spring IOC源码 - Bean的实例化过程源码及过程

一时失言乱红尘 2023-07-25 08:41 1阅读 0赞

昨天看了看getBean的源码，今天来写一下Bean的实例化过程源码及流程

# 源码 #

首先来看一下getBean源码:

org.springframework.beans.factory.support.AbstractBeanFactory\#getBean(java.lang.String, java.lang.Class<T>)

@Override
    	public <T> T getBean(String name, @Nullable Class<T> requiredType) throws BeansException {
    		return doGetBean(name, requiredType, null, false);
    	}

这个类没有具体实现，来看一下doGetBean源码

org.springframework.beans.factory.support.AbstractBeanFactory\#doGetBean

protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,
    			@Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {
    
    		final String beanName = transformedBeanName(name);//转换beanName（别名）  源码
    		Object bean;
    
    		// Eagerly check singleton cache for manually registered singletons.
    		Object sharedInstance = getSingleton(beanName);//这句代码是springIOC最核心的一句 源码
    		if (sharedInstance != null && args == null) {
    			if (logger.isDebugEnabled()) {
    				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 + "'");
    				}
    			}
    			bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);// 对缓存中的bean进行后置处理 源码
    		}
    
    		else {
    			// Fail if we're already creating this bean instance:
    			// We're assumably within a circular reference.
    			if (isPrototypeCurrentlyInCreation(beanName)) {//原型正在创建，这里是拦截多例对象循环依赖抛异常的地方，为什么不能解决循环依赖，因为没有缓存
    				throw new BeanCurrentlyInCreationException(beanName);
    			}
    
    			// Check if bean definition exists in this factory.  这面是判断是否有父工厂，如果有就去父工厂找，没有的话自己创建
    			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 {
    					// No args -> delegate to standard getBean method.
    					return parentBeanFactory.getBean(nameToLookup, requiredType);
    				}
    			}
    
    			if (!typeCheckOnly) {
    				markBeanAsCreated(beanName);
    			}
    
    			try {
    				final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
    				checkMergedBeanDefinition(mbd, beanName, args);//abstract 抽象类不能被实例化
    
    				// Guarantee initialization of beans that the current bean depends on.
    				String[] dependsOn = mbd.getDependsOn(); //@DependsOn (value = "Aaa") 这个标签 Aaa这个类先创建
    				if (dependsOn != null) {  //这里主要是@DependsOn标签，以及A与B两个类相互依赖情况下会抛异常
    					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()) {  //如果是单例
    					sharedInstance = getSingleton(beanName, () -> { //创建singletonFactory，当调用singletonFactory的getBean方法时候，就是调用createBean  去获取单例  看一下这个源码
    						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); //对创建的 bean进行后续的加工
    				}
    
    				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) {
    				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;
    	}

在doGetBean源码中，首先会调用ransformedBeanName转换beanName（别名），然后再调用getSingleton(beanName)方法从缓存中获取对象，然后我们第一次进来的时候，是没有对象的，所以获取不到，这时候会进入getSingleton(String beanName, ObjectFactory<?> singletonFactory) 中的beforeSingletonCreation(beanName)方法首先将beanName放入singletonsCurrentlyInCreation（不放入不会创建bean），然后getSingleton(String beanName, ObjectFactory<?> singletonFactory)会调用singletonFactory.getObject()会回调createBean方法中的doCreateBean(beanName, mbdToUse, args)方法来正常创建bean，下面来看一doCreateBean(beanName, mbdToUse, args)方法源码：

org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory\#doCreateBean

protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
    			throws BeanCreationException {
    
    		// Instantiate the bean.
    		BeanWrapper instanceWrapper = null;
    		if (mbd.isSingleton()) {
    			instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
    		}
    		if (instanceWrapper == null) {		//如果是@Bean形式的，通过instantiateUsingFactoryMethod方法来进行调用创建对象；如果是@Autowired标注，就通过autowireConstructor进行反射调用；如果通过普通的包扫描，就调用默认的构造方法进行反射对象
    			instanceWrapper = createBeanInstance(beanName, mbd, args);//1.很垃圾的一段代码，看不懂  通过构造方法或者工厂方法把对象实例化出来
    		}
    		final Object bean = instanceWrapper.getWrappedInstance();
    		Class<?> beanType = instanceWrapper.getWrappedClass();
    		if (beanType != NullBean.class) {
    			mbd.resolvedTargetType = beanType;
    		}
    
