深入理解Spring系列之六：bean初始化

《深入理解Spring系列之四：BeanDefinition装载前奏曲》中提到，对于非延迟单例bean的初始化在finishBeanFactoryInitialization(beanFactory)中完成。进入这个方法，代码如下。

protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) { // Initialize conversion service for this context. if (beanFactory.containsBean(CONVERSION_SERVICE_BEAN_NAME) && beanFactory.isTypeMatch(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class)) { beanFactory.setConversionService( beanFactory.getBean(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class)); } // Initialize LoadTimeWeaverAware beans early to allow for registering their transformers early. String[] weaverAwareNames = beanFactory.getBeanNamesForType(LoadTimeWeaverAware.class, false, false); for (String weaverAwareName : weaverAwareNames) { getBean(weaverAwareName); } // Stop using the temporary ClassLoader for type matching. beanFactory.setTempClassLoader(null); // Allow for caching all bean definition metadata, not expecting further changes. beanFactory.freezeConfiguration(); // Instantiate all remaining (non-lazy-init) singletons. beanFactory.preInstantiateSingletons(); }

关注最后一行代码，beanFactory.preInstantiateSingletons()完成初始化所有非延迟的单例bean，进入这个方法的具体实现，代码如下。

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<String>(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(new PrivilegedAction<Boolean>() { @Override public Boolean run() { return ((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(new PrivilegedAction<Object>() { @Override public Object run() { smartSingleton.afterSingletonsInstantiated(); return null; } }, getAccessControlContext()); }else { smartSingleton.afterSingletonsInstantiated(); } } } }

从上面的代码中看到，只会对非延迟单例bean进行初始化，scope为其它值的bean会在使用到的时候进行初始化，如prototype。这里关注getBean方法，这个方法看着很眼熟，其实就是《深入理解Spring系列之一：开篇》示例代码中用到的getBean，Spring对这个方法做了重复使用。getBean方法的具体实现在doGetBean方法中，这个方法的代码很长就不贴代码了。在doGetBean中，首先会初始化其依赖的bean，然后进行自身的初始化，这个方法里关注如下的代码段。

// Create bean instance. if (mbd.isSingleton()) { sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() { @Override public Object getObject() throws BeansException { try { return createBean(beanName, mbd, args); } catch (BeansException ex) { // Explicitly remove instance from singleton cache: It might have been put there // eagerly by the creation process, to allow for circular reference resolution. // Also remove any beans that received a temporary reference to the bean. destroySingleton(beanName); throw ex; } } }); bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd); }

这段代码完成了单例bean的初始化，追踪代码进入doCreateBean方法中，在这个方法中进行bean实例的创建、属性填充、将bean实例加入单例bean实例的缓存中。doCreateBean方法中有如下代码段。

if (instanceWrapper == null) { instanceWrapper = createBeanInstance(beanName, mbd, args); }

createBeanInstance方法里完成bean实例的创建，具体过程可继续追踪代码查看，其实就是使用反射进行实例对象的创建。