Injecting Collections with @Autowired and Applying Strategy Pattern in Spring

Spring developers are familiar with the @Autowired annotation for automatic bean injection, but it can also inject collections such as arrays, List, Set, and even Map of a specific type.

Consider a payment system with a PayService interface and three implementations: AliPayService, WechatPayService, and BankCardPayService. To obtain all PayService beans, you can use the traditional approaches:

String[] names = ctx.getBeanNamesForType(PayService.class);
List<PayService> anotherPayService = Lists.newArrayList();
for (String beanName : names) {
    anotherPayService.add(ctx.getBean(beanName, PayService.class));
}
// or
Map<String, PayService> beansOfType = ctx.getBeansOfType(PayService.class);
for (Map.Entry<String, PayService> entry : beansOfType.entrySet()) {
    anotherPayService.add(entry.getValue());
}

With collection injection, the same goal is achieved more concisely:

@Autowired
List<PayService> payServices;

@Autowired
PayService[] payServicesArray;

Using this capability, the strategy pattern becomes straightforward. Define a common PayService interface, implement it for each payment channel, and inject all implementations as a collection. A RouteService can then map a request's channelNo to the appropriate bean.

@Service
public class RouteService {
    @Autowired
    Map<String, PayService> payServiceMap;

    public PayResult epay(PayRequest payRequest) {
        PayService payService = payServiceMap.get(payRequest.getChannelNo());
        return payService.epay(payRequest);
    }
}

To avoid relying on bean names that match numeric channel codes, extend the PayService interface with a String channel() method, let each implementation return its unique identifier, and build the map programmatically after injection:

@Service
public class RouteService {
    @Autowired
    Set<PayService> payServiceSet;
    Map<String, PayService> payServiceMap;

    @PostConstruct
    public void init() {
        payServiceMap = new HashMap<>();
        for (PayService payService : payServiceSet) {
            payServiceMap.put(payService.channel(), payService);
        }
    }

    public PayResult epay(PayRequest payRequest) {
        PayService payService = payServiceMap.get(payRequest.getChannelNo());
        return payService.epay(payRequest);
    }
}

This approach cleanly separates payment channel differences, making the system easier to maintain and extend.