Spring（三）IOC容器的依赖注入流程

• • 一、基于XML的依赖注入
  • • 1.1 依赖注入发生的时间
    • 1.2 寻找获取 Bean 的入口
    • 1.3 开始实例化
    • 1.4 选择 Bean 实例化策略
    • 1.5 执行 Bean 实例化
    • 1.6 准备依赖注入
    • 1.7 解析属性注入规则
    • 1.8 注入赋值
    • 1.9 小结
  • 二、基于Annotation的依赖注入
  • • 2.1 定位Bean扫描路径
    • 2.2 读取Annotation元数据
    • 2.3 扫描指定包并解析为BeanDefinition
    • 2.4 注册注解BeanDefinition

  当Spring IOC容器完成了Bean定义资源的定位、载入和解析注册以后，IOC容器中已经管理了 Bean定义的相关数据，但是此时IOC容器还没有对所管理的Bean进行依赖注入，依赖注入在以下两种情况发生：

1. 用户第一次调用getBean()方法时，IOC容器触发依赖注入。
2. 当用户在配置文件中将元素配置了lazy-init=false属性，即让容器在解析注册Bean定义时进行预实例化，触发依赖注入。

  BeanFactory 接口中定义了几个getBean()方法，就是用户向IOC容器索取管理的Bean 的方法，我们通过分析其子类的具体实现，理解Spring IOC容器在用户索取 Bean 时如何完成依赖注入。在BeanFactory中我们可以看到getBean(String…)方法，这些方法的具体实现在AbstractBeanFactory中。

  AbstractBeanFactory的 getBean()相关方法的源码：

	//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的够哦成
		return doGetBean(name, null, null, false);
	}
	//通过name和指定类型从IOC容器获取Bean
	@Override
	public  T getBean(String name, Class requiredType) throws BeansException {
		return doGetBean(name, requiredType, null, false);
	}
	//通过name和参数从IOC容器获取Bean
	@Override
	public Object getBean(String name, Object... args) throws BeansException {
		return doGetBean(name, null, args, false);
	}
	//通过name、类型和参数从IOC容器获取Bean
	public  T getBean(String name, @Nullable Class requiredType, @Nullable Object... args)
			throws BeansException {

		return doGetBean(name, requiredType, args, false);
	}

	 //从IOC容器中获取Bean,此方法触发依赖注入
	@SuppressWarnings("unchecked")
	protected  T doGetBean(final String name, @Nullable final Class requiredType,
			@Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {

		//根据指定的名称获取被管理的Bean的名称
		//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;

		//直接尝试从缓存获取是否已经有被创建过的单例的Bean
		Object sharedInstance = getSingleton(beanName);
		if (sharedInstance != null && args == null) {
			if (logger.isTraceEnabled()) {
				//如果单例模式的Bean被创建，则直接返回
				if (isSingletonCurrentlyInCreation(beanName)) {
					logger.trace("Returning eagerly cached instance of singleton bean '" + beanName +
							"' that is not fully initialized yet - a consequence of a circular reference");
				}
				else {
					logger.trace("Returning cached instance of singleton bean '" + beanName + "'");
				}
			}
			//获取给定Bean的实例对象，主要完成FactoryBean的相关处理
			bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
		}

		else {
			//缓存中没有单例模式的Bean,缓存中已经有原型模式的Bean,
			//但是由于循环引用导致实例化对象失败
			if (isPrototypeCurrentlyInCreation(beanName)) {
				throw new BeanCurrentlyInCreationException(beanName);
			}

			//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中获取所需要的Bean，
			//如果不能再委托当前容器的父容器去查找，如果还是找不到则沿着继承关系向父容器查找
			BeanFactory parentBeanFactory = getParentBeanFactory();
			//当前容器的父容器存在，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);
				}
				//递归到beanFactory中寻找
				else if (args != null) {
					//委派父容器根据指定名称和显式的参数查找
					return (T) parentBeanFactory.getBean(nameToLookup, args);
				}
				else if (requiredType != null) {
					//委派父容器根据指定名称和类型查找
					return parentBeanFactory.getBean(nameToLookup, requiredType);
				}
				else {
					//委派父容器根据指定名称查找
					return (T) parentBeanFactory.getBean(nameToLookup);
				}
			}

			//创建的Bean是否需要进行类型验证
			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Bean定义，主要解决Bean继承时子类和父类公共属性问题
				final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
				checkMergedBeanDefinition(mbd, beanName, args);

