There's more...

When registering the application on Facebook, we also configured the redirect URI to be http://localhost:8080/connect. Why not use http://localhost:8080/callback? By using the /connect endpoint, we take advantage of the endpoints defined by ConnectController. If you do not want to use Spring Social, you are supposed to validate the authorization code through the use of state parameters by yourself. When using Spring Social, we also take advantage of callbacks which are particular to the Facebook provider as De-authorize Callback URL's which might be set up in the settings from the Facebook Login product.

Even though we are using Spring Social Facebook we are still creating some classes that are also provided by Spring Social Facebook. As you could realize, the name of some classes begin with Custom. That's because we can customize how to create an instance of OAuth2Template as well as the FacebookTemplate class. It's important because the version supported at the time of this writing was 2.5, which was to be deprecated soon, and that's the version defined inside the Facebook provider for Spring Social.

There is an important thing to be mentioned about the interactions between the client and the OAuth 2.0 Provider, which in this case is Facebook. As you may realize, we are registering the redirect URI without using TLS/SSL. The URI we've registered is HTTP instead of HTTPS. All the recipes in this book are using such an approach just to ease the creation of the examples. Be sure to use HTTPS in production to protect the integrity and confidentiality of data transferred between your application and any other provider.

Another valuable improvement which might be done is to use a Relational Database Management System (RDBMS) to persist connections with providers. As the application does not explicitly define the strategy for connection persistence, Spring Social provides the in-memory version, so whenever you restart your server the user's connections will be lost. If you want to try using a database, you might declare a bean of type JdbcUsersConnectionRepository, and create the following table within the database of your choice:

create table UserConnection (userId varchar(255) not null, 
    providerId varchar(255) not null, 
    providerUserId varchar(255), 
    rank int not null, 
    displayName varchar(255), 
    profileUrl varchar(512), 
    imageUrl varchar(512), 
    accessToken varchar(512) not null, 
    secret varchar(512), 
    refreshToken varchar(512), 
    expireTime bigint, 
    primary key (userId, providerId, providerUserId)); 
create unique index UserConnectionRank on UserConnection(userId, providerId, rank);

To get more details about this, look at Spring Social's official documentation at http://docs.spring.io/spring-social/docs/1.1.4.RELEASE/reference/htmlsingle/#section_establishingConnections.