Create implementation interface PlayListService and implement all
method.

void savePlaylistSong(int song_id, int playlist_id) which save
PlaylistSong to database.

PlaylistSong getPlaylistSongById(int playlistSongId) which load
PlayListSong from database.
Create service interface which will be connect which PlayListSong Dao
Layer and call method from there.
Create Controller PlaylistController Responsible for all operation on
song in Playlist
Find JQuery script responsible for take selected playlist and song and
save this relationship between them.
Mark this class as Service and add annotation @Transactional to all
method in this class
Mark this class as Repository