在Android開發中我們經常要把一些比較看重安全或者計算效率的東西通過JNI呼叫C/C++程式碼來實現,如果需要實現的功能簡單或者你的C/C++程式碼能力比較強,但是目前還是有很多功能強大的第三方庫的,比如openssl、FFmpeg等,呼叫這些第三方實現顯然比重複造輪子實際的多。
本教程適合將原始的動態庫(.so),即沒有包含JNI方法因而java無法直接呼叫的庫連結到自己的C/C++程式碼中,然後提供呼叫。
連結系統標準庫
# 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 )
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連結靜態庫
## libpng動態庫的設定
add_library( # Sets the name of the library.
png
# Sets the library as ashared library.
STATIC
# Provides a relative pathto your source file(s).
IMPORTED)
set_target_properties(
png
PROPERTIES IMPORTED_LOCATION
${CMAKE_SOURCE_DIR}/libs/${ANDROID_ABI}/libpng.a)
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連結動態庫
## 引入libssl動態庫
add_library(ssl SHARED IMPORTED)
set_target_properties(
ssl
PROPERTIES IMPORTED_LOCATION
${CMAKE_SOURCE_DIR}/libs/${ANDROID_ABI}/libssl.so)
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新增連結外部靜態庫和動態庫的流程差不多,用STATIC
和SHARED
來區分
${CMAKE_SOURCE_DIR}
是CMake 中預定義的常量,指當前工程的 CMake 檔案所在路徑,其他比較有用的常量:
-
CMAKE_CURRENT_SOURCE_DIR
: 指當前 CMake 檔案所在的資料夾路徑 -
CMAKE_CURRENT_LIST_FILE
: 指當前 CMake 檔案的完整路徑 -
PROJECT_SOURCE_DIR
:指當前工程的路徑
最後將所有庫連結起來就行了:
target_link_libraries( # Specifies the target library.
native-lib
png
openssl
ssl
android
# Links the target library to the log library
# included in the NDK.
${log-lib} )
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實戰:引入openssl庫供android使用
openssl是一款強大的加解密庫,提供了RSA、AES、MD5等常用的加密演算法,網上也有很多編譯openssl動態庫和靜態庫的方法。 專案的檔案結構目錄如下:
- 拷貝openssl的so檔案到lib資料夾下
- 拷貝openssl的標頭檔案到cpp的include目錄
- 編寫cmake檔案
# 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)
#配置載入標頭檔案
include_directories(./src/main/cpp/include)
file(GLOB mian_src "src/main/cpp/*.cpp")
# 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.
cipher
# Sets the library as a shared library.
SHARED
# Provides a relative path to your source file(s).
${mian_src} )
#動態方式載入
add_library(openssl SHARED IMPORTED )
add_library(ssl SHARED IMPORTED )
#引入第三方.so庫
set_target_properties(openssl PROPERTIES IMPORTED_LOCATION ${CMAKE_SOURCE_DIR}/libs/${ANDROID_ABI}/libcrypto.so)
set_target_properties(ssl PROPERTIES IMPORTED_LOCATION ${CMAKE_SOURCE_DIR}/libs/${ANDROID_ABI}/libssl.so)
# 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.
cipher
openssl
ssl
android
# Links the target library to the log library
# included in the NDK.
${log-lib} )
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- 編寫程式碼呼叫openssl標頭檔案裡面提供的方法:
extern "C"
JNIEXPORT jstring JNICALL
Java_org_hik_arraytest_MainActivity_md5(JNIEnv *env, jobject instance, jbyteArray src_) {
Log_d("MD5->資訊摘要演算法第五版");
jbyte *src = env->GetByteArrayElements(src_, NULL);
jsize src_Len = env->GetArrayLength(src_);
char buff[3] = {'\0'};
char hex[33] = {'\0'};
unsigned char digest[MD5_DIGEST_LENGTH];
MD5_CTX ctx;
MD5_Init(&ctx);
Log_d("MD5->進行MD5資訊摘要運算");
MD5_Update(&ctx, src, src_Len);
MD5_Final(digest, &ctx);
strcpy(hex, "");
Log_d("MD5->把雜湊值按%%02x格式定向到緩衝區");
for (int i = 0; i != sizeof(digest); i++) {
sprintf(buff, "%02x", digest[i]);
strcat(hex, buff);
}
Log_d("MD5->%s", hex);
Log_d("MD5->從jni釋放資料指標");
env->ReleaseByteArrayElements(src_, src, 0);
return env->NewStringUTF(hex);
}
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- java裡呼叫jni方法
public class MainActivity extends AppCompatActivity {
String TAG = "my_openssl";
static {
System.loadLibrary("cipher");
}
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Log.i(TAG, "MD5資訊摘要->" + md5("1111".getBytes()).toUpperCase());
}
/**
* MD5編碼
*/
public native String md5(byte[] src);
}
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結果如下:
遇到的錯誤:
java.lang.UnsatisfiedLinkError:
dlopen failed: library "/system/lib64/libcrypto.so" needed or dlopened by "/system/lib64/libnativeloader.so"
is not accessible for the namespace "classloader-namespace"
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java.lang.UnsatisfiedLinkError: dlopen failed: library "libcrypto.so" not found
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在app的build.gradle
中指定第三方so檔案目錄,不然生成apk檔案的時候不會包含這些so檔案。
sourceSets {
main {
jniLibs.srcDirs = ['libs']
}
}
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