JNI学习大纲

一、JNI编程入门

二、Android Studio中使用cmake开发JNI实战

第一章节我们介绍了JNI的开发步骤，那这一章节我们就开始在Android Studio中实战一下吧，Let's Start。

1. Android Studio中安装CMake插件

• AS中菜单栏选择Tools>SDK Manager
• 在Android SDK中选择SDK Tools，安装CMake和NDK。

2. JNI开发

2.1 编写JNI代码

在项目工程下的src/main创建cpp目录，编写native-lib.cpp（JNI代码实现文件）和对应的CMakeLists.txt（JNI代码编译配置）。

// native-lib.cpp

#include <jni.h>
#include <android/log.h>
#include <string>

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

// CMakeLists.txt

# Sets the minimum version of CMake required to build the native library.
cmake_minimum_required(VERSION 3.10.2)

# Declares and names the project.
project("jnitest")

# 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.
set(project_root_dir ${CMAKE_CURRENT_SOURCE_DIR}/../../../..)
include_directories(${project_root_dir}/common)

add_library( 
    # Sets the name of the library.
    jnitest

    # Sets the library as a shared library.
    SHARED

    # Provides a relative path to your source file(s).
    native-lib.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.

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

2.2 gradle中编译配置

android {
    ...
    defaultConfig {
        ...
        externalNativeBuild {
            // 设置生成so的arm架构
            cmake {
                cppFlags ''
                abiFilters 'arm64-v8a'
            }
        }
    }

    externalNativeBuild {
        cmake {
            // 编译
            path file('src/main/cpp/CMakeLists.txt')
            version '3.10.2'
        }
    }
}