首先，我们最基本的常识是从ApplicationContext入手，从AbstractApplicationContext的refresh()作为入口，找到finishBeanFactoryInitialization()方法，这个方法的作用是实例化非懒加载的Bean实例。那么我们就可以进入AbstractBeanFactory的getBean()方法。
@Override public Object getBean(String name) throws BeansException { return doGetBean(name, null, null, false); } @Override public <T> T getBean(String name, Class<T> requiredType) throws BeansException { return doGetBean(name, requiredType, null, false); } @Override public Object getBean(String name, Object... args) throws BeansException { return doGetBean(name, null, args, false); } 复制代码
下面来看看doGetBean()方法：
protected <T> T doGetBean( String name, @Nullable Class<T> requiredType, @Nullable Object[] args, boolean typeCheckOnly) throws BeansException { // 转换一下，比如有别名、&开头的名字等这些请求，全部都转换成统一的beanName String beanName = transformedBeanName(name); Object bean; // Eagerly check singleton cache for manually registered singletons. // 从缓存中去。先从一级缓存中取，不行再从二级缓存取、最后从三级缓存取 Object sharedInstance = getSingleton(beanName); if (sharedInstance != null && args == null) { if (logger.isTraceEnabled()) { if (isSingletonCurrentlyInCreation(beanName)) { logger.trace("Returning eagerly cached instance of singleton bean '" + beanName + "' that is not fully initialized yet - a consequence of a circular reference"); } else { logger.trace("Returning cached instance of singleton bean '" + beanName + "'"); } } // 扩展点---------扩展点-----------扩展点------------- // 判断name是不是&开头的，如果是&开头的话，并且sharedInstance是FactoryBean类型，就直接返回 // 如果不是&开头，也不是FactoryBean类型，直接返回 // 如果不是&开头，是FactoryBean类型，调用getObject()方法返回Bean对象 // 也就是说，我们可以通过实现FactoryBean来创建Bean对象 bean = getObjectForBeanInstance(sharedInstance, name, beanName, null); } else { // Fail if we're already creating this bean instance: // We're assumably within a circular reference. if (isPrototypeCurrentlyInCreation(beanName)) { throw new BeanCurrentlyInCreationException(beanName); } // 下面是从父容器中找对应的Bean实例 // 注意：容器是存在父子关系的。spring mvc就是一个父子关系的容器 // Check if bean definition exists in this factory. BeanFactory parentBeanFactory = getParentBeanFactory(); if (parentBeanFactory != null && !containsBeanDefinition(beanName)) { // Not found -> check parent. String nameToLookup = originalBeanName(name); if (parentBeanFactory instanceof AbstractBeanFactory) { return ((AbstractBeanFactory) parentBeanFactory).doGetBean( nameToLookup, requiredType, args, typeCheckOnly); } else if (args != null) { // Delegation to parent with explicit args. return (T) parentBeanFactory.getBean(nameToLookup, args); } else if (requiredType != null) { // No args -> delegate to standard getBean method. return parentBeanFactory.getBean(nameToLookup, requiredType); } else { return (T) parentBeanFactory.getBean(nameToLookup); } } if (!typeCheckOnly) { markBeanAsCreated(beanName); } // 下面准备查找当前Bean对象依赖的其他Bean。比如在创建当前这个Bean之前必须要创建另外一个Bean try { RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName); checkMergedBeanDefinition(mbd, beanName, args); // 找到所有在这个Bean创建之前一定要创建出来的依赖，依次调用getBean()创建 // Guarantee initialization of beans that the current bean depends on. String[] dependsOn = mbd.getDependsOn(); if (dependsOn != null) { for (String dep : dependsOn) { if (isDependent(beanName, dep)) { throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Circular depends-on relationship between '" + beanName + "' and '" + dep + "'"); } registerDependentBean(dep, beanName); try { getBean(dep); } catch (NoSuchBeanDefinitionException ex) { throw new BeanCreationException(mbd.getResourceDescription(), beanName, "'" + beanName + "' depends on missing bean '" + dep + "'", ex); } } } // Create bean instance. if (mbd.isSingleton()) { // ******这是重点******这是重点******这是重点******这是重点****** // 这里理解起来有点绕。注意这个getSingleton(String beanName,ObjectFactory<?> singletonFactory)方法。 // 第二个参数使用lamble表达式，可以先忽略 // getSingleton()里面就做了两件事情： // 1.通过lamble表达式创建Bean对象 // 2.把创建好的Bean对象放到一级缓存中 // 最后返回创建好的Bean对象 sharedInstance = getSingleton(beanName, () -> { try { // 一定要先理清楚getSingleton()方法，再来看createBean()方法 // ******这是重点******这是重点******这是重点******这是重点***** 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. // 没创建成功就要销毁。