Android的JNI【实战教程】5⃣️---Android Studio 2.2 以上 NDK开发

先知先觉

发布于 2019-01-21 14:37:41

5910

发布于 2019-01-21 14:37:41

今天介绍一下Android Studio 2.2 下 NDK开发，那叫一个顺溜—-纵享丝滑! 虽然现在AS 2.2 之后，jni开发配置相当方便，但是还是建议大家从我的第一篇文章看起，从基础知识入手，并且要了解之前是如何配置NDK工程的，这是一个循序渐进的过程。

今天主要介绍一下如何分别在新工程和老工程中创建最新NDK项目。

新工程创建Ndk Project

创建

创建时候勾选 include C++ support。

在AS 2.2 之后，当我们创建工程的时候会在下面多出一个勾选框，是否支持C++，这里我们勾选，然后一路next。

然后最后一步我们会看到如下界面：

这里不一定所有的同学都能跑起来，是因为我在之前的博客 http://blog.csdn.net/github_33304260/article/details/62891083 里面配置过NDK，这里就不相信讲解了，大家可以参考之前的文章，如果有问题可以留言。

好啦，就这么简单创建成功啦，已经可以运行啦，我们跑起来看一下。

就是这么纵享丝滑，不过很多童鞋对于新的结构可能不熟悉那么接下来就行详细讲解一下新的结构和文件。

项目一览

1.目录结构

勾选了include C++ support 就会多出以下几个目录及文件。

.externalNativeBuild —> cmake 编译好的文件, 显示支持的各种硬件等信息存放 .so 文件等Cmake相关文件
cpp —> 众所周知 JNI Folder 。C 语言程序的逻辑部分, native-lib.cpp 文件名可自行修改
CMakeLists.txt —> CMake 脚本配置的文件, 具体可查阅 CMake官网的资料

2.build.gradle中的Cmake

    externalNativeBuild {
         cmake {
            cppFlags ""
         }
   }

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

Tips: 在Gradle里面多了以上两部分内容。当然这里默认的配置信息比较少，配置配置指令具体可查阅 CMake 官网。

3.CMakeLists.txt文件中的具体配置

# 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  # 这个是jni编译生产的so库的名字

             # Sets the library as a shared library.
             SHARED

             # Provides a relative path to your source file(s).
             src/main/cpp/native-lib.cpp ) # 要编译的c/c++文件列表 文件路径想对于cmake文件路径

# 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 # 依赖的系统so库

              # 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} )

4·MainAvtivity中新增的方法

1、在java代码中增加引用so库的代码,使代码生效

    // Used to load the 'native-lib' library on application startup.
    static {
        //此处的form库的名称需要和CMakeLists.txt中配置的相同
        System.loadLibrary("native-lib");
    }

2、定义方法

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

5· native-lib.cpp

里面可以直接的创建cpp源代码,和ndkBuild一样,用C/C++所写的源代码中的方法名称必须是全路径的方法名,然后以Java开头,分割使用下划线.

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

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

extern “C” ：暴露给外部使用。确保所有Java需要调用的C方法都放在extern “C”中，这样CMake才会帮我们正确编译。

好啦到这里新鲜东西就已经都讲完了。接下来看看如何在旧工程中添加最新NDK Project。

老工程导入Ndk Project

1 选择app—> 右键 New —> Folder —> JNI Folder 。

选中 Change Folder Location 将JNI更换成cpp （为了统一，系统自动生成的是cpp文件 CMakeLists.txt里面配置也是cpp路径）

cpp文件夹这个文件夹颜色和java一样说明你创建对了。

2 选择 jni —> New —> C/C++Source File

3 完善native-lib.cpp文件

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

extern "C"

}

4 在APP目录下添加 CMakeLists.txt 文件

代码和上面创建后生成的一样，具体功能自行添加。 切记： jni编译生产的so库的名字和路径要正确

# 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  # 这个是jni编译生产的so库的名字

             # Sets the library as a shared library.
             SHARED

             # Provides a relative path to your source file(s).
             src/main/cpp/native-lib.cpp ) # 要编译的c/c++文件列表 文件路径想对于cmake文件路径

# 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 # 依赖的系统so库

              # 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} )

5 make Project

6 关联CMakeLists.txt app — > Link C++ Project with Gradle

再次查看Gradle 发现多了如下代码：

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

或者手动在Gradle添加上面代码。

7 引用so库

在MainActivity中引用：

public class MainActivity extends AppCompatActivity {
    //引用so库
    static {
            System.loadLibrary("native-lib");
    }

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

8 编写第一个jni方法

public native String stringFromJNI();

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