Android NDK开发之引入第三方库

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 )

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)

链接动态库

## 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)

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} )

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} )

• 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);
}

• 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);

}

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"

 java.lang.UnsatisfiedLinkError: dlopen failed: library "libcrypto.so" not found

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']
        }
    }