Spring源码分析—延迟加载

lazy-init 延迟加载机制分析

对于被修饰为lazy-init的bean Spring 容器初始化阶段不会进⾏ init 并且依赖注⼊，当第⼀次进⾏getBean时候才进⾏初始化并依赖注⼊对于⾮懒加载的bean，getBean的时候会从缓存⾥头获取，因为容器初始化阶段 Bean 已经初始化完成并缓存了起来。

public void preInstantiateSingletons() throws BeansException {
    if (logger.isDebugEnabled()) {
        logger.debug("Pre-instantiating singletons in " + this);
    }
    //所有bean的名字
    List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);

    // 触发所有非延迟加载单例beans的初始化，主要步骤为调用getBean
    for (String beanName : beanNames) {
        //合并父BeanDefinition
        RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
        // 不是抽象类、是单例的且不是懒加载的
        if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
            // 处理 FactoryBean
            if (isFactoryBean(beanName)) {
                //如果是FactoryBean则加上&
                Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);
                if (bean instanceof FactoryBean) {
                    final FactoryBean<?> factory = (FactoryBean<?>) bean;
                    // 判断当前 FactoryBean 是否是 SmartFactoryBean 的实现
                    boolean isEagerInit;
                    if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
                        isEagerInit = AccessController.doPrivileged((PrivilegedAction<Boolean>)
                                        ((SmartFactoryBean<?>) factory)::isEagerInit,
                                getAccessControlContext());
                    }
                    else {
                        isEagerInit = (factory instanceof SmartFactoryBean &&
                                ((SmartFactoryBean<?>) factory).isEagerInit());
                    }
                    if (isEagerInit) {
                        getBean(beanName);
                    }
                }
            }
            else {
                // 不是FactoryBean的直接使用此方法进行初始化
                getBean(beanName);
            }
        }
    }

    // 如果bean实现了 SmartInitializingSingleton 接口的，那么在这里得到回调
    for (String beanName : beanNames) {
        Object singletonInstance = getSingleton(beanName);
        if (singletonInstance instanceof SmartInitializingSingleton) {
            final SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance;
            if (System.getSecurityManager() != null) {
                AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
                    smartSingleton.afterSingletonsInstantiated();
                    return null;
                }, getAccessControlContext());
            }
            else {
                smartSingleton.afterSingletonsInstantiated();
            }
        }
    }
}