				//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
				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 + "'");
						}
						//把能依赖Bean注册给当前依赖的Bean
						registerDependentBean(dep, beanName);
						try {
							//把被依赖Bean注册给当前依赖的Bean
							getBean(dep);
						}
						catch (NoSuchBeanDefinitionException ex) {
							throw new BeanCreationException(mbd.getResourceDescription(), beanName,
									"'" + beanName + "' depends on missing bean '" + dep + "'", ex);
						}
					}
				}

				//创建单例模式的Bean的实例对象
				if (mbd.isSingleton()) {
					//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) {
							//从单例模式的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容器创建原型模式的Bean实例对象
				else if (mbd.isPrototype()) {
					//原型模式每次都会创建一个新的对象
					Object prototypeInstance = null;
					try {
						//回调方法，默认的功能是注册当前创建的原型对象
						beforePrototypeCreation(beanName);
						//创建指定Bean的对象实例
						prototypeInstance = createBean(beanName, mbd, args);
					}
					finally {
						//回调方法，默认功能是：告诉IOC容器不再创建指定Bean的原型对象
						afterPrototypeCreation(beanName);
					}
					//获取指定Bean的实例对象
					bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
				}

				else {
					//创建的Bean既不是单例模式也不是原型模式，创建其他生命周期的Bean
					//比如request、session等生命周期
					String scopeName = mbd.getScope();
					final Scope scope = this.scopes.get(scopeName);
					//如果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 {
						//调用匿名内部类，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);
							}
						});
						//获取指定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;
			}
		}

		// 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.isTraceEnabled()) {
					logger.trace("Failed to convert bean '" + name + "' to required type '" +
							ClassUtils.getQualifiedName(requiredType) + "'", ex);
				}
				throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
			}
		}
		return (T) bean;
	}

  通过上面对向IOC容器获取Bean方法的分析，我们可以看到在Spring中，如果Bean定义的单例模式(Singleton)，则容器在创建之前先从缓存中查找，以确保整个容器中只存在一个实例对象。如果 Bean定义的是原型模式(Prototype)，则容器每次都会创建一个新的实例对象。
  上面的源码只是定义了根据 Bean 定义的模式，采取的不同创建 Bean 实例对象的策略，具体的Bean实例 对象的创 建过程 由实现了ObjectFactory接口的匿名内部类的createBean()方法完成，ObjectFactory使用委派模式，具体的Bean实例创建过程交由其实现类 AbstractAutowireCapableBeanFactory 完成，我们继续分析AbstractAutowireCapableBeanFactory的 createBean()方法的源码，理解其创建 Bean 实例的具体实现过程。

  AbstractAutowireCapableBeanFactory 类实现了ObjectFactory接口，创建容器指定的Bean实例对象，同时还对创建的Bean实例对象进行初始化处理。其创建 Bean 实例对象的方法源码：

	 //创建Bean实例对象
	@Override
	protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
			throws BeanCreationException {

		if (logger.isTraceEnabled()) {
			logger.trace("Creating instance of bean '" + beanName + "'");
		}
		RootBeanDefinition mbdToUse = mbd;

		// 判断需要创建的Bean是否可以实例化，即是否可以通过当前的类加载器加载
		Class resolvedClass = resolveBeanClass(mbd, beanName);
		if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
			mbdToUse = new RootBeanDefinition(mbd);
			mbdToUse.setBeanClass(resolvedClass);
		}

		//验证及准备覆盖的方法
		try {
			mbdToUse.prepareMethodOverrides();
		}
		catch (BeanDefinitionValidationException ex) {
			throw new BeanDefinitionStoreException(mbdToUse.getResourceDescription(),
					beanName, "Validation of method overrides failed", ex);
		}

		try {
			// 如果Bean配置了初始化前后的处理器，则返回一个需要创建Bean的代理对象
			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 {
			//创建bean的入口
			Object beanInstance = doCreateBean(beanName, mbdToUse, args);
			if (logger.isTraceEnabled()) {
				logger.trace("Finished creating instance of bean '" + beanName + "'");
			}
			return beanInstance;
		}
		catch (BeanCreationException | ImplicitlyAppearedSingletonException ex) {
			throw ex;
		}
		catch (Throwable ex) {
			throw new BeanCreationException(
					mbdToUse.getResourceDescription(), beanName, "Unexpected exception during bean creation", ex);
		}
	}

