SpringBoot源码解析(七)@Primary

在SpringBoot中有许多类使用到了@Primary注解，关于用法，请看这篇博客：在spring中常被忽视的注解 @Primary。

这次我们不说用法，从源码层面来看下：

一、入口

在创建bean的时候，我们会获取bean的依赖bean，这个方法就是DefaultListableBeanFactory的doResolveDependency方法，如果匹配的bean大于1，会执行如下方法：

			if (matchingBeans.size() > 1) {
				autowiredBeanName = determineAutowireCandidate(matchingBeans, descriptor);

二、查找

最后会进入DefaultListableBeanFactory的如下方法，进行bean的过滤

	protected String determinePrimaryCandidate(Map<String, Object> candidates, Class<?> requiredType) {
		String primaryBeanName = null;
		for (Map.Entry<String, Object> entry : candidates.entrySet()) {
			String candidateBeanName = entry.getKey();
			Object beanInstance = entry.getValue();
			if (isPrimary(candidateBeanName, beanInstance)) {
				if (primaryBeanName != null) {
					boolean candidateLocal = containsBeanDefinition(candidateBeanName);
					boolean primaryLocal = containsBeanDefinition(primaryBeanName);
					if (candidateLocal && primaryLocal) {
						throw new NoUniqueBeanDefinitionException(requiredType, candidates.size(),
								"more than one 'primary' bean found among candidates: " + candidates.keySet());
					}
					else if (candidateLocal) {
						primaryBeanName = candidateBeanName;
					}
				}
				else {
					primaryBeanName = candidateBeanName;
				}
			}
		}
		return primaryBeanName;
	}

isPrimary方法是判断是否有@Primary注解，如果有多个满足条件的bean有@Primary注解就会抛出异常：

throw new NoUniqueBeanDefinitionException(requiredType, candidates.size(),
								"more than one 'primary' bean found among candidates: " + candidates.keySet());