Android項目開發中使用CMake進行JNI開發筆記

想弄個Demo進行JNI練習，由於使用的是之前創建的項目，且創建項目時，並沒有包含C++，然後就回顧了Android筆記之使用CMake進行JNI開發（Android Studio）這篇文章。按着這個文章步驟做，以前是可以的，但是在編譯時報了configrure Error，最後找到了Android studio 同步工程失敗：External Native Build Issues: Error configuring，說是gradle插件版本的問題，按着改了gradle插件版本就解決了問題。

這裏重新記錄下當前環境和實現步驟，以後好作個對比更新。

環境：

Android Studio 3.1.4
gradle插件是3.2.1 ，可在項目的根目錄的build.gradle中查看

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
}

添加步驟

1.添加CMakeLists.txt
2.模塊配置CMakeLists.txt
3.編寫C/CPP文件
4.java調用

這裏假設已經在src/main/cpp下編寫好了C++文件JniLib.cpp

1.在工程模塊下新建一個CMakeLists.txt
這裏以app模塊爲例，如下圖：

CMakeLists.txt內容如下：

# 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.
			#庫名,可修改
             JniLib 

             # Sets the library as a shared library.
             SHARED

             # Provides a relative path to your source file(s).
             # c++源代碼文件路徑
             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.

# 不是必須添加的
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.

# 不是必須添加的
target_link_libraries( # Specifies the target library.
                       JniLib

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

2.配置模塊支持jni調用
對項目模塊右鍵，選擇Link C++ Project with Gradle

在彈出的提示框中選擇剛新建配置的CMakeLists.txt文件：

等待編譯完成，會在模塊的android節點中生成如下內容

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

注意： gradle插件3.2.0以下，添加添加了externalNativeBuild節點，編譯時會報configure error。

4.編寫C/CPP文件
JniLIb.cpp

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

extern "C" JNIEXPORT jstring // 指定方法的返回類型，不設置的話會報錯
JNICALL
Java_com_sharedream_demo_activity_JNIDemoActivity_getMessage(
      JNIEnv *env,
      jobject /* this */) {
  std::string hello = "Hello from C++";
  return env->NewStringUTF(hello.c_str());
}

注意： 每個JNI方法，都需要 extern “C” JNIEXPORT <JNI類型> ,JNIEXPORT <JNI類型> 用於控制函數的返回類型，否則會報錯
5.java調用

public class JNIDemoActivity extends BaseActivity {

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

    static {
        // 1.導入對應的so庫
        System.loadLibrary("JniLib");
    }

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

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

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

參考

Android筆記之使用CMake進行JNI開發（Android Studio）