Question

I have an application config file that looks something like this:

database:
  type: [db-type]
  username: [name]
  password: [pw]
  server: [ip]
  database: [db-name]

db-type can be any of the following: {postgresql, mysql, mssql, file}.
I wanted to configure the binding, such that (it’s hibernate based) the app loads a special Provider of SessionFactory depending on which of the values is used, i.e. a PostgresqlSessionFactoryProvider.

The problem is, that Guice also takes care of injecting an instance of Config.class into classes that need access to it. Now I need to access the config, while setting up the binding… It’s sort of a chicken-egg problem.

How do I get around that?

Answer 1

I have found a way to do this. It might not be the best way to do it, so if you know better, I will still look at the answers.

The point is, that the binder does first have to be completely configured, so it can provide an instance of Config.class.

So I came up with a custom Provider for SessionFactory.class that needs a config and the guice injector itself, so that when all the info has been slurped by guice, it has the means to provide a different implementation based on the config.

public class SessionFactoryProvider implements Provider<SessionFactory> {

    private Config config;
    private Injector injector;

    @Inject
    public SessionFactoryProvider(Config config, Injector injector) {
        this.config = config;
        this.injector = injector;
    }

    @Override
    public SessionFactory get() {
        switch (config.database.type) {
        case postgresql:
            return injector.getInstance(PostgresqlSessionFactoryProvider.class).get();
        case mysql:
            return injector.getInstance(MysqlSessionFactoryProvider.class).get();
        case file:
            return injector.getInstance(FileBasedSessionFactoryProvider.class).get();
            /* some more providers... */
        default:
            return injector.getInstance(FileBasedSessionFactoryProvider.class).get();
        }
    }
}

What do you think? Is this a good way to do this?