1. 程式人生 > >Spring原始碼分析(三)(IoC容器的依賴注入)(1)

Spring原始碼分析(三)(IoC容器的依賴注入)(1)

    依賴注入的過程是使用者第一次向IoC容器索要Bean時才觸發的,當然也有例外,可以在BeanDefinition資訊中通過控制lazy-init屬性來讓容器完成對Bean的預例項化。這個預例項化實際上也是一個完成依賴注入的過程,但它是在初始化的過程中完成的。當用戶向IoC容器索要Bean時。基本的loC容器介面BeanFactory中,有一個getBean的介面定義,這個介面的實現就是觸發依賴注入發生的地方。 下面從DefaultListableBeanFactory的基類AbstractBeanFactory入手去看看getBean()的實現,如程式碼所示:

	@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);
	}

	public <T> T getBean(String name, Class<T> requiredType, Object... args) throws BeansException {
		return doGetBean(name, requiredType, args, false);
	}

	//實際取得Bean的地方,也是觸發依賴注入發生的地方
	protected <T> T doGetBean(
			final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)
			throws BeansException {

		final String beanName = transformedBeanName(name);
		Object bean;

		// Eagerly check singleton cache for manually registered singletons.
		//從快取中取得Bean,處理那些已經被建立過得單例模式的Bean,對這種Bean的請求不需要重複建立
		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 + "'");
				}
			}
			//這裡的getObjectForBeanInstance完成的是FactoryBean的相關處理,以取得FactoryBean的生產結果,BeanFactory和FactoryBean的區別:
			// BeanFactory是個Factory,也就是IOC容器或物件工廠,FactoryBean是個Bean。在Spring中,所有的Bean都是由BeanFactory(也就是IOC容器)來進行管理的。
			// 但對FactoryBean而言,這個Bean是一個能生產或者修飾物件生成的工廠Bean
			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);
			}

			// Check if bean definition exists in this factory.
			//這裡對IoC容器中的BeanDefintion是否存在進行檢查,檢查是否能在當前BeanFactory中取得需要的Bean。
			// 如果當前的工廠中取不到,則到雙親BeanFactory中取,如果當前的雙親工廠取不到,那就順著雙親BeanFactory鏈一直向上找
			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);
			}

			try {
				//根據Bean的名字取得BeanDefinition
				final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
				checkMergedBeanDefinition(mbd, beanName, args);
				// Guarantee initialization of beans that the current bean depends on.
				//獲取當前Bean的所有依賴Bean,這樣會觸發getBean的遞迴呼叫,直到取到一個沒有任何依賴的Bean位置
				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.
				//這裡會通過呼叫crateBean方法建立Singleton bean的例項,這裡有一個回撥函式
				//下面會進入createBean進行詳細分析
				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);
				}
				//建立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;
			}
		}
		//這裡對建立的Bean進行型別檢查,如果沒問題就返回這個新建立Bean,這個Bean已經是包含了依賴關係的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;
	}

    這個就是依賴注入的入口,在這裡觸發了依賴注入,而依賴注人的發生是在容器中的BeanDefinition資料已經建立好的前提下進行的。“ 程式=資料+演算法 ”很經典的一句話,前面的BeanDefinition就是資料,下面看看這些資料是怎樣為依賴注入服務的。雖然依賴注入的過程不涉及複雜的演算法問題,但這個過程也不簡單,因為對於IoC容器的使用,Spring提供了許多的引數配置,每一個引數配置實際上代表了一個IoC容器的實現特性,這些特性的實現很多都需要在依賴注入的過程中或者對Bean進行生命週期管理的過程中完成。儘管可以用最簡單的方式來描述loC容器,將它視為一個hashMap ,但只能說這個hashMap是容器的最基本的資料結構,而不是IoC容器的全部。Spring IoC容器作為一個產品,其價值體現在一系列相關的產品特性上,這些產品特性以依賴反轉模式的實現為核心,為使用者更好地使用依賴反轉提供便利,從而實現了一個完整的loC容器產品。這些產品特性的實現並不是一個簡單的過程,它提供了一個成熟的IoC容器產品供使用者使用。所以,儘管Spring IoC容器沒有什麼獨特的演算法, 但卻可以看成是一個成功的軟體工程產品。

