Springboot详细讲解循环依赖

@Component
public class A {
    private B b;
    @Autowired
    public A(B b) {
        this.b = b;
    }
}

@Component
public class B {
    private A a;
    @Autowired
    public B(A a) {
        this.a = a;
    }
}

@Component
public class A {
    private B b;
    @Autowired
    @Lazy
    public A(B b) {
        this.b = b;
    }
}

@Component
public class B {
    private A a;
    @Autowired
    @Lazy
    public B(A a) {
        this.a = a;
    }
}

protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
			throws BeanCreationException {
		if (earlySingletonExposure) {
			Object earlySingletonReference = getSingleton(beanName, false);
			if (earlySingletonReference != null) {
				if (exposedObject == bean) {
					exposedObject = earlySingletonReference;
				}
				else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
					String[] dependentBeans = getDependentBeans(beanName);
					Set<String> actualDependentBeans = new LinkedHashSet<>(dependentBeans.length);
					// 重点在这里，这里会遍历所有依赖的bean，如果beanA依赖beanB和缓存中的beanB不相等
					// 也就是说beanA本来依赖的是一个原始对象beanB，但是这个时候发现beanB是一个代理对象，就会增加到actualDependentBeans
					for (String dependentBean : dependentBeans) {
						if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
							actualDependentBeans.add(dependentBean);
						}
					}
					// 发现actualDependentBeans不为空，就发生了我们最开始截图的错误
					if (!actualDependentBeans.isEmpty()) {
						throw new BeanCurrentlyInCreationException(beanName,
								"Bean with name '" + beanName + "' has been injected into other beans [" +
								StringUtils.collectionToCommaDelimitedString(actualDependentBeans) +
								"] in its raw version as part of a circular reference, but has eventually been " +
								"wrapped. This means that said other beans do not use the final version of the " +
								"bean. This is often the result of over-eager type matching - consider using " +
								"'getBeanNamesOfType' with the 'allowEagerInit' flag turned off, for example.");
					}
				}
			}
		}
		// ReGISter bean as disposable.
		try {
			registerDisposableBeanIfNecessary(beanName, bean, mbd);
		}
		catch (BeanDefinitionValidationException ex) {
			throw new BeanCreationException(
					mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
		}
		return exposedObject;
	}