spring解决循环依赖的源码依据

矫情吗；* 2022-08-31 01:45 182阅读 0赞

spring的ioc容器在处理循环依赖的时候采用的是三级缓存，其实就是将已经初始化的spring  bean 放到一级缓存singletonObjects，一级缓存又被称作"单例池";将已经实例化但还未初始化的spring  bean 放到二级缓存earlySingletonObjects中；将存放创建spring bean的原始工厂存放到三级缓存singletonFactories，所谓的三级缓存其实就三个Map。

源码查看org.springframework.beans.factory.support.DefaultSingletonBeanRegistry这个类即可：

/** Cache of singleton objects: bean name to bean instance. */
    	private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);
    
    	/** Cache of singleton factories: bean name to ObjectFactory. */
    	private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16);
    
    	/** Cache of early singleton objects: bean name to bean instance. */
    	private final Map<String, Object> earlySingletonObjects = new ConcurrentHashMap<>(16);

三级缓存使用的的过程，先去查一级缓存，以及缓存没有再去查找二级缓存，二级缓存没有的话从三级缓存获取，具体源码逻辑如下：

/**
    	 * Return the (raw) singleton object registered under the given name.
    	 * <p>Checks already instantiated singletons and also allows for an early
    	 * reference to a currently created singleton (resolving a circular reference).
    	 * @param beanName the name of the bean to look for
    	 * @param allowEarlyReference whether early references should be created or not
    	 * @return the registered singleton object, or {@code null} if none found
    	 */
    	@Nullable
    	protected Object getSingleton(String beanName, boolean allowEarlyReference) {
    		// Quick check for existing instance without full singleton lock
    		Object singletonObject = this.singletonObjects.get(beanName);
    		if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
    			singletonObject = this.earlySingletonObjects.get(beanName);
    			if (singletonObject == null && allowEarlyReference) {
    				synchronized (this.singletonObjects) {
    					// Consistent creation of early reference within full singleton lock
    					singletonObject = this.singletonObjects.get(beanName);
    					if (singletonObject == null) {
    						singletonObject = this.earlySingletonObjects.get(beanName);
    						if (singletonObject == null) {
    							ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
    							if (singletonFactory != null) {
    								singletonObject = singletonFactory.getObject();
    								this.earlySingletonObjects.put(beanName, singletonObject);
    								this.singletonFactories.remove(beanName);
    							}
    						}
    					}
    				}
    			}
    		}
    		return singletonObject;
    	}