To integrate nmslib and faiss approximate k-NN functionality (implemented in C++) into the k-NN plugin (implemented in Java), we created a Java Native Interface, which lets the k-NN plugin make calls to the native libraries. The interface includes three libraries:
libopensearchknn_nmslib, the JNI library that interfaces with nmslib,
libopensearchknn_faiss, the JNI library that interfaces with faiss, and
libopensearchknn_common, a library containing common shared functionality between native libraries.
The Lucene library is not implemented using a native library.
libopensearchknn_nmslib are lazily loaded when they are first called in the plugin. This means that if you are only planning on using one of the libraries, the plugin never loads the other library.
To build the libraries from source, refer to the DEVELOPER_GUIDE.
For more information about JNI, see Java Native Interface on Wikipedia.