不是重点 destroySingleton(beanName); throw ex; } }); // 同样的，检查一下是不是FactoryBean，并且name是否以&开头 // 扩展点---------扩展点-----------扩展点------------- // 判断name是不是&开头的，如果是&开头的话，并且sharedInstance是FactoryBean类型，就直接返回 // 如果不是&开头，也不是FactoryBean类型，直接返回 // 如果不是&开头，是FactoryBean类型，调用getObject()方法返回Bean对象 // 也就是说，我们可以通过实现FactoryBean来创建Bean对象 bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd); } // 其实到这里就可以不用看非单例的情况了，因为基本都差不多。重点关注如何创建Bean对象。 else if (mbd.isPrototype()) { // It's a prototype -> create a new instance. Object prototypeInstance = null; try { beforePrototypeCreation(beanName); // 还是创建Bean对象，这是重点 // ******这是重点******这是重点******这是重点******这是重点***** prototypeInstance = createBean(beanName, mbd, args); } finally { afterPrototypeCreation(beanName); } // 同上，检查是否是FactoryBean bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd); } // 这里也一样，其他scope，重点还是在createBean() else { String scopeName = mbd.getScope(); if (!StringUtils.hasLength(scopeName)) { throw new IllegalStateException("No scope name defined for bean ´" + beanName + "'"); } Scope scope = this.scopes.get(scopeName); if (scope == null) { throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'"); } try { Object scopedInstance = scope.get(beanName, () -> { beforePrototypeCreation(beanName); try { // 重点都在这里 // ******这是重点******这是重点******这是重点******这是重点***** return createBean(beanName, mbd, args); } finally { afterPrototypeCreation(beanName); } }); bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd); } catch (IllegalStateException ex) { throw new BeanCreationException(beanName, "Scope '" + scopeName + "' is not active for the current thread; consider " + "defining a scoped proxy for this bean if you intend to refer to it from a singleton", ex); } } } catch (BeansException ex) { cleanupAfterBeanCreationFailure(beanName); throw ex; } } //下面都不是重点，可忽略 // Check if required type matches the type of the actual bean instance. if (requiredType != null && !requiredType.isInstance(bean)) { try { T convertedBean = getTypeConverter().convertIfNecessary(bean, requiredType); if (convertedBean == null) { throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass()); } return convertedBean; } catch (TypeMismatchException ex) { if (logger.isTraceEnabled()) { logger.trace("Failed to convert bean '" + name + "' to required type '" + ClassUtils.getQualifiedName(requiredType) + "'", ex); } throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass()); } } return (T) bean; } 复制代码
下面简单分析一下getSingleton():
public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) { Assert.notNull(beanName, "Bean name must not be null"); synchronized (this.singletonObjects) { // 先从一级缓存中取，先试试 Object singletonObject = this.singletonObjects.get(beanName); if (singletonObject == null) { if (this.singletonsCurrentlyInDestruction) { throw new BeanCreationNotAllowedException(beanName, "Singleton bean creation not allowed while singletons of this factory are in destruction " + "(Do not request a bean from a BeanFactory in a destroy method implementation!)"); } if (logger.isDebugEnabled()) { logger.debug("Creating shared instance of singleton bean '" + beanName + "'"); } // 这不用看，就是做一个数据校验 beforeSingletonCreation(beanName); boolean newSingleton = false; boolean recordSuppressedExceptions = (this.suppressedExceptions == null); if (recordSuppressedExceptions) { this.suppressedExceptions = new LinkedHashSet<>(); } try { // ******这是重点******这是重点******这是重点******这是重点***** // 这里就是调用lamble表示式里面的逻辑了，千万要理清楚 // lamble表达式里面就是做了createBean()的逻辑 singletonObject = singletonFactory.getObject(); newSingleton = true; } // 处理创建Bean对象出异常的情况，不用管 catch (IllegalStateException ex) { // Has the singleton object implicitly appeared in the meantime -> // if yes, proceed with it since the exception indicates that state. singletonObject = this.singletonObjects.get(beanName); if (singletonObject == null) { throw ex; } } catch (BeanCreationException ex) { if (recordSuppressedExceptions) { for (Exception suppressedException : this.suppressedExceptions) { ex.addRelatedCause(suppressedException); } } throw ex; } finally { if (recordSuppressedExceptions) { this.suppressedExceptions = null; } afterSingletonCreation(beanName); } if (newSingleton) { // 最后就是把创建好的Bean对象放到一级缓存中。因为这个Bean对象已经是可以安全发布的了。 addSingleton(beanName, singletonObject); } } // 返回创建好的Bean对象 return singletonObject; } } 复制代码
