Spring源码解析（2）：IOC过程上

在测试代码中手动调用FileSystemXmlApplicationContext的getBean方法，以查看Spring的Ioc过程

public class MaiClass {
    public static void main(String[] args){
        FileSystemXmlApplicationContext fileSystemXmlApplicationContext =
                new FileSystemXmlApplicationContext("classpath:spring-config.xml");
        TestBean testBean = (TestBean) fileSystemXmlApplicationContext.getBean("TestBean");
        testBean.print();
    }
}

FileSystemXmlApplicationContext调用了AbstractBeanFactory的getBean方法，进而调用了

doGetBean方法，这个方法中，获取如果是单例Bean，先从缓存中取得实例，若实例为空则执行创建新

的Bean，若不是单例bean，则直接创建。同时判断循环依赖，Spring仅能处理单例对象之间基于setter

方式注入的循环依赖，否则抛出BeanCurrentlyInCreationException异常。

protected <T> T doGetBean(//这里是实际取得Bean的地方，也是出发依赖注入发生的地方
			final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)
			throws BeansException {
//取beanName 去掉开头的&？？如果是别名，获取真实的beanName
		final String beanName = transformedBeanName(name);
		Object bean;
//1.先从缓存中取得Bean，处理那些已经被创建过的单例模式的Bean，对这种Bean请求不需要重复创建,不加锁判断，提高效率
		// Eagerly check singleton cache for manually registered singletons.
		Object sharedInstance = getSingleton(beanName);
		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 + "'");
				}
			}//完成FactoryBean的相关处理，取的FactoryBean的生产结果
			bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
		}

		else {
			// Spring 仅能够处理单例对象之间基于 setter 注入方式造成的循环依赖，
			// 除此之外全部抛出 BeanCurrentlyInCreationException 异常。
			if (isPrototypeCurrentlyInCreation(beanName)) {
				throw new BeanCurrentlyInCreationException(beanName);
			}
//这里IOC容器中的BeanDefinition是否存在进行检查，检查是否能在当前的BeanFactory中取的需要的Bean。
//如果在当前工厂取不到，则至双亲BeanFactory中取，如果双亲取不到，则沿着双亲一直向上查找
			BeanFactory parentBeanFactory = getParentBeanFactory();
			if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
				// Not found -> check parent.
				String nameToLookup = originalBeanName(name);
				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);//标记bean已经被创建
			}

			try {//这里根据Bean的名字取的RootBeanDefinition
				final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
				checkMergedBeanDefinition(mbd, beanName, args);
//获取当前Bean的所有依赖Bean，这样会触发getBean的递归调用，知道渠道一个没有任何以来的Bean位置
				// 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);
						}
					}
				}
//这里通过调用createBean方法创建Singleton bean实例，这里有一个回调函数getObject，会在getSingleton中调用ObjectFactory的createBean
// 此处还会对缓存中是否存有单例对象进行加锁判断，若确实无对象，调用ObjectFactory的createBean创建对象
				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);
				}
//这里是创建prototype bean的地方
				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, new ObjectFactory<Object>() {
							@Override
							public Object getObject() throws BeansException {
								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;
			}
		}
//对创建的Bean类型检查，如果没有问题，就返回这个新船建的Bean，这个Bean已经是包含了依赖关系的Bean
		// Check if required type matches the type of the actual bean instance.
		if (requiredType != null && bean != null && !requiredType.isInstance(bean)) {
			try {
				return getTypeConverter().convertIfNecessary(bean, requiredType);
			}
			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;
	}

查看AbstractAutowireCapableBeanFactory的createBean方法，这个方法简单来说，就是根据bean是否配置过aop来觉定bean的实例化过程

protected Object createBean(String beanName, RootBeanDefinition mbd, Object[] args) throws BeanCreationException {
		if (logger.isDebugEnabled()) {
			logger.debug("Creating instance of bean '" + beanName + "'");
		}
		RootBeanDefinition mbdToUse = mbd;
//这里判断要创建的Bean是否可以实例化，这个类是否可以通过类装载器载入
		// 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);
		}

		// 准备和验证配置的方法注入
		try {
			mbdToUse.prepareMethodOverrides();
		}
		catch (BeanDefinitionValidationException ex) {
			throw new BeanDefinitionStoreException(mbdToUse.getResourceDescription(),
					beanName, "Validation of method overrides failed", ex);
		}

		try {//跟aop相关
			// 如果Bean配置了PostPeocessor，那么这里返回的是一个proxy
			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);
		}
//这里是创建Bean的调用
		Object beanInstance = doCreateBean(beanName, mbdToUse, args);
		if (logger.isDebugEnabled()) {
			logger.debug("Finished creating instance of bean '" + beanName + "'");
		}
		return beanInstance;
	}

