解释一下spring如何解决循环依赖 并贴出相关源代码

Spring通过三级缓存解决循环依赖问题。当Spring创建一个Bean时，它会将该Bean放入第一级缓存中，然后检查该Bean所依赖的其他Bean是否已经创建，如果已经创建，则将这些Bean注入到该Bean中。如果其他Bean还没有创建，则将该Bean放入第二级缓存中，等待其他Bean创建完成后再进行注入。如果其他Bean也依赖于该Bean，则将该Bean放入第三级缓存中，等待其他Bean创建完成后再进行注入。

以下是相关的源代码：

public abstract class AbstractBeanFactory extends FactoryBeanRegistrySupport implements ConfigurableBeanFactory {

    private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);

    private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16);

    private final Map<String, Object> earlySingletonObjects = new ConcurrentHashMap<>(16);

    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;
    }

    protected void addSingletonFactory(String beanName, ObjectFactory<?> singletonFactory) {
        Assert.notNull(singletonFactory, "Singleton factory must not be null");
        synchronized (this.singletonObjects) {
            if (!this.singletonObjects.containsKey(beanName)) {
                this.singletonFactories.put(beanName, singletonFactory);
                this.earlySingletonObjects.remove(beanName);
            }
        }
    }

    protected void addSingleton(String beanName, Object singletonObject) {
        synchronized (this.singletonObjects) {
            this.singletonObjects.put(beanName, singletonObject);
            this.singletonFactories.remove(beanName);
            this.earlySingletonObjects.remove(beanName);
        }
    }
}

请注意，这只是Spring中解决循环依赖问题的一部分代码。完整的代码可以在Spring源代码中找到。