核心重点createBean()方法：
@Override protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) throws BeanCreationException { if (logger.isTraceEnabled()) { logger.trace("Creating instance of bean '" + beanName + "'"); } RootBeanDefinition mbdToUse = mbd; // Make sure bean class is actually resolved at this point, and // clone the bean definition in case of a dynamically resolved Class // which cannot be stored in the shared merged bean definition. Class<?> resolvedClass = resolveBeanClass(mbd, beanName); if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) { mbdToUse = new RootBeanDefinition(mbd); mbdToUse.setBeanClass(resolvedClass); } // Prepare method overrides. try { mbdToUse.prepareMethodOverrides(); } catch (BeanDefinitionValidationException ex) { throw new BeanDefinitionStoreException(mbdToUse.getResourceDescription(), beanName, "Validation of method overrides failed", ex); } try { // ********重点1，这是一个扩展点，此时还没有任何Bean实例被创建出来 // ********这里就是把Bean的创建机会开放给开发者 Object bean = resolveBeforeInstantiation(beanName, mbdToUse); if (bean != null) { return bean; } } catch (Throwable ex) { throw new BeanCreationException(mbdToUse.getResourceDescription(), beanName, "BeanPostProcessor before instantiation of bean failed", ex); } try { // 如果开发者自己不创建Bean对象，那么就spring自己来亲自处理 // 重点2，这是spring框架创建Bean的过程 Object beanInstance = doCreateBean(beanName, mbdToUse, args); if (logger.isTraceEnabled()) { logger.trace("Finished creating instance of bean '" + beanName + "'"); } return beanInstance; } catch (BeanCreationException | ImplicitlyAppearedSingletonException ex) { // A previously detected exception with proper bean creation context already, // or illegal singleton state to be communicated up to DefaultSingletonBeanRegistry. throw ex; } catch (Throwable ex) { throw new BeanCreationException( mbdToUse.getResourceDescription(), beanName, "Unexpected exception during bean creation", ex); } } 复制代码
扩展点1：开发者自己创建Bean
@Nullable protected Object resolveBeforeInstantiation(String beanName, RootBeanDefinition mbd) { Object bean = null; if (!Boolean.FALSE.equals(mbd.beforeInstantiationResolved)) { // Make sure bean class is actually resolved at this point. if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) { Class<?> targetType = determineTargetType(beanName, mbd); if (targetType != null) { // 这里就是创建Bean的入口 bean = applyBeanPostProcessorsBeforeInstantiation(targetType, beanName); if (bean != null) { // 如果真的把Bean实例创建出来了，那么再继续后续的生命周期的逻辑调用 bean = applyBeanPostProcessorsAfterInitialization(bean, beanName); } } } mbd.beforeInstantiationResolved = (bean != null); } return bean; } @Nullable protected Object applyBeanPostProcessorsBeforeInstantiation(Class<?> beanClass, String beanName) { for (BeanPostProcessor bp : getBeanPostProcessors()) { // 一定要是InstantiationAwareBeanPostProcessor的实现类，才会在Bean没有实例化的时候被调用 if (bp instanceof InstantiationAwareBeanPostProcessor) { InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp; Object result = ibp.postProcessBeforeInstantiation(beanClass, beanName); if (result != null) { return result; } } } return null; } @Override public Object applyBeanPostProcessorsAfterInitialization(Object existingBean, String beanName) throws BeansException { Object result = existingBean; // 这里面可以对已经创建出来的Bean实例增强 for (BeanPostProcessor processor : getBeanPostProcessors()) { Object current = processor.postProcessAfterInitialization(result, beanName); if (current == null) { return result; } result = current; } return result; } 复制代码
1.实现了InstantiationAwareBeanPostProcessor接口的postProcessBeforeInstantiation()方法，就有机会自己创建Bean对象
2.实现了BeanPostProcessor接口的postProcessAfterInitialization()方法就能在Bean对象创建出来后，还有机会对创建出来的Bean对象进行增强或者扩展。注意：可以在这个点做AOP增强。
