001_Android Studio使用CMake工具和C语言混合编译

1.官方例子

Android Studio到2.2版本之后，在新建工程时，界面上多了一个Include C++ Support的选项。勾选它之后将会创建一个默认的C++与JAVA混编的示例工程。下面简单介绍一下这个工程

1.1 新建工程

勾选"include C++ support",然后点击"next",选择“Empty Activity”，最后点击“Finish”,这样和C++混合编译的工程就创建完毕。

.

1.2 混和编译的差异

项目打开后选择“Project”选项查看目录结构，工程里多出了.externalNativeBuild文件夹、cpp文件夹、CMakeLists.txt，如下图：

• 这三项说明如下：

  1. externalNativeBuild文件夹：通过cmake工具编译输出的文件, 显示支持的各种硬件等信息。系统生成。
  2. cpp文件夹：存放C/C++代码文件，native-lib.cpp文件是该示例工程中自带的，可以根据自己需求修改。
  3. CMakeLists.txt文件：CMake工具编译C语言的脚本配置的文件。需要根据自己需求进行相应的修改。

• app目录下的build.gradle文件中有两处不一样，如下图所示：
  !

• MainActiviy文件中调用C++文件内容的方式如下图：

  • 前面表示加载哪个C++文件；
  • 后面表示对C++文件中的函数进行引用说明。
    

package com.zhoujiazhao.myapplication;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity {

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

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        // Example of a call to a native method
        TextView tv = (TextView) findViewById(R.id.sample_text);
        tv.setText(stringFromJNI());
    }

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

• 此外，C++文件中的函数名要和自己工程的包名和引用的类名要对应，我的是Java_com_zhoujiazhao_myapplication_MainActivity

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

extern "C"
JNIEXPORT jstring

JNICALL
Java_com_zhoujiazhao_myapplication_MainActivity_stringFromJNI(
        JNIEnv *env,
        jobject /* this */) {
    std::string hello = "Hello from C++";
    return env->NewStringUTF(hello.c_str());
}

2.在其它工程中添加和C混合编译

2.1 创建“cpp"文件夹和“c"源码

这里文件名为“hello.c”

//
// Created by jiazhao on 2018-06-28.
//

#include <jni.h>


jint Java_com_zhoujiazhao_zhaoc_MainActivity_Add(
        JNIEnv *env,
        jobject obj, jint a, jint b) {
    return a + b;
}

2.2 build.gradle文件中添加如下代码：

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

2.3 创建CMakeLists.txt文件

这里部分代码可以从官方示例工程里面直接拷贝过来，进行修改，需根据C文件名进行修改，如下图所示：

# 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.
             hello

             # Sets the library as a shared library.
             SHARED

             # Provides a relative path to your source file(s).
             src/main/cpp/hello.c )

# 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.
                       hello

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

2.4 C文件中函数的调用的方法和官方示例工程类似，MainActivity文件如下：

package com.zhoujiazhao.zhaoc;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity {

    static {
        System.loadLibrary("hello");
    }

    private TextView mTextView = null;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        mTextView = (TextView)findViewById(R.id.hello_text_view);
        mTextView.setText("aaa:" + Integer.toString(Add(3, 5)));
    }

    public native int Add(int a, int b);
}

2.5生产xx.so库

点击菜单“Build -> Make project”生成.so文件

.so文件目录：app-build-intermediates-cmake-debug-obj

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