spring循環(huán)依賴的解決方案

Spring IOC循環(huán)依賴解決方案分析

這里Spring主要用了三層緩存來完成對(duì)循環(huán)依賴的實(shí)現(xiàn)。

下面的屬性來源于DefaultSingletonBeanRegistry類

    /** Cache of singleton objects: bean name --> bean instance */
    private final Map<String, Object> singletonObjects = new ConcurrentHashMap<String, Object>(64);

    /** Cache of singleton factories: bean name --> ObjectFactory */
    private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<String, ObjectFactory<?>>(16);

    /** Cache of early singleton objects: bean name --> bean instance */
    private final Map<String, Object> earlySingletonObjects = new HashMap<String, Object>(16);

當(dāng)獲取一個(gè)實(shí)例對(duì)象的時(shí)候，會(huì)調(diào)用此方法

    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 != NULL_OBJECT ? singletonObject : null);
    }

先從singletonObjects 找，如果找不到會(huì)從earlySingletonObjects中查詢，再找不到去singletonFactories 中去查詢，如果找到的話會(huì)放在earlySingletonObjects中，那么問題來了，singletonFactories的對(duì)象是什么時(shí)候放進(jìn)去的。

研究Spring構(gòu)造類實(shí)例的時(shí)候，通過AbstractAutowireCapableBeanFactory的doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args)方法中調(diào)用addSingletonFactory方法將A類曝光到singletonFactories中。

    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);
            }
        }
    }

Spring注入一個(gè)類的大體步驟分為兩部分，一是先完成對(duì)類的構(gòu)造工作，二是會(huì)對(duì)類的屬性進(jìn)行設(shè)置和填充
那么關(guān)于的單例的類實(shí)例之間的互相引用的問題就清晰了

假設(shè)A對(duì)象有個(gè)屬性B 則在A初始化的時(shí)候，構(gòu)造完成之后就放在了singletonFactories中，當(dāng)發(fā)現(xiàn)去set屬性的時(shí)候發(fā)現(xiàn)B沒有初始化，于是接著初始化B，設(shè)置屬性的時(shí)候在緩存中都可以拿到各自對(duì)象的引用，所以不會(huì)有循環(huán)依賴的報(bào)錯(cuò)