How can I link a JNI file to an external Android C library?

Question

I'm trying to use mbedtls in my Android project. It uses CMake, conveniently and it seems like the easiest way to use it is to keep it as-is, and then add an extra C file that contains JNI wrappers.

I've downloaded the source to ./app/mbedtls and added this to ./app/build.gradle :

defaultConfig {
    ...
    externalNativeBuild {
        cmake {
            arguments "-DENABLE_TESTING=OFF" // Testing requires Perl which I don't have.
        }
    }
}
...
externalNativeBuild {
    cmake {
        path "mbedtls/CMakeLists.txt"
    }
}

After doing that the gradle build finishes successfully (somewhat amazingly!) although it doesn't actually generate any libx.a files.

I have also created a file ./app/CMakeLists.txt containing this:

cmake_minimum_required(VERSION 3.4.1)
add_library(mbedtls_jni SHARED src/main/cpp/mbedtls_jni.cpp)

And I have created that file. But how can I get it to link that file and mbedtls together into one library? Ideally without modifying anything in mbedtls.

Answer 1

Try this:

1. ./app/src/main/cpp/mbedtls

Copy all mbedtls library files(*.c and *.h) to mbedtls folder then rename all *.c to *.cpp

2. ./app/src/main/cpp/yourmain.cpp

Your main program (yourmain.cpp and yourmain.h) in cpp folder.

3. ./app/CMakeLists.txt

cmake_minimum_required(VERSION 3.4.1)
    option(USE_SHARED_MBEDTLS_LIBRARY "Build mbedtls TLS shared library." ON)

    find_library( log-lib
                  log )

    set(src_mbed
        src/main/cpp/mbedtls/aes.cpp
        src/main/cpp/mbedtls/aesni.cpp
        src/main/cpp/mbedtls/arc4.cpp
        src/main/cpp/mbedtls/asn1.cpp
        src/main/cpp/mbedtls/asn1write.cpp
        src/main/cpp/mbedtls/base64.cpp
        src/main/cpp/mbedtls/bignum.cpp
        src/main/cpp/mbedtls/blowfish.cpp
        src/main/cpp/mbedtls/camellia.cpp
        src/main/cpp/mbedtls/ccm.cpp
        src/main/cpp/mbedtls/cipher.cpp
        src/main/cpp/mbedtls/cipher_wrap.cpp
        src/main/cpp/mbedtls/cmac.cpp
        src/main/cpp/mbedtls/ctr_drbg.cpp
        src/main/cpp/mbedtls/des.cpp
        src/main/cpp/mbedtls/dhm.cpp
        src/main/cpp/mbedtls/ecdh.cpp
        src/main/cpp/mbedtls/ecdsa.cpp
        src/main/cpp/mbedtls/ecjpake.cpp
        src/main/cpp/mbedtls/ecp.cpp
        src/main/cpp/mbedtls/ecp_curves.cpp
        src/main/cpp/mbedtls/entropy.cpp
        src/main/cpp/mbedtls/entropy_poll.cpp
        src/main/cpp/mbedtls/error.cpp
        src/main/cpp/mbedtls/gcm.cpp
        src/main/cpp/mbedtls/havege.cpp
        src/main/cpp/mbedtls/hmac_drbg.cpp
        src/main/cpp/mbedtls/md.cpp
        src/main/cpp/mbedtls/md2.cpp
        src/main/cpp/mbedtls/md4.cpp
        src/main/cpp/mbedtls/md5.cpp
        src/main/cpp/mbedtls/md_wrap.cpp
        src/main/cpp/mbedtls/memory_buffer_alloc.cpp
        src/main/cpp/mbedtls/oid.cpp
        src/main/cpp/mbedtls/padlock.cpp
        src/main/cpp/mbedtls/pem.cpp
        src/main/cpp/mbedtls/pk.cpp
        src/main/cpp/mbedtls/pk_wrap.cpp
        src/main/cpp/mbedtls/pkcs12.cpp
        src/main/cpp/mbedtls/pkcs5.cpp
        src/main/cpp/mbedtls/pkparse.cpp
        src/main/cpp/mbedtls/pkwrite.cpp
        src/main/cpp/mbedtls/platform.cpp
        src/main/cpp/mbedtls/ripemd160.cpp
        src/main/cpp/mbedtls/rsa.cpp
        src/main/cpp/mbedtls/sha1.cpp
        src/main/cpp/mbedtls/sha256.cpp
        src/main/cpp/mbedtls/sha512.cpp
        src/main/cpp/mbedtls/threading.cpp
        src/main/cpp/mbedtls/timing.cpp
        src/main/cpp/mbedtls/version.cpp
        src/main/cpp/mbedtls/version_features.cpp
        src/main/cpp/mbedtls/xtea.cpp
        src/main/cpp/mbedtls/certs.cpp
        src/main/cpp/mbedtls/pkcs11.cpp
        src/main/cpp/mbedtls/x509.cpp
        src/main/cpp/mbedtls/x509_create.cpp
        src/main/cpp/mbedtls/x509_crl.cpp
        src/main/cpp/mbedtls/x509_crt.cpp
        src/main/cpp/mbedtls/x509_csr.cpp
        src/main/cpp/mbedtls/x509write_crt.cpp
        src/main/cpp/mbedtls/x509write_csr.cpp
        src/main/cpp/mbedtls/debug.cpp
        src/main/cpp/mbedtls/net_sockets.cpp
        src/main/cpp/mbedtls/ssl_cache.cpp
        src/main/cpp/mbedtls/ssl_ciphersuites.cpp
        src/main/cpp/mbedtls/ssl_cli.cpp
        src/main/cpp/mbedtls/ssl_cookie.cpp
        src/main/cpp/mbedtls/ssl_srv.cpp
        src/main/cpp/mbedtls/ssl_ticket.cpp
        src/main/cpp/mbedtls/ssl_tls.cpp
        src/main/cpp/yourmain.cpp
        )

    add_library(yourmain SHARED ${src_mbed})
    set_target_properties(yourmain PROPERTIES VERSION 2.4.2 SOVERSION 0)
    target_link_libraries(yourmain ${mbedtls} ${log-lib})

4. ./app/buid.grandle

apply plugin: 'com.android.application'

android {
    compileSdkVersion 26

    buildToolsVersion "27.0.3"

    defaultConfig {
        applicationId "yourdomain.yourapp"
        minSdkVersion 22
        targetSdkVersion 26
        versionCode 1
        versionName "1.0"
        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"

        externalNativeBuild {
            cmake {
                cppFlags "-fexceptions"
                cppFlags "-fvisibility=hidden"
                cppFlags "-fvisibility-inlines-hidden"
            }
        }
    }

    externalNativeBuild {
        cmake {
            path "CMakeLists.txt"  // this is required
        }
    }

    buildTypes {
        debug {
            debuggable true
            jniDebuggable true
        }
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }

    productFlavors {
    }
}

dependencies {
    compile fileTree(include: ['*.jar'], dir: 'libs')
    androidTestCompile('com.android.support.test.espresso:espresso-core:2.2.2', {
        exclude group: 'com.android.support', module: 'support-annotations'
    })
    compile 'com.android.support:appcompat-v7:26.0.0-alpha1'
    compile 'com.android.support:design:26.0.0-alpha1'
    compile 'com.android.support:cardview-v7:26.0.0-alpha1'
    testCompile 'junit:junit:4.12'
}