Spring bean的加载

Spring bean的加载

AbstractBeanFactory#doGetBean

• doGetBean才是真正向IoC容器获取被管理Bean的过程

AbstractAutowireCapableBeanFactory#doCreateBean

• 真正创建bean

//获取IOC容器中指定名称的Bean
	@Override
	public Object getBean(String name) throws BeansException {
     
		//doGetBean才是真正向IoC容器获取被管理Bean的过程
		return doGetBean(name, null, null, false);
	}

@SuppressWarnings("unchecked")
	//真正实现向IOC容器获取Bean的功能，也是触发依赖注入功能的地方
	protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,
			@Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {
     

		//根据指定的名称获取被管理Bean的名称，剥离指定名称中对容器的相关依赖
		//如果指定的是别名，将别名转换为规范的Bean名称
		final String beanName = transformedBeanName(name);
		Object bean;

		// Eagerly check singleton cache for manually registered singletons.
		//先从缓存中取是否已经有被创建过的单态类型的Bean
		//对于单例模式的Bean整个IOC容器中只创建一次，不需要重复创建
		Object sharedInstance = getSingleton(beanName);
		//IOC容器创建单例模式Bean实例对象
		if (sharedInstance != null && args == null) {
     
			if (logger.isDebugEnabled()) {
     
				//如果指定名称的Bean在容器中已有单例模式的Bean被创建
				//直接返回已经创建的Bean
				if (isSingletonCurrentlyInCreation(beanName)) {
     
					logger.debug("Returning eagerly cached instance of singleton bean '" + beanName +
							"' that is not fully initialized yet - a consequence of a circular reference");
				}
				else {
     
					logger.debug("Returning cached instance of singleton bean '" + beanName + "'");
				}
			}
			//获取给定Bean的实例对象，主要是完成FactoryBean的相关处理
			//注意：BeanFactory是管理容器中Bean的工厂，而FactoryBean是
			//创建创建对象的工厂Bean，两者之间有区别
			bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
		}

		else {
     
			// Fail if we're already creating this bean instance:
			// We're assumably within a circular reference.
			//缓存没有正在创建的单例模式Bean
			//缓存中已经有已经创建的原型模式Bean
			//但是由于循环引用的问题导致实例化对象失败
			if (isPrototypeCurrentlyInCreation(beanName)) {
     
				throw new BeanCurrentlyInCreationException(beanName);
			}

			// Check if bean definition exists in this factory.
			//对IOC容器中是否存在指定名称的BeanDefinition进行检查，首先检查是否
			//能在当前的BeanFactory中获取的所需要的Bean，如果不能则委托当前容器
			//的父级容器去查找，如果还是找不到则沿着容器的继承体系向父级容器查找
			BeanFactory parentBeanFactory = getParentBeanFactory();
			//当前容器的父级容器存在，且当前容器中不存在指定名称的Bean
			if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
     
				// Not found -> check parent.
				//解析指定Bean名称的原始名称
				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.
					//委派父级容器根据指定名称和显式的参数查找
					return (T) parentBeanFactory.getBean(nameToLookup, args);
				}
				else {
     
					// No args -> delegate to standard getBean method.
					//委派父级容器根据指定名称和类型查找
					return parentBeanFactory.getBean(nameToLookup, requiredType);
				}
			}

			//创建的Bean是否需要进行类型验证，一般不需要
			if (!typeCheckOnly) {
     
				//向容器标记指定的Bean已经被创建
				markBeanAsCreated(beanName);
			}

			try {
     
				//根据指定Bean名称获取其父级的Bean定义
				//主要解决Bean继承时子类合并父类公共属性问题
				final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
				checkMergedBeanDefinition(mbd, beanName, args);

