Android JNI使用

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经常做Android开发的朋友不可避免地会接触到一个词：JNI，其英文全称为Java-Native Interface，JNI的作用即在于通过JNI可以让我们从Java代码调用到C++代码。

一般我们在开发APP时，由于Framework层没有对应的框架实现，可能需要调用C/C++编写的代码，或者是出于安全需要将核心代码使用C/C++编写，进而通过JNI来调用。

使用JNI需要提前配置NDK，因为C++代码依赖于NDK环境。在AndroidStudio中配置NDK时首先需要通过File->Settings->System Settings->Android SDK->SDK Tools内选择NDK进行下载，并通过File->Project Structure->Android NDK location配置其路径。

AndroidStudio中配置使用JNI大致可遵循如下步骤：

1.在app/src/main目录下创建cpp目录，并添加CMakeLists.txt,其内容大致如下：

# 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.
# 设置cmake 的最小版本  一般系统自动生成
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.
#设定目标产物类型、名称、源码等信息
add_library( # Sets the name of the library.
             # 设置生成.so 的文件名，最终产物为libnative-lib.so
             native-lib
             # Sets the library as a shared library.
             #设置库的类型  一种静态文件  STATIC .a   一种动态文件  SHARED .so
             SHARED
             # Provides a relative path to your source file(s).
             # 需要编译的c/c++ 文件，以CMakeLists.txt所在目录为基准的相对目录
             src/main/cpp/main.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.
# 依赖的NDK第三方库--
find_library( # Sets the name of the path variable.
              dependencies-lib
              # Specifies the name of the NDK library that
              # you want CMake to locate.
              EGL, #依赖libEGL.so
              nativehelper #依赖libnativehelper.so
              log )#依赖liblog.so
 
# 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.
                       # 指定链接的目标库
                       native-lib
 
                       # Links the target library to the log library
                       # included in the NDK.
                       ${dependencies-lib } )

关于CMake的更多使用信息，可以参考我的这篇博客。

2.在app/build.gradle中将gradle与cmake进行关联，如下所示：

android {
   defaultConfig {
	externalNativeBuild{
      cmake{
	  cflags “_D”
	  cppflags “-frtti”,”-fexceptions”
	  abiFilters ‘x86’,’x86_64’,’armabi’,’armeabi-v7a’,’arm64-v8a’
      }
   }
    .......
    externalNativeBuild {
        cmake {
	    //通过path关键字指定CMakeLists.txt路径
	    //该路径是是以build.gradle为基准的相对路径
            path "CMakeLists.txt"
        }
    }
}

3.添加关键代码，关键代码的添加分为两部分，一部分是C/C++代码，另外一部分是Java代码。

首先是Java代码,包括加载JNI库，同时声明native 函数。如下示例：

//here we will load jni library
static {
     System.loadLibrary("natiave-lib"); //该名称对应cmake中的add_library所添加的库名
}
// Native method declaration
private native boolean nUpdateTexture(HardwareBuffer buffer, int textureId);

然后我们需要编写C/C++代码：

//必须要引用jni.h头文件
#include <jni.h>

//此外有两个重要的函数JNI_OnLoad、JNI_OnUnload是需要去完成的
//当native library被加载时（如调用System.loadLibrary）,JAVA VM将会调用JNI_Onload,返回JNI版本
JNIEXPORT jint JNI_OnLoad(JavaVM* vm, void* /*reserved*/) {

}

//当JAVA类加载器发生GC时，该函数将会被JAVA VM进行调用。通常该函数将用于清理工作。
JNIEXPORT void JNI_OnUnload(JavaVM* vm, void* /*reserved*/) {

}

此外我们还需要了解一下JNI中的数据类型，以便将Java中的数据类型与native原生类型做匹配:

Java Type	Native Type	Description
boolean	jboolean	unsigned 8 bits
byte	jbyte	signed 8 bits
char	jchar	unsigned 16 bits
short	jshort	signed 16 bits
int	jint	signed 32 bits
long	jlong	signed 64 bits
float	jfloat	32 bits
double	jdouble	64 bits
void	void	N/A