Android逆向：AndroidStudio JNI编写SO

Android逆向：AndroidStudio JNI编写SO

本文是在​​这篇博客​​​的基础上开发的so。 另外，也可以自己从头开发​​​Native C++​​应该更简单一些，不会有这么多坑。

JNI（Java Native Interface），他是java平台的特性，不是安卓系统提供的。他定义了一些JNI函数，来让开发者可以通过调用这些函数来实现java代码调用C／C++代码。

C++实现

新建java 类文件：​​src\main\java\com\lilongsy\TestJNI.java​​，用Java实现逻辑。

package com.lilongsy;public class TestJNI { static { System.loadLibrary("JniTest"); } public static native String sayHello();}

在java目录下，运行javah命令，生成​​com_lilongsy_TestJNI.h​​头文件：

D:\java-test\MyApplication\app\src\main\java>javah com.lilongsy.TestJNI

/* DO NOT EDIT THIS FILE - it is machine generated */#include /* Header for class com_lilongsy_TestJNI */#ifndef#define#ifdefextern "C" {#endif/* * Class: com_lilongsy_TestJNI * Method: sayHello * Signature: ()Ljava/lang/String; */JNIEXPORT jstring JNICALL Java_com_lilongsy_TestJNI_sayHello (JNIEnv *, jclass);#ifdef}#endif#endif

在main下新建​​jni​​文件夹

把​​com_lilongsy_TestJNI.h​​​头文件剪切到jni文件夹下，并新建​​main.cpp​​文件，其代码如下：

#include "com_lilongsy_TestJNI.h"JNIEXPORT jstring JNICALL Java_com_lilongsy_TestJNI_sayHello (JNIEnv *env, jclass obj) { return env->NewStringUTF((char *)"登录成功_JNI");};

设置CMake编译环境

在AndroidStudio File > Settings > Android SDK > SDK Tools下，安装NDK、CMake、LLDB（国内需要单独下载到cmake同级目录，并用lldb小写）。

另外一种方法是手动下载NDK，下载地址：​​For more information about using CMake with Android Studio, read the# documentation: Sets the minimum version of CMake required to build the native library.#CMakeLists.txtcmake_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. # 设置so文件名称. JniTest # Sets the library as a shared library. SHARED # 设置这个so文件为共享. # Provides a relative path to your source file(s). # 设置这个so文件为共享. src/main/jni/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.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. # 制定目标库. JniTest # Links the target library to the log library # included in the NDK. ${log-lib} )

配置build.gradle

为了自动构建时，调用cmake，在APP>​​build.gradle​​​的​​android​​加入如下代码：

externalNativeBuild { cmake { path file('CMakeLists.txt') }}

为了让cmake生成多个abi下的so文件，在在APP>​​build.gradle​​​的​​defaultConfig​​加入如下代码：

externalNativeBuild { cmake { cppFlags "" //生成多个版本的so文件 abiFilters 'arm64-v8a','armeabi-v7a','x86','x86_64' }}

重构（rebuild project）

在​​build​​​下，​​rebuild project​​：

会在\app\build\intermediates\merged_native_libs目录下生成不同abi的so文件。

调用so

在app\src\main目录下新建​​jniLIB​​文件夹，并把上文生成的lib下的所有内容都复制到jniLIB下：

在MainActivity，加入如下代码，测试​​TestJNI.sayHello()​​的返回值：

Toast.makeText(MainActivity.this, TestJNI.sayHello(), Toast.LENGTH_SHORT).show();

运行APK，登录成功会有Toast提示：​​登录成功_JNI​​。

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