    		// Allow post-processors to modify the merged bean definition.
    		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.
    		boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&//allowCircularReferences允许循环引用
    				isSingletonCurrentlyInCreation(beanName));
    		if (earlySingletonExposure) {
    			if (logger.isDebugEnabled()) {
    				logger.debug("Eagerly caching bean '" + beanName +
    						"' to allow for resolving potential circular references");
    			}
    			addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));//2.把beanName封装成ObjectFactory放入singletonFactories
    		}
    
    		// Initialize the bean instance.
    		Object exposedObject = bean;
    		try {
    			populateBean(beanName, mbd, instanceWrapper);//3. 给我bean赋值
    			exposedObject = initializeBean(beanName, exposedObject, mbd);//4.对 bean进行初始化  源码
    		}
    		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) {
    			Object earlySingletonReference = getSingleton(beanName, false);
    			if (earlySingletonReference != null) {
    				if (exposedObject == bean) {
    					exposedObject = earlySingletonReference;
    				}
    				else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
    					String[] dependentBeans = getDependentBeans(beanName);
    					Set<String> actualDependentBeans = new LinkedHashSet<>(dependentBeans.length);
    					for (String dependentBean : dependentBeans) {
    						if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
    							actualDependentBeans.add(dependentBean);
    						}
    					}
    					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.
    		try {
    			registerDisposableBeanIfNecessary(beanName, bean, mbd);//如果bean销毁 注册销毁的bean后置处理器
    		}
    		catch (BeanDefinitionValidationException ex) {
    			throw new BeanCreationException(
    					mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
    		}
    
    		return exposedObject;
    	}

doCreateBean此方法创建对象一共分为4部：

createBeanInstance（调用构造函数创建对象）

addSingletonFactory（把我们的早期对象放入缓存中，用户解决循环依赖）

populateBean（给创建的bean进行赋值）

initializeBean（对bean进行初始化）：此方法是对bean进行初始化，我们来看一下这个方法的源码：

org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory\#initializeBean(java.lang.String, java.lang.Object, org.springframework.beans.factory.support.RootBeanDefinition)

protected Object initializeBean(final String beanName, final Object bean, @Nullable RootBeanDefinition mbd) {
    		if (System.getSecurityManager() != null) {
    			AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
    				invokeAwareMethods(beanName, bean);
    				return null;
    			}, getAccessControlContext());
    		}
    		else {
    			invokeAwareMethods(beanName, bean);//1.调用Awre接口，进行回调
    		}
    
    		Object wrappedBean = bean;
    		if (mbd == null || !mbd.isSynthetic()) {
    			wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);//2.执行后置处理器的before方法
    		}
    
    		try {
    			invokeInitMethods(beanName, wrappedBean, mbd);	//3.调用初始化方法 源码
    		}
    		catch (Throwable ex) {
    			throw new BeanCreationException(
    					(mbd != null ? mbd.getResourceDescription() : null),
    					beanName, "Invocation of init method failed", ex);
    		}
    		if (mbd == null || !mbd.isSynthetic()) {
    			wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);// 4. 动态代理aop 	后置处理器的after方法
    		}
    
    		return wrappedBean;
    	}

此方法也分为四步：

1. invokeAwareMethods(调用XXAware接口)

2. applyBeanPostProcessorsBeforeInitialization(执行我们的后置处理器的before方法)

3. invokeInitMethods(对象的初始化方法)

4. applyBeanPostProcessorsAfterInitialization(后置处理器的after方法)

经过了以上步骤，我们的对象就初始化了，但是还有重要的一步，那就是把实例化好的bean加载到singletonObjects（单例缓存池）缓存中，这一部是非常重要的，因为singletonObjects，earlySingletonObjects，singletonFactories多级缓存是用来解决spring单实例循环依赖问题的。

这样，我们的实例化对象就创建了，然后我画了张图来总结这个过程，下面进行上图

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM4NjMwODEw_size_16_color_FFFFFF_t_70][]

[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM4NjMwODEw_size_16_color_FFFFFF_t_70]: /images/20230528/fd81ab8785f344c9930381178e55c923.png