# @Fallback

`@Fallback()` makes a bean the one used only when no other bean of the type is there.

## Why

It is the opposite of [@Primary](primary.md). A `@Primary` bean wins over the others. A `@Fallback` bean loses to any other candidate.

Use it for a default implementation that something else may replace: a test profile, an optional module, an application that brings its own. The code that adds the replacement does not need to know about the default or mark itself primary.

## Example

```dart
import 'package:spring_core_dart/spring_core_dart.dart';

abstract interface class Clock {
  DateTime now();
}

// The normal clock. It steps aside when another Clock exists.
@Component()
@Fallback()
class SystemClock implements Clock {
  @override
  DateTime now() => DateTime.now();
}

// Exists only with the `test` profile, and then wins.
@Component()
@Profile(['test'])
class FixedClock implements Clock {
  @override
  DateTime now() => DateTime(2026, 1, 1);
}

@Service()
class Invoices {
  Invoices(this.clock); // SystemClock, or FixedClock in tests
  final Clock clock;
}
```

## Parameters

None.

## In Spring (Java)

Spring 6.2 and later:

```java
@Component
@Fallback
class SystemClock implements Clock { ... }
```

The meaning is the same.

## See also

- [@Primary](primary.md)
- [@Qualifier](qualifier.md)
- [@Profile](profile.md)
- [@Conditional](conditional.md)