spring bean的构造器参数

spring组件的构造器参好神奇

这里有一个接口：FruitService和两个实现类：AppleService、BananaService，假设我想在其中一个实现类实例化中获取当前Spring容器的Bean怎么办？

只需要重写该实现类的构造方法：

@Component
public class AppleService implements FruitService {

	public AppleService(List<Object> list) {
		System.out.println(list);
	}

}

list可以获取当前spring实例化好的Beans:

我能不能只拿某个Bean呢，答案是可以的，只需要重写将Objent替换成对应的类即可：

@Component
public class AppleService implements FruitService {

	public AppleService(List<FruitService> list) {
		System.out.println(list);
	}

}

不仅如此，还可以将参数转换成Map和Collection类型，spring也会自动帮忙处理，但是如果是非上述类型，则会在创建的时候抛出异常。

源码

由于spring源码非常深，这里直接列出处理这个人性化的地方最核心一个方法：org.springframework.beans.factory.support.DefaultListableBeanFactory#resolveMultipleBeans

@Nullable
	private Object resolveMultipleBeans(DependencyDescriptor descriptor, @Nullable String beanName,
			@Nullable Set<String> autowiredBeanNames, @Nullable TypeConverter typeConverter) {

		final Class<?> type = descriptor.getDependencyType();

		if (descriptor instanceof StreamDependencyDescriptor) {
			Map<String, Object> matchingBeans = findAutowireCandidates(beanName, type, descriptor);
			if (autowiredBeanNames != null) {
				autowiredBeanNames.addAll(matchingBeans.keySet());
			}
			Stream<Object> stream = matchingBeans.keySet().stream()
					.map(name -> descriptor.resolveCandidate(name, type, this))
					.filter(bean -> !(bean instanceof NullBean));
			if (((StreamDependencyDescriptor) descriptor).isOrdered()) {
				stream = stream.sorted(adaptOrderComparator(matchingBeans));
			}
			return stream;
		}
		else if (type.isArray()) {
			Class<?> componentType = type.getComponentType();
			ResolvableType resolvableType = descriptor.getResolvableType();
			Class<?> resolvedArrayType = resolvableType.resolve(type);
			if (resolvedArrayType != type) {
				componentType = resolvableType.getComponentType().resolve();
			}
			if (componentType == null) {
				return null;
			}
			Map<String, Object> matchingBeans = findAutowireCandidates(beanName, componentType,
					new MultiElementDescriptor(descriptor));
			if (matchingBeans.isEmpty()) {
				return null;
			}
			if (autowiredBeanNames != null) {
				autowiredBeanNames.addAll(matchingBeans.keySet());
			}
			TypeConverter converter = (typeConverter != null ? typeConverter : getTypeConverter());
			Object result = converter.convertIfNecessary(matchingBeans.values(), resolvedArrayType);
			if (result instanceof Object[]) {
				Comparator<Object> comparator = adaptDependencyComparator(matchingBeans);
				if (comparator != null) {
					Arrays.sort((Object[]) result, comparator);
				}
			}
			return result;
		}
		else if (Collection.class.isAssignableFrom(type) && type.isInterface()) {
			Class<?> elementType = descriptor.getResolvableType().asCollection().resolveGeneric();
			if (elementType == null) {
				return null;
			}
			Map<String, Object> matchingBeans = findAutowireCandidates(beanName, elementType,
					new MultiElementDescriptor(descriptor));
			if (matchingBeans.isEmpty()) {
				return null;
			}
			if (autowiredBeanNames != null) {
				autowiredBeanNames.addAll(matchingBeans.keySet());
			}
			TypeConverter converter = (typeConverter != null ? typeConverter : getTypeConverter());
			Object result = converter.convertIfNecessary(matchingBeans.values(), type);
			if (result instanceof List) {
				if (((List<?>) result).size() > 1) {
					Comparator<Object> comparator = adaptDependencyComparator(matchingBeans);
					if (comparator != null) {
						((List<?>) result).sort(comparator);
					}
				}
			}
			return result;
		}
		else if (Map.class == type) {
			ResolvableType mapType = descriptor.getResolvableType().asMap();
			Class<?> keyType = mapType.resolveGeneric(0);
			if (String.class != keyType) {
				return null;
			}
			Class<?> valueType = mapType.resolveGeneric(1);
			if (valueType == null) {
				return null;
			}
			Map<String, Object> matchingBeans = findAutowireCandidates(beanName, valueType,
					new MultiElementDescriptor(descriptor));
			if (matchingBeans.isEmpty()) {
				return null;
			}
			if (autowiredBeanNames != null) {
				autowiredBeanNames.addAll(matchingBeans.keySet());
			}
			return matchingBeans;
		}
		else {
			return null;
		}
	}

spring会一一匹配指定类型，假如没有匹配成功，则会返回null，匹配成功但是没有数据，则会返回一个空对象。