Injecting Providers

With normal dependency injection, each type gets exactly one instance of each of its dependent types. The RealBillingService gets one CreditCardProcessor and one TransactionLog. When this flexibility is necessary, Guice binds a provider. Providers produce a value when the get() method is invoked:

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

The provider's type is parameterized to differentiate a Provider<TransactionLog> from a Provider<CreditCardProcessor>. Wherever you inject a value you can inject a provider for that value.

public class RealBillingService implements BillingService {
  private final Provider<CreditCardProcessor> processorProvider;
  private final Provider<TransactionLog> transactionLogProvider;

  @Inject
  public RealBillingService(Provider<CreditCardProcessor> processorProvider,
      Provider<TransactionLog> transactionLogProvider) {
    this.processorProvider = processorProvider;
    this.transactionLogProvider = transactionLogProvider;
  }

  public Receipt chargeOrder(PizzaOrder order, CreditCard creditCard) {
    CreditCardProcessor processor = processorProvider.get();
    TransactionLog transactionLog = transactionLogProvider.get();

    /* use the processor and transaction log here */
  }
}

For every binding, annotated or not, the injector has a built-in binding for its provider.

Providers for multiple instances

Use providers when you need multiple instances of the same type. Suppose your application saves a summary entry and a details when a pizza charge fails. With providers, you can get a new entry whenever you need one:

public class LogFileTransactionLog implements TransactionLog {

  private final Provider<LogFileEntry> logFileProvider;

  @Inject
  public LogFileTransactionLog(Provider<LogFileEntry> logFileProvider) {
    this.logFileProvider = logFileProvider;
  }

  public void logChargeResult(ChargeResult result) {
    LogFileEntry summaryEntry = logFileProvider.get();
    summaryEntry.setText("Charge " + (result.wasSuccessful() ? "success" : "failure"));
    summaryEntry.save();

    if (!result.wasSuccessful()) {
      LogFileEntry detailEntry = logFileProvider.get();
      detailEntry.setText("Failure result: " + result);
      detailEntry.save();
    }
  }

Providers for lazy loading

If you've got a dependency on a type that is particularly expensive to produce, you can use providers to defer that work. This is especially useful when you don't always need the dependency:

public class DatabaseTransactionLog implements TransactionLog {
  
  private final Provider<Connection> connectionProvider;

  @Inject
  public DatabaseTransactionLog(Provider<Connection> connectionProvider) {
    this.connectionProvider = connectionProvider;
  }

  public void logChargeResult(ChargeResult result) {
    /* only write failed charges to the database */
    if (!result.wasSuccessful()) {
      Connection connection = connectionProvider.get();
    }
  }

Providers for Mixing Scopes

It is an error to depend on an object in a narrower scope. Suppose you have a singleton transaction log that needs on the request-scoped current user. Should you inject the user directly, things break because the user changes from request to request. Since providers can produce values on-demand, they enable you to mix scopes safely:

@Singleton
public class ConsoleTransactionLog implements TransactionLog {
  
  private final AtomicInteger failureCount = new AtomicInteger();
  private final Provider<User> userProvider;

  @Inject
  public ConsoleTransactionLog(Provider<User> userProvider) {
    this.userProvider = userProvider;
  }

  public void logConnectException(UnreachableException e) {
    failureCount.incrementAndGet();
    User user = userProvider.get();
    System.out.println("Connection failed for " + user + ": " + e.getMessage());
    System.out.println("Failure count: " + failureCount.incrementAndGet());
  }