在Android开发中我们经常要把一些比较看重安全或者计算效率的东西通过JNI调用C/C++代码来实现，如果需要实现的功能简单或者你的C/C++代码能力比较强，但是目前还是有很多功能强大的第三方库的，比如openssl、FFmpeg等，调用这些第三方实现显然比重复造轮子实际的多。
本教程适合将原始的动态库（.so），即没有包含JNI方法因而java无法直接调用的库链接到自己的C/C++代码中，然后提供调用。
链接系统标准库 # 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 ) 复制代码 链接静态库 ## libpng动态库的设置 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) 复制代码 链接静态库 ## 引入libssl动态库 add_library(ssl SHARED IMPORTED) set_target_properties( ssl PROPERTIES IMPORTED_LOCATION ${CMAKE_SOURCE_DIR}/libs/${ANDROID_ABI}/libssl.so) 复制代码添加链接外部静态库和动态库的流程差不多，用 STATIC 和 SHARED 来区分
${CMAKE_SOURCE_DIR} 是CMake 中预定义的常量，指当前工程的 CMake 文件所在路径，其他比较有用的常量：
CMAKE_CURRENT_SOURCE_DIR ： 指当前 CMake 文件所在的文件夹路径
CMAKE_CURRENT_LIST_FILE ： 指当前 CMake 文件的完整路径
PROJECT_SOURCE_DIR ：指当前工程的路径
最后将所有库链接起来就行了：
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} ) 复制代码 实战：引入openssl库供android使用openssl 是一款强大的加解密库，提供了RSA、AES、MD5等常用的加密算法，网上也有很多编译openssl动态库和静态库的方法。 项目的文件结构目录如下：
拷贝openssl的so文件到lib文件夹下 拷贝openssl的头文件到cpp的include目录 编写cmake文件 # 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) #配置加载头文件 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} ) #动态方式加载 add_library(openssl SHARED IMPORTED ) add_library(ssl SHARED IMPORTED ) #引入第三方.so库 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} ) 复制代码 编写代码调用openssl头文件里面提供的方法： extern "C" JNIEXPORT jstring JNICALL Java_org_hik_arraytest_MainActivity_md5(JNIEnv *env, jobject instance, jbyteArray src_) { Log_d("MD5->信息摘要算法第五版"); 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->进行MD5信息摘要运算"); MD5_Update(&ctx, src, src_Len); MD5_Final(digest, &ctx); strcpy(hex, ""); Log_d("MD5->把哈希值按%%02x格式定向到缓冲区"); for (int i = 0; i != sizeof(digest); i++) { sprintf(buff, "%02x", digest[i]); strcat(hex, buff); } Log_d("MD5->%s", hex); Log_d("MD5->从jni释放数据指针"); env->ReleaseByteArrayElements(src_, src, 0); return env->NewStringUTF(hex); } 复制代码 java里调用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信息摘要->" + md5("1111".getBytes()).toUpperCase()); } /** * MD5编码 */ public native String md5(byte[] src); } 复制代码
结果如下：
遇到的错误： 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" 复制代码 java.lang.UnsatisfiedLinkError: dlopen failed: library "libcrypto.so" not found 复制代码
在app的 build.gradle 中指定第三方so文件目录，不然生成apk文件的时候不会包含这些so文件。
sourceSets { main { jniLibs.srcDirs = ['libs'] } } 复制代码