Provider Bindings

When your @Provides methods start to grow complex, you may consider moving them to a class of their own. The provider class implements Guice's Provider interface, which is a simple, general interface for supplying values:

public interface Provider<T> {
  T get();
}

Our provider implementation class has dependencies of its own, which it receives via its @Inject-annotated constructor. It implements the Provider interface to define what's returned with complete type safety:

public class DatabaseTransactionLogProvider implements Provider<TransactionLog> {
  private final Connection connection;

  @Inject
  public DatabaseTransactionLogProvider(Connection connection) {
    this.connection = connection;
  }

  public TransactionLog get() {
    DatabaseTransactionLog transactionLog = new DatabaseTransactionLog();
    transactionLog.setConnection(connection);
    return transactionLog;
  }
}

Finally we bind to the provider using the .toProvider clause:

public class BillingModule extends AbstractModule {
  @Override
  protected void configure() {
    bind(TransactionLog.class)
        .toProvider(DatabaseTransactionLogProvider.class);
  }

If your providers are complex, be sure to test them!

Throwing Exceptions

Guice does not allow exceptions to be thrown from Providers. The Provider interface does not allow for checked exception to be thrown. RuntimeExceptions may be wrapped in a ProvisionException or CreationException and may prevent your Injector from being created. If you need to throw an exception for some reason, you may want to use the ThrowingProviders extension.