AbstractAutowireCapableBeanFactory
protected Object createBean(String beanName, RootBeanDefinition mbd, Object[] args) throws BeanCreationException {
		if (logger.isDebugEnabled()) {
			logger.debug("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.
		//判斷需要建立的Bean是否可以例項化,這個類是否可以通過類裝載器來載入
		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.
			//如果Bean配置了PostProcessor,那麼這裡返回的是一個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);
		}
		//建立Bean的呼叫
		Object beanInstance = doCreateBean(beanName, mbdToUse, args);
		if (logger.isDebugEnabled()) {
			logger.debug("Finished creating instance of bean '" + beanName + "'");
		}
		return beanInstance;
	}



	//然後到doCreteBean中看看Bean是如何生成的
	protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args)
			throws BeanCreationException {

		// Instantiate the bean.
		//BeanWrapper是用來持有創建出來的Bean物件的
		BeanWrapper instanceWrapper = null;
		//如果是Singleton先把快取中同名的清除
		if (mbd.isSingleton()) {
			instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
		}
		//這是建立Bean的地方由createBeanInstance來完成
		if (instanceWrapper == null) {
			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 {
					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 &&
				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);
				}
			});
		}

		// Initialize the bean instance.
		//這裡是對Bean的初始化,依賴注入往往在這裡發生,
		// 這個exposedObject在初始化處理完後會返回作為依賴注入完成後的bean
		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;
	}


    在這裡我們看到,與依賴注入關係特別密切的方法有createBeanlnstance和populateBean,下面分別介紹這兩個方法。在createBeanlnstance中生成了Bean所包含的Java物件,這個物件的生成有很多種不同的方式,可以通過工廣方法生成,也可以通過容器的autowire特性生成,這些生成方式都是自相關的BeanDefinition來指定的。如以下程式碼所示,可以看到不同生成方式對應的實現:

	protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, Object[] args) {
		// Make sure bean class is actually resolved at this point.
		//確認需要建立的Bean例項的類可以例項化
		Class<?> beanClass = resolveBeanClass(mbd, beanName);
		//這裡使用工廠對Bean進行例項化
		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());
		}

		if (mbd.getFactoryMethodName() != null)  {
			return instantiateUsingFactoryMethod(beanName, mbd, args);
		}

		// Shortcut when re-creating the same bean...
		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);
			}
		}
		//使用建構函式進行例項化
		// Need to determine the constructor...
		Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
		if (ctors != null ||
				mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_CONSTRUCTOR ||
				mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args))  {
			return autowireConstructor(beanName, mbd, ctors, args);
		}
		//使用預設的建構函式對Bean進行例項化
		// No special handling: simply use no-arg constructor.
		return instantiateBean(beanName, mbd);
	}


	//最常見的例項化過程instantiateBean
	protected BeanWrapper instantiateBean(final String beanName, final RootBeanDefinition mbd) {
		try {
			//使用預設的例項化策略對Bean進行例項化,預設例項策略是
			// CglibSubclassingInstantiationStrategy,CGLib來對Bean進行例項化
			Object beanInstance;
			final BeanFactory parent = this;
			if (System.getSecurityManager() != null) {
				beanInstance = AccessController.doPrivileged(new PrivilegedAction<Object>() {
					@Override
					public Object run() {
						return getInstantiationStrategy().instantiate(mbd, beanName, parent);
					}
				}, getAccessControlContext());
			}
			else {
				beanInstance = getInstantiationStrategy().instantiate(mbd, beanName, parent);
			}
			BeanWrapper bw = new BeanWrapperImpl(beanInstance);
			initBeanWrapper(bw);
			return bw;
		}
		catch (Throwable ex) {
			throw new BeanCreationException(
					mbd.getResourceDescription(), beanName, "Instantiation of bean failed", ex);
		}

    這裡用CGLIB對Bean進行例項化。CGLIB是一個常用的位元組碼生成器的類庫,它提供了一系列的API來提供生成和轉換java的位元組碼的功能。在Spring AOP中也使用CGLIB對Java的位元組碼進行增強。在loC容器中,要了解怎樣使用CGLIB來生成Bean物件,需要看一下SimpleInstantiationStrategy類。這個Strategy是Spring用來生成Bean物件的預設類,它提供了兩種例項化Java物件的方法,一種是通過BeanUtils ,它使用了JVM的反射功能, 一種是通過前面提到的CGLIB來生成,如程式碼所示:

