spring获取单例bean之从三级缓存(一)_spring从三级缓存加载单例的时机-优快云博客

本文详细分析了Spring中getBean方法的工作原理，特别是如何处理单例bean的循环依赖问题。通过三级缓存（singletonObjects、earlySingletonObjects和singletonFactories）的使用，Spring能够在bean实例化过程中检测并解决循环依赖，确保应用正常运行。在getBean过程中，首先尝试从singletonObjects获取bean，如果不存在，则检查earlySingletonObjects，最后从singletonFactories中通过ObjectFactory获取并创建bean实例。

摘要生成于 C知道，由 DeepSeek-R1 满血版支持，前往体验 >

getBean

spring中调用方式 内联代码片。

// 1.获取bean
MyTestBean bean=(MyTestBean) bf.getBean( "myTestBean " )

// 2.跟进bean方法
	@Override
	public Object getBean(String name) throws BeansException {
		return doGetBean(name, null, null, false);
	}

// 3.跟进doGetBean方法，方法中有getSingleton方法
Object sharedInstance = getSingleton(beanName);

// 4.三级缓存
	@Nullable
	protected Object getSingleton(String beanName, boolean allowEarlyReference) {
		Object singletonObject = this.singletonObjects.get(beanName);
		if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
			synchronized (this.singletonObjects) {
				singletonObject = this.earlySingletonObjects.get(beanName);
				if (singletonObject == null && allowEarlyReference) {
					ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
					if (singletonFactory != null) {
						singletonObject = singletonFactory.getObject();
						this.earlySingletonObjects.put(beanName, singletonObject);
						this.singletonFactories.remove(beanName);
					}
				}
			}
		}
		return singletonObject;
	}

分析下getSingleton()

// 检查缓存中或者实例工厂中是否有对应的实例，为什么首先会使用这段代码呢？
//因为在创建单例bean的时候会存在依赖注入的情况，而在创建依赖的时候为了
//避免循环依赖。spring创建bean的原则是不等bean创建完成就会将创建bean的ObjectFactory提早曝光，也就是将ObjectFactory加入到缓存中，一但下个bean创建时候需要依赖上个bean则直接使用objectFactory
	protected Object getSingleton(String beanName, boolean allowEarlyReference) {
		Object singletonObject = this.singletonObjects.get(beanName);
		if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
			synchronized (this.singletonObjects) {
				singletonObject = this.earlySingletonObjects.get(beanName);
				if (singletonObject == null && allowEarlyReference) {
					ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
					if (singletonFactory != null) {
						singletonObject = singletonFactory.getObject();
						this.earlySingletonObjects.put(beanName, singletonObject);
						this.singletonFactories.remove(beanName);
					}
				}
			}
		}
		return singletonObject;
	}
//这个方法首先尝试从singletonObjects里面获取实例，如果获取不到再从
//earlySingletonObjects里面获取，如果还获取不到，再尝试从singleton-
//Factories里面获取beanName对应的ObjectFactory,然后调用ObjectFac-
//tory的getObject来创建bean,并放到earlySingletonObjects里面，并
//从singletonFactories里面remove掉这个ObjectFactory。

// 1.三个map
//用于保存beanName和创建bean实例之间的关系，bean name --> bean instance
private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);
//也是保存beanName和创建bean实例之间的关系，与singletonObjects的不同之处在于，当一个单例bean被放到这里后，那么当bean还在创建过程中，就可以通过getBean方法获取到了，其目的是为了用来检测循环依赖,bean name --> bean instance
private final Map<String, Object> earlySingletonObjects = new HashMap<>(16);
//用来保存beanName和创建bean的工厂之间的关系,bean name --> ObjectFactory
private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16);