上一节分析了Spring从缓存中获取单例bean的过程，相对于创建一个全新的单例bean，该过程还是很简单的，本节接着分析创建单例bean的准备工作。
引
protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType, @Nullable final Object[] args, boolean typeCheckOnly) throws BeansException { // 1、转换bean的名称,去掉&前缀,且如果bean有别名的话,优先使用别名 final String beanName = transformedBeanName(name); Object bean; // 2、从缓存中获取bean Object sharedInstance = getSingleton(beanName); if (sharedInstance != null && args == null) { bean = getObjectForBeanInstance(sharedInstance, name, beanName, null); } // 3、如果未能从缓存中获取到bean,则要重新创建bean else { // Fail if we're already creating this bean instance: We're assumably within a circular reference. // 3.1判断指定的原型模式的bean是否当前正在创建(在当前线程内),如果是->则抛出异常(Spring不会解决原型模式bean的循环依赖) if (isPrototypeCurrentlyInCreation(beanName)) { throw new BeanCurrentlyInCreationException(beanName); } // Check if bean definition exists in this factory. // 3.2 检测bean definition是否存在beanFactory中 BeanFactory parentBeanFactory = getParentBeanFactory(); // 如果当前BeanFactory中不包含给定beanName的beanDefinition定义,且父beanFactory不为空,则去父beanFactory中再次查找 if (parentBeanFactory != null && !containsBeanDefinition(beanName)) { // 3.3 将name转换为原始beanName // 因为这里的name已经经过beanName的规范处理,例如:&myBean-->规范-->myBean // 所以当我们再次去父beanFactory查找时,要将beanName再次转换为原始的beanName,myBean-->回转-->&myBean String nameToLookup = originalBeanName(name); // 3.4 下面会递归调用各种getBean的方法重载,从当前bean的父factoryBean中加载bean if (parentBeanFactory instanceof AbstractBeanFactory) { return ((AbstractBeanFactory) parentBeanFactory).doGetBean(nameToLookup, requiredType, args, typeCheckOnly); } else if (args != null) { // 参数不为空,则委托parentBeanFactory使用显式参数调动 return (T) parentBeanFactory.getBean(nameToLookup, args); } else if (requiredType != null) { // 参数为空,则委托parentBeanFactory使用标准的getBean方法获取bean return parentBeanFactory.getBean(nameToLookup, requiredType); } else { // 否则委托parentBeanFactory使用默认的getBean方法 return (T) parentBeanFactory.getBean(nameToLookup); } } // 3.3 如果当前bean不是用于类型检查,则将该bean标记为已经被创建或者即将被创建 if (!typeCheckOnly) { markBeanAsCreated(beanName); } try { // 3.4 合并beanDefinition,如果指定的bean是一个子bean的话,则遍历其所有的父bean final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName); // 校验合并的beanDefinition,如果验证失败,则抛出异常 checkMergedBeanDefinition(mbd, beanName, args); // 3.5 确保初始化当前bean所依赖的bean。 String[] dependsOn = mbd.getDependsOn(); if (dependsOn != null) { // 循环所有的依赖bean,并递归实例化 for (String dep : dependsOn) { if (isDependent(beanName, dep)) { throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Circular depends-on relationship between '" + beanName + "' and '" + dep + "'"); } // 注册依赖 registerDependentBean(dep, beanName); try { // 实例化依赖的bean getBean(dep); } catch (NoSuchBeanDefinitionException ex) { throw new BeanCreationException(mbd.getResourceDescription(), beanName, "'" + beanName + "' depends on missing bean '" + dep + "'", ex); } } } // 3.6 创建单例bean if (mbd.isSingleton()) { sharedInstance = getSingleton(beanName, () -> { try { // 创建bean 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); } // 创建原型模式bean else if (mbd.isPrototype()) { // It's a prototype -> create a new instance. Object prototypeInstance = null; try { beforePrototypeCreation(beanName); prototypeInstance = createBean(beanName, mbd, args); } finally { afterPrototypeCreation(beanName); } bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd); } else { String scopeName = mbd.getScope(); final 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 { 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; } } // 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) { throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass()); } } return (T) bean; }
在到达步骤3.6之前的工作，都是为创建当前单例bean的准备工作。其中包括了：对BeanDefinition的检测、如果当前BeanFactory中不包含BeanDefinition则对父BeanFactory的检测、BeanDefinition的合并、依赖bean的初始化等等工作。
1.创建单例bean
// 3.6 创建单例bean if (mbd.isSingleton()) { sharedInstance = getSingleton(beanName, () -> { try { // 创建bean 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); }
- getSingleton方法分析
public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) { synchronized (this.singletonObjects) { // 1、尝试从缓存中获取bean 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!)"); } // 2、创建bean之前的回调方法 beforeSingletonCreation(beanName); boolean newSingleton = false; boolean recordSuppressedExceptions = (this.suppressedExceptions == null); if (recordSuppressedExceptions) { this.suppressedExceptions = new LinkedHashSet<>(); } try { // 3、通过getObject创建bean的实例 singletonObject = singletonFactory.getObject(); newSingleton = true; } 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; } // 4、创建bean之后的回调方法 afterSingletonCreation(beanName); } if (newSingleton) { // 5、将新创建的单例bean加入缓存 addSingleton(beanName, singletonObject); } } return singletonObject; } }
该方法中完成了对单例bean的创建工作，并在创建bean之前和之后进行方法回调进行一些异常信息的检查，最后将单例bean加入缓存中。其中关键的步骤就是第三步
通过getObject创建bean的实例，singletonObject = singletonFactory.getObject();，这里初学者阅读起来可能稍微有些绕，singletonFactory.getObject();方法执行后会回到3.6中的return createBean(beanName, mbd, args)；完成对bean的创建。
- createBean方法分析
protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) throws BeanCreationException { 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. // 确保此时bean类已经被解析，并且在动态解析类不能存储在共享合并bean定义中时克隆bean定义。 Class<?> resolvedClass = resolveBeanClass(mbd, beanName); if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) { mbdToUse = new RootBeanDefinition(mbd); mbdToUse.setBeanClass(resolvedClass); } // 准备和验证配置的方法注入 // 注意:这里处理的是replace-method和lookup-method方法注入,而不是@Override注解 try { mbdToUse.prepareMethodOverrides(); } catch (BeanDefinitionValidationException ex) { throw new BeanDefinitionStoreException(mbdToUse.getResourceDescription(),beanName, "Validation of method overrides failed", ex); } try { // Give BeanPostProcessors a chance to return a proxy instead of the target bean instance. // 让beanPostprocessor有机会返回一个代理而不是目标bean实例。 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 { // 执行bean创建,并返回bean的实例 Object beanInstance = doCreateBean(beanName, mbdToUse, args); return beanInstance; } catch (BeanCreationException | ImplicitlyAppearedSingletonException ex) { throw ex; } catch (Throwable ex) { throw new BeanCreationException(mbdToUse.getResourceDescription(), beanName, "Unexpected exception during bean creation", ex); } }
从方法名上好像已经接触到了IoC的核心-->bean的创建，但是很遗憾，该方法中依然只是例行检查工作，真正的创建方法在doCreateBean中，从这里也能看出spring代码阅读的一个小技巧，真正干活儿的往往是doXXX开头的方法，而在doXXX之前都是做一些准备工作