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

		//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;
		}

		//调用PostProcessor后置处理器
		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;
			}
		}

		// 向容器中缓存单例模式的Bean对象，以防止循环引用
		boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
				isSingletonCurrentlyInCreation(beanName));
		if (earlySingletonExposure) {
			if (logger.isTraceEnabled()) {
				logger.trace("Eagerly caching bean '" + beanName +
						"' to allow for resolving potential circular references");
			}
			//为避免后期循环依赖，尽早持有对象的引用
			addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
		}

		//Bean对象的初始化，依赖注入在此触发，这个对象在初始化完成之后返回依赖注入完成后的Bean
		Object exposedObject = bean;
		try {
			//将Bean实例对象封装，并且将Bean定义中配置的属性值赋给实例对象
			populateBean(beanName, mbd, instanceWrapper);
			//初始化Bean对象,Bean实例对象的依赖注入完成之后，为Bean实例对象
			//应用BeanPostProcessor后置处理器
			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的Bean对象
			Object earlySingletonReference = getSingleton(beanName, false);
			if (earlySingletonReference != null) {
				//根据名称获取已注册的Bean和正在实例化的Bean是同一个
				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创建新的实例对象
				else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
					String[] dependentBeans = getDependentBeans(beanName);
					Set 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);
						}
					}
					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.");
					}
				}
			}
		}

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

		return exposedObject;
	}

  通过上面的源码注释，我们看到具体的依赖注入实现其实就在以下两个方法中：

1. createBeanInstance()方法，生成Bean所包含的java对象实例。
2. populateBean()方法，对Bean属性的依赖注入进行处理。

  下面继续分析这两个方法的代码实现。

  在 createBeanInstance()方法中，根据指定的初始化策略，使用简单工厂、工厂方法或者容器的自动装配特性生成 Java 实例对象，创建对象的源码：

	 //创建Bean的实例对象
	protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) {
		// 确认Bean是可实例化的
		Class beanClass = resolveBeanClass(mbd, beanName);

		// 使用工厂方法对Bean进行实例化
		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());
		}

		Supplier instanceSupplier = mbd.getInstanceSupplier();
		if (instanceSupplier != null) {
			return obtainFromSupplier(instanceSupplier, beanName);
		}

		//调用工厂方法进行实例化
		if (mbd.getFactoryMethodName() != null) {
			return instantiateUsingFactoryMethod(beanName, mbd, args);
		}

		// 使用容器的自动装配方法进行实例化
		boolean resolved = false;
		boolean autowireNecessary = false;
		if (args == null) {
			synchronized (mbd.constructorArgumentLock) {
				//一个类有多个构造函数，每个构造函数都会不同的参数，所以调用前需要先根据参数
				//寻找构造函数和对应的工厂方法
				if (mbd.resolvedConstructorOrFactoryMethod != null) {
					resolved = true;
					autowireNecessary = mbd.constructorArgumentsResolved;
				}
			}
		}

		if (resolved) {
			if (autowireNecessary) {
				//配置了自动装配属性，使用容器的自动装配进行实例化，容器的自动装配
				//根据参数类型匹配Bean的构造方法
				return autowireConstructor(beanName, mbd, null, null);
			}
			else {
				//使用默认构造函数构造
				return instantiateBean(beanName, mbd);
			}
		}

		// 使用Bean的构造方法进行实例化
		Constructor[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
		if (ctors != null || mbd.getResolvedAutowireMode() == AUTOWIRE_CONSTRUCTOR ||
				mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args)) {
			//使用容器的自动装配特性，调用匹配的构造方法进行实例化
			return autowireConstructor(beanName, mbd, ctors, args);
		}

		ctors = mbd.getPreferredConstructors();
		if (ctors != null) {
			return autowireConstructor(beanName, mbd, ctors, null);
		}

		//使用默认构造函数构造
		return instantiateBean(beanName, mbd);
	}
	//使用默认的无参构造方法实例化Bean对象
	protected BeanWrapper instantiateBean(final String beanName, final RootBeanDefinition mbd) {
		try {
			Object beanInstance;
			final BeanFactory parent = this;
			//获取系统的安全管理接口、JDK标准的安全管理API
			if (System.getSecurityManager() != null) {
				//这是一个匿名内部类，根据实例化策略创建实例对象
				beanInstance = AccessController.doPrivileged((PrivilegedAction