Implementing the Strategy Pattern in Spring with Built‑in @Autowired

In a Spring Boot project, the author demonstrates how to avoid extensive if‑else statements by combining the strategy pattern with a factory that selects a concrete rule based on an enum key.

Factory with @Autowired Map

The SignalLightRulesFactory is annotated with @Component and declares a field

Map<String, SignalLightRules> signalLightRulesStrategy;

which is marked with @Autowired. Spring automatically injects every bean that implements the SignalLightRules interface; the map key is the bean name (the value of the @Component annotation) and the value is the corresponding strategy instance. The factory method simply looks up the map:

public SignalLightRules getSignalLightRules(String signalLightKey) {
    return signalLightRulesStrategy.get(signalLightKey);
}

Strategy Interface and Example Implementation

The strategy interface is defined as

public interface SignalLightRules {
    List<SignalrightDevsDTO> getSignalrightDevsDtoList(String mineApiCode);
}

An example implementation is annotated with a custom name:

@Component(Constants.SIFANGJISIGNAL)
public class SiFangJiSignalLightRules implements SignalLightRules {
    @Override
    public List<SignalrightDevsDTO> getSignalrightDevsDtoList(String mineApiCode) {
        List<SignalrightDevsDTO> signalrightDevsDtoList = null;
        // ... omitted logic ...
        return signalrightDevsDtoList;
    }
}

How @Autowired Works for Collections

The article extracts the Javadoc of org.springframework.beans.factory.annotation.Autowired, highlighting that when the injection point is an array, java.util.Collection, or java.util.Map, the container autowires all matching beans. For a map, the keys must be of type String and are resolved to bean names. Ordering follows @Order or Ordered values if present.

Key Points from the Annotation Source

The annotation can be placed on constructors, fields, setter methods, config methods, and (since Spring 5.0) on individual parameters. The required attribute defaults to true, controlling whether missing beans cause a failure. Autowiring is performed by AutowiredAnnotationBeanPostProcessor, which cannot be used inside BeanPostProcessor or BeanFactoryPostProcessor implementations.

Finally, the author notes that the same injection technique works for arrays and other collection types, and points readers to the source code of @Autowired for deeper understanding.