C++";
  return env->NewStringUTF(hello.c_str());
}

Note: Each JNI method requires extern "C" JNIEXPORT <JNI type> , JNIEXPORT <JNI type> is used to control the return type of the function, otherwise an error will be reported
5.java call

public class JNIDemoActivity extends BaseActivity {
    
    

    @BindView(R.id.tv_jni_message)
    TextView tvJNIMessage;

    static {
    
    
        // 1.导入对应的so库
        System.loadLibrary("JniLib");
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
    
    
        super.onCreate(savedInstanceState);
        // 3.调用
        String message = getMessage();
        Logger.d("message = " + message);
        tvJNIMessage.setText(message);
    }

    @Override
    protected int getLayoutId() {
    
    
        return R.layout.activity_jni_demo;
    }

    // 2.声明本地方法
    public static native String getMessage();
}

reference

Android Notes: Using CMake for JNI Development (Android Studio)