前面的文章也提到過，IOC容器初始化過程一般不包含Bean載入的實現。Bean載入一般發生在應用第一次通過getBean向容器索取Bean的時候。但有一個例外：如果在XML檔案中為Bean定義了lazy-init屬性，那麼Bean的載入在IOC容器初始化時就預先完成了，我們將其稱為預例項化。預例項化雖然會對容器初始化的效能有一些影響，但卻能夠提高應用第一次取得Bean的效能。

在上下文的初始化過程中，也就是refresh中的程式碼實現，可以看到預例項化是整個refresh初始化IoC容器的一個步驟。在AbstractApplicationContext中看一下refresh的實現。

@Override
 

public void refresh() throws BeansException, IllegalStateException {
    ...
    ...
            // Instantiate all remaining (non-lazy-init) singletons.
            finishBeanFactoryInitialization(beanFactory);
    ...
    ...
}
protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
    ...
    ...
    beanFactory.preInstantiateSingletons();
    ...
    ...
}
 

在finishBeanFactoryInitialization的方法中，封裝了對lazy-init屬性的處理，實際的處理是在DefaultListableBeanFactory這個基本容器的preInstantiateSingletons方法中完成的。

@Override
public void preInstantiateSingletons() throws BeansException {
    if (this.logger.isDebugEnabled()) {
        this.logger.debug("Pre-instantiating singletons in "
 
 + this);
    }

    // Iterate over a copy to allow for init methods which in turn register new bean definitions.
    // While this may not be part of the regular factory bootstrap, it does otherwise work fine.
    List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);

    // Trigger initialization of all non-lazy singleton beans...
    for (String beanName : beanNames) {
        RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
        if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
            if (isFactoryBean(beanName)) {
                final FactoryBean<?> factory = (FactoryBean<?>) getBean(FACTORY_BEAN_PREFIX + beanName);
                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 {
                getBean(beanName);
            }
        }
    }

    // Trigger post-initialization callback for all applicable beans...
    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();
            }
        }
    }
}