SimpleInstantiationStrategy
	public Object instantiate(RootBeanDefinition bd, String beanName, BeanFactory owner) {
		// Don't override the class with CGLIB if no overrides.
		if (bd.getMethodOverrides().isEmpty()) {
			//這裡取得指定的構造器或者生成物件的工廠方法來對Bean進行例項化
			Constructor<?> constructorToUse;
			synchronized (bd.constructorArgumentLock) {
				//這裡取得指定的構造器或者生成物件的工廠方法來對Bean進行例項化
				constructorToUse = (Constructor<?>) bd.resolvedConstructorOrFactoryMethod;
				if (constructorToUse == null) {
					final Class<?> clazz = bd.getBeanClass();
					if (clazz.isInterface()) {
						throw new BeanInstantiationException(clazz, "Specified class is an interface");
					}
					try {
						if (System.getSecurityManager() != null) {
							constructorToUse = AccessController.doPrivileged(new PrivilegedExceptionAction<Constructor<?>>() {
								@Override
								public Constructor<?> run() throws Exception {
									return clazz.getDeclaredConstructor((Class[]) null);
								}
							});
						}
						else {
							constructorToUse =	clazz.getDeclaredConstructor((Class[]) null);
						}
						bd.resolvedConstructorOrFactoryMethod = constructorToUse;
					}
					catch (Throwable ex) {
						throw new BeanInstantiationException(clazz, "No default constructor found", ex);
					}
				}
			}
			//通過BeanUtils進行例項化,這個BeanUtils的例項化通過Constructor來例項化Bean,
			// 在BeanUtils中可以看到具體的呼叫ctor.newInstance(args);
			return BeanUtils.instantiateClass(constructorToUse);
		}
		else {
			// Must generate CGLIB subclass.
			//使用CGLIB來例項化物件
			return instantiateWithMethodInjection(bd, beanName, owner);
		}
	}

    在CglibSubclassinglnstantiationStrategy 中可以看到具體的例項化過程和CGLIB的使用方法這裡的Enhancer類, 是CGLIB的類,通過這個Enhancer生成Java物件,使用的是Enhancer的create方法,如程式碼所示。

CglibSubclassingInstantiationStrategy
	protected Object instantiateWithMethodInjection(RootBeanDefinition bd, String beanName, BeanFactory owner) {
		return instantiateWithMethodInjection(bd, beanName, owner, null);
	}


	protected Object instantiateWithMethodInjection(RootBeanDefinition bd, String beanName, BeanFactory owner,
			Constructor<?> ctor, Object... args) {

		// Must generate CGLIB subclass...
		return new CglibSubclassCreator(bd, owner).instantiate(ctor, args);
	}


		public Object instantiate(Constructor<?> ctor, Object... args) {
			//生成Enhancer物件,併為Enhancer物件設定生成Java物件的引數,比如基類、回撥方法等
			Class<?> subclass = createEnhancedSubclass(this.beanDefinition);
			Object instance;
			if (ctor == null) {
//				通過BeanUtils進行例項化,這個BeanUtils的例項化通過Constructor來例項化Bean
				instance = BeanUtils.instantiateClass(subclass);
			}
			else {
				try {
					Constructor<?> enhancedSubclassConstructor = subclass.getConstructor(ctor.getParameterTypes());
					instance = enhancedSubclassConstructor.newInstance(args);
				}
				catch (Exception ex) {
					throw new BeanInstantiationException(this.beanDefinition.getBeanClass(),
							"Failed to invoke constructor for CGLIB enhanced subclass [" + subclass.getName() + "]", ex);
				}
			}
			// SPR-10785: set callbacks directly on the instance instead of in the
			// enhanced class (via the Enhancer) in order to avoid memory leaks.
			//設定回撥
			Factory factory = (Factory) instance;
			factory.setCallbacks(new Callback[] {NoOp.INSTANCE,
					new LookupOverrideMethodInterceptor(this.beanDefinition, this.owner),
					new ReplaceOverrideMethodInterceptor(this.beanDefinition, this.owner)});
			return instance;
		}

    到這裡已經分析了例項化Bean物件的整個過程。在例項化Bean物件生成的基礎上,再介紹一下Spring是怎樣對這些物件進行處理的,也就是Bean物件生成以後,怎樣把這些Bean物件的依賴關係設定好,完成整個依賴注入過程。這個過程涉及對各種Bean物件的屬性的處理過程(即依賴關係處理的過程),這些依賴關係處理的依據就是已經解析得到的BeanDefinition。要詳細瞭解這個過程,需要回到前面的populateBean方法,這個方法在AbstractAutowireCapableBeanFactory中的實現如程式碼所示。
 

參考《SPRING技術內幕》