How to Dynamically Autowire Beans in Spring: Two Practical Approaches

1. Introduction

In this short tutorial we show how to dynamically autowire a Bean in Spring.

We first present a realistic use case where dynamic autowiring is useful, then demonstrate two different ways to solve the problem.

2. Use Cases for Dynamic Autowiring

Dynamic autowiring is handy wherever the execution logic of a Spring Bean needs to change at runtime, especially when a runtime variable decides which code to run.

To illustrate, we create an application that controls servers in different regions. The interface looks like:

public interface RegionService {
    boolean isServerActive(int serverId);
    String getISOCountryCode();
}

And two implementations:

@Service("GBregionService")
public class GBRegionService implements RegionService {
    @Override
    public boolean isServerActive(int serverId) {
        return false;
    }

    @Override
    public String getISOCountryCode() {
        return "GB";
    }
}

@Service("USregionService")
public class USRegionService implements RegionService {
    @Override
    public boolean isServerActive(int serverId) {
        return true;
    }

    @Override
    public String getISOCountryCode() {
        return "US";
    }
}

Assume a website lets users check whether a server in a specific region is active. We want to switch the RegionService implementation based on user input – a perfect scenario for dynamic bean autowiring.

3. Using BeanFactory

BeanFactory is the root interface for accessing the Spring Bean container. Because BeanFactory itself is a Spring Bean, we can autowire it and retrieve beans by name:

@Service
public class BeanFactoryDynamicAutowireService {
    private static final String SERVICE_NAME_SUFFIX = "regionService";
    private final BeanFactory beanFactory;

    @Autowired
    public BeanFactoryDynamicAutowireService(BeanFactory beanFactory) {
        this.beanFactory = beanFactory;
    }

    public boolean isServerActive(String isoCountryCode, int serverId) {
        RegionService service = beanFactory.getBean(getRegionServiceBeanName(isoCountryCode), RegionService.class);
        return service.isServerActive(serverId);
    }

    private String getRegionServiceBeanName(String isoCountryCode) {
        return isoCountryCode + SERVICE_NAME_SUFFIX;
    }
}

We use the overloaded getBean() method to obtain the bean with the given name and type. While this works, a more idiomatic approach is preferred.

4. Using a Map of Implementations

Spring can collect all implementations of an interface into a Map where the key is the bean name. This allows us to inject the map and look up the appropriate implementation:

@Service
public class CustomMapFromListDynamicAutowireService {
    private final Map<String, RegionService> servicesByCountryCode;

    @Autowired
    public CustomMapFromListDynamicAutowireService(List<RegionService> regionServices) {
        servicesByCountryCode = regionServices.stream()
                .collect(Collectors.toMap(RegionService::getISOCountryCode, Function.identity()));
    }

    public boolean isServerActive(String isoCountryCode, int serverId) {
        RegionService service = servicesByCountryCode.get(isoCountryCode);
        return service.isServerActive(serverId);
    }
}

We build a map of implementations keyed by ISO country code in the constructor and use it to retrieve the correct service at runtime.

5. Summary

This quick tutorial demonstrated two ways to dynamically autowire beans in Spring: using BeanFactory directly and injecting a map of all implementations.