Spring-bean循环依赖-蒲公英云

循环依赖指的是两个或以上bean相互存在引用。

Spring中循环依赖的几种情况：
1.构造器参数循环依赖；无法解决，只能避免
2.setter方式，单例；
3.setter方法，prototype原型；无法解决，只能避免

在bean初始化一文中，可以知道，创建bean需要经过一下几个步骤：
1.实例化bean；
2.populate bean，填充bean的属性；
3.initialize bean，初始化bean；

对于作用域为singleton的bean，创建过程中以及创建完成会缓存该bean；
从DefaultSingletonBeanRegistry的源码中可以看出：

/** Cache of singleton objects: bean name --> bean instance */
private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);
/** Cache of singleton factories: bean name --> ObjectFactory */
private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16);
/** Cache of early singleton objects: bean name --> bean instance */
private final Map<String, Object> earlySingletonObjects = new HashMap<>(16);
//标记bean正在创建，创建完成后会remove
/** Names of beans that are currently in creation */
private final Set<String> singletonsCurrentlyInCreation =
        Collections.newSetFromMap(new ConcurrentHashMap<>(16));

spring容器通过三级缓存解决循环依赖问题。
容器在创建单例bean前，会先判断缓存中是否存在；
AbstractBeanFactory#doGetBean：

protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,
        @Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {
    final String beanName = transformedBeanName(name);
    Object bean;
    // Eagerly check singleton cache for manually registered singletons.
    //从缓存中获取
    Object sharedInstance = getSingleton(beanName);
    if (sharedInstance != null && args == null) {
        if (logger.isDebugEnabled()) {
            if (isSingletonCurrentlyInCreation(beanName)) {
                logger.debug("Returning eagerly cached instance of singleton bean '" + beanName +
                        "' that is not fully initialized yet - a consequence of a circular reference");
            }
            else {
                logger.debug("Returning cached instance of singleton bean '" + beanName + "'");
            }
        }
        bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
    } else {
        //不存在，创建bean
        ...
    }
}

再看看getSingleton方法：

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;
}

先从一级缓存singletonObjects获取，没有则从earlySingletonObjects获取，再没有就从singletonFactories获取，如果有，则移到二级缓存earlySingletonObjects；

如果缓存没有，则创建bean。bean在实例化bean之后，会将bean添加到singletonObjects中，并将bean从singletonFactories和earlySingletonObjects中移除，DefaultSingletonBeanRegistry#addSingleton：

/**
 * Add the given singleton factory for building the specified singleton
 * if necessary.
 * <p>To be called for eager registration of singletons, e.g. to be able to
 * resolve circular references.
 * @param beanName the name of the bean
 * @param singletonFactory the factory for the singleton object
 */
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);
            this.registeredSingletons.add(beanName);
        }
    }
}

这是解决循环依赖的关键，该方法调用在createBeanInstance之后，populateBean之前，它的作用是提前暴露对象，但该bean未完成属性填充以及初始化。
比如A和B存在循环依赖，bean A在实例化后，将自己保存到singletonFactories，此时进行下一步填充属性，发现需要引用到B，接着容器创建bean B，在实例化bean B后，填充属性时发现依赖到A，此时尝试从容器中获取A，由于此时A初始化尚未完成，但是A提前暴露了对象，所以可以从singletonFactories中提前获取A对象引用，接着B完成自身初始化，完成后再返回A，继续进行属性填充及初始化。

从上面也可以知道，为什么构造器参数循环依赖不行，因为要先实例化bean。容器也不能解决原型bean的循环依赖问题，因为容器没有缓存原型bean，也就不能提前暴露对象，只能去避免。

参考