spring組件的構造器參好神奇
這里有一個接口：FruitService和兩個實現類：AppleService、BananaService，假設我想在其中一個實現類實例化中獲取當前Spring容器的Bean怎么辦？
只需要重寫該實現類的構造方法：
@Component
public class AppleService implements FruitService {
public AppleService(List<Object> list) {
System.out.println(list);
}
}
list可以獲取當前spring實例化好的Beans:
我能不能只拿某個Bean呢，答案是可以的，只需要重寫將Objent替換成對應的類即可：
@Component
public class AppleService implements FruitService {
public AppleService(List<FruitService> list) {
System.out.println(list);
}
}
不僅如此，還可以將參數轉換成Map和Collection類型，spring也會自動幫忙處理，但是如果是非上述類型，則會在創建的時候拋出異常。
源碼
調用棧：
由於spring源碼非常深，這里直接列出處理這個人性化的地方最核心一個方法：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會一一匹配指定類型，假如沒有匹配成功，則會返回null，匹配成功但是沒有數據，則會返回一個空對象。