This is perhaps the simplest possible example project that one can make demonstrating how to use IPOPT in Android. To accomplish this, you will need the precompiled IPOPT libraries for Android that are available in the release section of this repo:

• https://github.com/jeti/android_ipopt

or you can build the libraries yourself with the script provided there. Up to you...

There are just a few steps necessary to get IPOPT working in an Android project, as demonstrated in this video. Here is a recap of what we did to create this repo (you can follow these steps to create your own app from scratch):

1. Create a new Android project with C++ support. This project should have the structure

   project/
   ---app/
   ------build.gradle
   ------CMakeLists.txt
   ---------libs/
   ---------src/
   ------------main/
   ------------cpp/   
   

2. Download the shared libraries from https://github.com/jeti/android_ipopt/releases, and extract them into the project/app/libs directory.

3. Open project/app/build.gradle.

   In android/defaultConfig, add the ABIs that you want to compile for. For example, add the lines

   ndk {
       abiFilters 'x86', 'x86_64', 'arm64-v8a', 'armeabi'
   }
   

   In the video, I add the 4 ABIs above, but I really only need x86_64 since that is the emulator I am testing in. However, in practice most new Android devices are arm64-v8a, so if you are going to put it on a real device, you probably want that option as well. Note that you can also add mips or mips64, but I have yet to see such a device in the wild.

   Next, in the main android section of the same file (outside of defaultConfig), add the lines

   sourceSets {
   	main {
   		// let gradle pack the shared library into apk
   		jniLibs.srcDirs = ['libs']
   	}
   }
   

   This is necessary so that the .so libraries get packaged with the app.

4. Go into the project/app/src/main/cpp folder and copy in your C++ code. You have to be very careful about your method naming here. If you are going to call a method from the stringFromJNI method of MainActivity class located in the io.jeti.ipopt package, then the C++ method should have the signature

   extern "C" JNIEXPORT jstring
   JNICALL
   Java_io_jeti_ipopt_MainActivity_stringFromJNI(
       JNIEnv *env,
       jobject /* this */) {
       ...
       return ...
    }
   
   

   But I am not here to give you a primer on the JNI. If you don't know what I am talking about, or are not familiar with JNI naming schemes, then please find a JNI tutorial.

5. Open project/app/CMakeLists.txt.

   You need to update this file to reflect the C++ code that you actually want to build, and to make sure that you are linking to ipopt.

   To accomplish the first part of this, find the add_library portion of the file. You want to change that portion so that it builds your code, not the default code that was created when you created the project. In my case, the auto-generated block looked like

   add_library( # Sets the name of the library.
                native-lib
   
                # Sets the library as a shared library.
                SHARED
   
                # Provides a relative path to your source file(s).
                src/main/cpp/native-lib.cpp)
   

   However, we replaced the C++ code src/main/cpp/native-lib.cpp with our own files

   • src/main/cpp/cpp_example.cpp
   • src/main/cpp/MyNLP.cpp

   Therefore we updated that block to read

   add_library( # Sets the name of the library.
                native-lib
   
                # Sets the library as a shared library.
                SHARED
   
                # Provides a relative path to your source file(s).
                src/main/cpp/cpp_example.cpp
                src/main/cpp/MyNLP.cpp)
   

   Finally, we had to link in the dependencies to IPOPT. Since IPOPT itself is dependent on some other libraries, we first need to tell cmake about our external libraries by adding lines of the form

   add_library(xyz SHARED IMPORTED)
   set_property(TARGET xyz PROPERTY IMPORTED_LOCATION  
                ${CMAKE_SOURCE_DIR}/libs/${ANDROID_ABI}/libxyz.so)
   

   for each of our external libraries, that is, is for xyz=blas,lapack,metis,mumps,ipopt. Then, since the includes where all placed in project/app/libs/include, we tell cmake where to find them

   # Location of header files
   include_directories(${CMAKE_SOURCE_DIR}/libs/include/coin
                       ${CMAKE_SOURCE_DIR}/libs/include/coin/ThirdParty)
   

   Lastly, we link the libraries to our project by updating the target_link_libraries call from something like

   target_link_libraries( # Specifies the target library.
                          native-lib
   
                          # Links the target library to the log library
                          # included in the NDK.
                          ${log-lib}
                          )
   

   to

   target_link_libraries( # Specifies the target library.
                          native-lib
   
                          # Links the target library to the log library
                          # included in the NDK.
                          ${log-lib}
   
                          # Link to IPOPT
                          blas
                          lapack
                          metis
                          mumps
                          ipopt
                          )
   

Happy optimizing...