若bean没有配置aop，调用doCreateBean创建bean实例，doCreateBean方法对Bean的实例进行创建，然后通过后置处理器对Bean中@Resource、@Autowired之类的注解进行预处理，并把处理后得到的信息注册到BeanDefinition中去，然后解决单例的循环依赖问题，最后调用populateBean(beanName, mbd, instanceWrapper)方法进行依赖注入。

protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args)
			throws BeanCreationException {

		// 这个BeanWrapper是是用来持有创建出来的Bean对象的
		BeanWrapper instanceWrapper = null;
		if (mbd.isSingleton()) {//如果是单例,先把缓存中的同名Bean清除
			instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
		}
		if (instanceWrapper == null) {//这里是创建Bean的地方,由createBeanInstance完成
			instanceWrapper = createBeanInstance(beanName, mbd, args);
		}
		final Object bean = (instanceWrapper != null ? instanceWrapper.getWrappedInstance() : null);
		Class<?> beanType = (instanceWrapper != null ? instanceWrapper.getWrappedClass() : null);
		mbd.resolvedTargetType = beanType;

		// Allow post-processors to modify the merged bean definition.
		synchronized (mbd.postProcessingLock) {
			if (!mbd.postProcessed) {
				try {//这个方法获取带有注解的成员变量和方法，注册到BeanDefinition里面，如果@Resource@Autowired等
					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单例bean解决循环依赖问题
		// even when triggered by lifecycle interfaces like BeanFactoryAware.单例&&允许循环依赖&&单例bean正在创建
		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");
			}
			addSingletonFactory(beanName, new ObjectFactory<Object>() {
				@Override
				public Object getObject() throws BeansException {
					return getEarlyBeanReference(beanName, mbd, bean);
				}
			});
		}
//这里是对Bean的初始化,依赖注入往往在这里发生,这个exposedObject在初始化完以后会返回作为依赖注入完成后的Bean
		// Initialize the bean instance.
		Object exposedObject = bean;
		try {
			populateBean(beanName, mbd, instanceWrapper);//!!!!!!!!!!!!
			if (exposedObject != null) {
				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) {
			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<String>(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);
		}
		catch (BeanDefinitionValidationException ex) {
			throw new BeanCreationException(
					mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
		}

		return exposedObject;
	}

doCreateBean调用createBeanInstance对Bean进行实例化

	protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, Object[] args) {
		// 确认需要创建的Bean实例的类可以实例化
		Class<?> beanClass = resolveBeanClass(mbd, beanName);
//判断bean是不是public
		if (beanClass != null && !Modifier.isPublic(beanClass.getModifiers()) && !mbd.isNonPublicAccessAllowed()) {
			throw new BeanCreationException(mbd.getResourceDescription(), beanName,
					"Bean class isn't public, and non-public access not allowed: " + beanClass.getName());
		}
//使用 factory-method方法对Bean进行实例化
		if (mbd.getFactoryMethodName() != null)  {
			return instantiateUsingFactoryMethod(beanName, mbd, args);
		}

		// Shortcut when re-creating the same bean... 重新创建相同bean的捷径？
		boolean resolved = false;
		boolean autowireNecessary = false;
		if (args == null) {
			synchronized (mbd.constructorArgumentLock) {
				if (mbd.resolvedConstructorOrFactoryMethod != null) {
					resolved = true;
					autowireNecessary = mbd.constructorArgumentsResolved;
				}
			}
		}
		if (resolved) {
			if (autowireNecessary) {
				return autowireConstructor(beanName, mbd, null, null);
			}
			else {
				return instantiateBean(beanName, mbd);
			}
		}
//使用构造函数进行实例化
		// 根据后置处理器获取构造器，@Lookup是啥
		Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
		if (ctors != null ||
				mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_CONSTRUCTOR ||
				mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args))  {
			return autowireConstructor(beanName, mbd, ctors, args);
		}
//使用默认的构造函数对Bean进行实例化
		// No special handling: simply use no-arg constructor.
		return instantiateBean(beanName, mbd);
	}

createBeanInstance有调用了AbstractAutowireCapableBeanFactory的instantiateBean方法对Bean进行实例化，此方法获取InstantiationStrategy的对象，然后通过InstantiationStrategy调用instantiate对Bean进行实例化。

InstantiationStrategy调用instantiate对Bean类型进行判断，如果是lookup-method和replace-method注入，则采用CGLIB创建对象，否则，采用构造器创建对象。

对象创建完成后，封装到BeanWrapper中。回到AbstractAutowireCapableBeanFactory的doCreateBean方法中，接着调用applyMergedBeanDefinitionPostProcessors对Bean中的@Autowired、@Resource之类的注解，通过后置处理器进行预解析，解析结果存入到BeanDefnition中。接着处理单例Bean的循环依赖问题。这两处就不再详细解释，后续单独说明。

完成这些操作，接下来调用populateBean(beanName, mbd, instanceWrapper)完成Bean的依赖注入，下节再说。