				// Guarantee initialization of beans that the current bean depends on.
				//获取当前Bean所有依赖Bean的名称
				String[] dependsOn = mbd.getDependsOn();
				//如果当前Bean有依赖Bean
				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 + "'");
						}
						//递归调用getBean方法，获取当前Bean的依赖Bean
						registerDependentBean(dep, beanName);
						//把被依赖Bean注册给当前依赖的Bean
						getBean(dep);
					}
				}

				// Create bean instance.
				//创建单例模式Bean的实例对象
				if (mbd.isSingleton()) {
     
					//这里使用了一个匿名内部类，创建Bean实例对象，并且注册给所依赖的对象
					sharedInstance = getSingleton(beanName, () -> {
     
						try {
     
							//创建一个指定Bean实例对象，如果有父级继承，则合并子类和父类的定义
							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.
							//显式地从容器单例模式Bean缓存中清除实例对象
							destroySingleton(beanName);
							throw ex;
						}
					});
					//获取给定Bean的实例对象
					bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
				}

				//IOC容器创建原型模式Bean实例对象
				else if (mbd.isPrototype()) {
     
					// It's a prototype -> create a new instance.
					//原型模式(Prototype)是每次都会创建一个新的对象
					Object prototypeInstance = null;
					try {
     
						//回调beforePrototypeCreation方法，默认的功能是注册当前创建的原型对象
						beforePrototypeCreation(beanName);
						//创建指定Bean对象实例
						prototypeInstance = createBean(beanName, mbd, args);
					}
					finally {
     
						//回调afterPrototypeCreation方法，默认的功能告诉IOC容器指定Bean的原型对象不再创建
						afterPrototypeCreation(beanName);
					}
					//获取给定Bean的实例对象
					bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
				}

				//要创建的Bean既不是单例模式，也不是原型模式，则根据Bean定义资源中
				//配置的生命周期范围，选择实例化Bean的合适方法，这种在Web应用程序中
				//比较常用，如：request、session、application等生命周期
				else {
     
					String scopeName = mbd.getScope();
					final Scope scope = this.scopes.get(scopeName);
					//Bean定义资源中没有配置生命周期范围，则Bean定义不合法
					if (scope == null) {
     
						throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'");
					}
					try {
     
						//这里又使用了一个匿名内部类，获取一个指定生命周期范围的实例
						Object scopedInstance = scope.get(beanName, () -> {
     
							beforePrototypeCreation(beanName);
							try {
     
								return createBean(beanName, mbd, args);
							}
							finally {
     
								afterPrototypeCreation(beanName);
							}
						});
						//获取给定Bean的实例对象
						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.
		//对创建的Bean实例对象进行类型检查
		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.isDebugEnabled()) {
     
					logger.debug("Failed to convert bean '" + name + "' to required type '" +
							ClassUtils.getQualifiedName(requiredType) + "'", ex);
				}
				throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
			}
		}
		return (T) bean;
	}

//真正创建Bean的方法
	protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
			throws BeanCreationException {
     

		// Instantiate the bean.
		//封装被创建的Bean对象
		BeanWrapper instanceWrapper = null;
		if (mbd.isSingleton()) {
     
			instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
		}
		if (instanceWrapper == null) {
     
			//根据指定bean 使用对应的策略创建新的实例，如工厂方法，构造函数自动注入，简单初始化
			instanceWrapper = createBeanInstance(beanName, mbd, args);
		}
		final Object bean = instanceWrapper.getWrappedInstance();
		//获取实例化对象的类型
		Class<?> beanType = instanceWrapper.getWrappedClass();
		if (beanType != NullBean.class) {
     
			mbd.resolvedTargetType = beanType;
		}

		// Allow post-processors to modify the merged bean definition.
		//调用PostProcessor后置处理器
		// MergedBeanDefinitionPostProcessor 应用 Autowired注解正是通过此方法实现诸如类型的预解析
		synchronized (mbd.postProcessingLock) {
     
			if (!mbd.postProcessed) {
     
				try {
     
					applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
				}
				catch (Throwable ex) {
     
					throw new BeanCreationException(mbd.getResourceDescription(), beanName,
							"Post-processing of merged bean definition failed", ex);
				}
				mbd.postProcessed = true;
			}
		}

