Avoiding recursive logins

If you run the application and try to run login first-username followed by login second-username, you’ll notice that it works! How/why is this working?

The values in the Map<String, Command> that are available in CommandProcessorFactory are [HelloWorldCommand, LoginCommand]. When we created the @Subcomponent for UserCommandsRouter, it inherited the modules from CommandProcessorFactory, and therefore also the values of Map<String, Command>. So the full set of values in the Map<String, Command> for UserCommandsRouter is [HelloWorldCommand, LoginCommand, DepositCommand, WithdrawCommand].

There is no way to remove values from the map, but we have another option. We can add an Optional<Account> dependency to LoginCommand to indicate if we’re in a state where an Account is available or not:

final class LoginCommand extends SingleArgCommand {
  private final Database database;
  private final Outputter outputter;
  private final UserCommandsRouter.Factory userCommandsRouterFactory;
  private final Optional<Account> account;

  @Inject
  LoginCommand(
      Database database,
      Outputter outputter,
      UserCommandsRouter.Factory userCommandsRouterFactory,
      Optional<Account> account) {
    this.database = database;
    this.outputter = outputter;
    this.userCommandsRouterFactory = userCommandsRouterFactory;
    this.account = account;
  }

  @Override
  public Result handleArg(String username) {
    if (account.isPresent()) {
      // Ignore "login <foo>" commands if we already have an account
      return Result.handled();
    }
  }
}

In order to tell Dagger how to supply this Optional<Account>, let’s add a @BindsOptionalOf method to LoginCommandModule:

@Module
interface LoginCommandModule {
  ...

  @BindsOptionalOf
  Account optionalAccount();
}

When Dagger sees a @BindsOptionalOf method, it will use Optional.of(<the account>) if an Account is available; if not, it will use Optional.empty() (or Optional.absent() if you’re using the Guava version of Optional).

Looking back, we can now see that within CommandProcessorFactory, there is no Account, but within UserCommandsRouter there is. Each will have its own LoginCommand that differs in this optional account, and we can therefore run different logic in both cases!

CONCEPTS

• When Dagger creates sets or maps of objects, the set or map will contain all values from any parent component(s) in addition to the values that are installed by bindings in the component itself.
• @BindsOptionalOf tells Dagger that it can construct instances of Optional<ReturnType>. The presence of the Optional is determined by whether Dagger knows how to create an instance of ReturnType, and it can be present in a subcomponent but absent in its parent.