		// Eagerly cache singletons to be able to resolve circular references
		// even when triggered by lifecycle interfaces like BeanFactoryAware.
		//向容器中缓存单例模式的Bean对象，以防循环引用 --》是否需要提早曝光。
		boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
				isSingletonCurrentlyInCreation(beanName));
		if (earlySingletonExposure) {
     
			if (logger.isDebugEnabled()) {
     
				logger.debug("Eagerly caching bean '" + beanName +
						"' to allow for resolving potential circular references");
			}
			//这里是一个匿名内部类，为了防止循环引用，尽早持有对象的引用
			// 	为避免后期的循环依赖，可以在初始化完成前将创建实例的ObjectFactory加入工厂。
			addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
		}

		// Initialize the bean instance.
		//Bean对象的初始化，依赖注入在此触发
		//这个exposedObject在初始化完成之后返回作为依赖注入完成后的Bean
		Object exposedObject = bean;
		try {
     
			//将Bean实例对象封装，并且Bean定义中配置的属性值赋值给实例对象
			populateBean(beanName, mbd, instanceWrapper);
			//初始化Bean对象
			exposedObject = initializeBean(beanName, exposedObject, mbd);
		}
		catch (Throwable ex) {
     
			if (ex instanceof BeanCreationException && beanName.equals(((BeanCreationException) ex).getBeanName())) {
     
				throw (BeanCreationException) ex;
			}
			else {
     
				throw new BeanCreationException(
						mbd.getResourceDescription(), beanName, "Initialization of bean failed", ex);
			}
		}

		if (earlySingletonExposure) {
     
			//获取指定名称的已注册的单例模式Bean对象
			Object earlySingletonReference = getSingleton(beanName, false);
			// earlySingletonReference 只有在检测到有循环依赖的情况下才会不为空。
			if (earlySingletonReference != null) {
     
				//根据名称获取的已注册的Bean和正在实例化的Bean是同一个,exposedObject 没有被增强。
				if (exposedObject == bean) {
     
					//当前实例化的Bean初始化完成
					exposedObject = earlySingletonReference;
				}
				//当前Bean依赖其他Bean，并且当发生循环引用时不允许新创建实例对象
				else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
     
					String[] dependentBeans = getDependentBeans(beanName);
					Set<String> actualDependentBeans = new LinkedHashSet<>(dependentBeans.length);
					//获取当前Bean所依赖的其他Bean
					for (String dependentBean : dependentBeans) {
     
						//对依赖Bean进行类型检查
						if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
     
							actualDependentBeans.add(dependentBean);
						}
					}
					 //因为bean创建后，其所依赖的bean 一定是已经创建的
					// actualDependentBeans不为空，表示当前bean 创建后其所依赖的bean 却没有全部创建完，就是说存在循环依赖。
					if (!actualDependentBeans.isEmpty()) {
     
						throw new BeanCurrentlyInCreationException(beanName,
								"Bean with name '" + beanName + "' has been injected into other beans [" +
								StringUtils.collectionToCommaDelimitedString(actualDependentBeans) +
								"] in its raw version as part of a circular reference, but has eventually been " +
								"wrapped. This means that said other beans do not use the final version of the " +
								"bean. This is often the result of over-eager type matching - consider using " +
								"'getBeanNamesOfType' with the 'allowEagerInit' flag turned off, for example.");
					}
				}
			}
		}

		// Register bean as disposable.
		//注册完成依赖注入的Bean
		try {
     
			// 根据scope注册bean
			registerDisposableBeanIfNecessary(beanName, bean, mbd);
		}
		catch (BeanDefinitionValidationException ex) {
     
			throw new BeanCreationException(
					mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
		}

		return exposedObject;
	}