Spring5源码分析（12）doGetBean

接上回， AbstractApplicationContext#refresh 调用 AbstractApplicationContext#finishBeanFactoryInitialization 来初始化所有的非懒加载单例Bean。在该 AbstractApplicationContext#finishBeanFactoryInitialization 方法内部通过调用 AbstractBeanFactory#doGetBean 来获取Spring容器所管理的Bean。

AbstractBeanFactory#doGetBean

AbstractBeanFactory#doGetBean 源码如下：

protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,
						  @Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {

	// 如果这个 name 是 FactoryBean 的beanName (&+beanName),就删除& , 返回beanName ,传入的name也可以是别名,也需要做转换
	// 注意 beanName 和 name 变量的区别,beanName是经过处理的,经过处理的beanName就直接对应singletonObjects中的key
	final String beanName = transformedBeanName(name);
	Object bean;

	// Eagerly check singleton cache for manually registered singletons.
	// 根据beanName尝试从singletonObjects获取Bean
	// 获取不到则再尝试从earlySingletonObjects,singletonFactories 从获取Bean
	// 这段代码和解决循环依赖有关
	Object sharedInstance = getSingleton(beanName);
	// 第一次进入sharedInstance肯定为null
	if (sharedInstance != null && args == null) {
		if (logger.isTraceEnabled()) {
			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 + "'");
			}
		}
		// 如果sharedInstance不为null,也就是非第一次进入
		// 为什么要调用 getObjectForBeanInstance 方法,判断当前Bean是不是FactoryBean,如果是,那么要不要调用getObject方法
		// 因为传入的name变量如果是(&+beanName),那么beanName变量就是(beanName),也就是说,程序在这里要返回FactoryBean
		// 如果传入的name变量(beanName),那么beanName变量也是(beanName),但是,之前获取的sharedInstance可能是FactoryBean,需要通过sharedInstance来获取对应的Bean
		// 如果传入的name变量(beanName),那么beanName变量也是(beanName),获取的sharedInstance就是对应的Bean的话,就直接返回Bean
		bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
	} else {
		// Fail if we're already creating this bean instance:
		// We're assumably within a circular reference.
		// 判断是否循环依赖
		if (isPrototypeCurrentlyInCreation(beanName)) {
			throw new BeanCurrentlyInCreationException(beanName);
		}

		// Check if bean definition exists in this factory.
		// 获取父BeanFactory,一般情况下,父BeanFactory为null,如果存在父BeanFactory,就先去父级容器去查找
		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 if (requiredType != null) {
				// No args -> delegate to standard getBean method.
				return parentBeanFactory.getBean(nameToLookup, requiredType);
			} else {
				return (T) parentBeanFactory.getBean(nameToLookup);
			}
		}

		// 创建的Bean是否需要进行类型验证,一般情况下都不需要
		if (!typeCheckOnly) {
			// 标记 bean 已经被创建
			markBeanAsCreated(beanName);
		}

		try {
			// 获取其父类Bean定义,子类合并父类公共属性
			final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
			checkMergedBeanDefinition(mbd, beanName, args);

			// Guarantee initialization of beans that the current bean depends on.
			// 获取当前Bean依赖的Bean的名称 ,@DependsOn
			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 + "'");
					}
					// 如果当前Bean依赖其他Bean,把被依赖Bean注册给当前Bean
					registerDependentBean(dep, beanName);
					try {
						// 先去创建所依赖的Bean
						getBean(dep);
					} catch (NoSuchBeanDefinitionException ex) {
						throw new BeanCreationException(mbd.getResourceDescription(), beanName,
								"'" + beanName + "' depends on missing bean '" + dep + "'", ex);
					}
				}
			}

			// Create bean instance.
			if (mbd.isSingleton()) {
				// 创建单例Bean
				sharedInstance = getSingleton(beanName, () -> {
					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);
			} else if (mbd.isPrototype()) {
				// It's a prototype -> create a new instance.
				// 创建prototype Bean,每次都会创建一个新的对象
				Object prototypeInstance = null;
				try {
					// 回调beforePrototypeCreation方法，注册当前创建的原型对象
					beforePrototypeCreation(beanName);
					// 创建对象
					prototypeInstance = createBean(beanName, mbd, args);
				} finally {
					// 回调 afterPrototypeCreation 方法，告诉容器该Bean的原型对象不再创建
					afterPrototypeCreation(beanName);
				}
				bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
			} else {
				// 如果既不是单例Bean,也不是prototype,则获取其Scope
				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 {
					// 创建对象
					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.
	// 对创建的Bean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;
}
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主要流程都已经在上述的源码中增加了注释，读者可以自己查阅。其中，我觉得最主要是要明白以下几点：

Object sharedInstance = getSingleton(beanName);

Object sharedInstance = getSingleton(beanName); 这段代码是解决循环依赖的关键，先大概看一下，留个印象，具体的等后面后空讲一下循环依赖吧。

public Object getSingleton(String beanName) {
	return getSingleton(beanName, true);
}

//getSingleton(beanName, true);源码
@Nullable
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
	//singletonObjects 就是Spring内部用来存放单例Bean的对象池,key为beanName，value为Bean
	Object singletonObject = this.singletonObjects.get(beanName);
	// singletonsCurrentlyInCreation 存放了当前正在创建的bean的BeanName
	if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
		synchronized (this.singletonObjects) {
			// earlySingletonObjects 是早期单例Bean的缓存池,此时Bean已经被创建(newInstance),但是还没有完成初始化
			// key为beanName，value为Bean
			singletonObject = this.earlySingletonObjects.get(beanName);
			//是否允许早期依赖
			if (singletonObject == null && allowEarlyReference) {
				//singletonFactories 单例工厂的缓存,key为beanName,value 为ObjectFactory
				ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
				if (singletonFactory != null) {
					//获取早期Bean
					singletonObject = singletonFactory.getObject();
					//将早期Bean放到earlySingletonObjects中
					this.earlySingletonObjects.put(beanName, singletonObject);
					this.singletonFactories.remove(beanName);
				}
			}
		}
	}
	return singletonObject;
}
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transformedBeanName和getObjectForBeanInstance的作用

调用 doGetBean 方法时会传入 name 变量，表明需要从容器中获取那个Bean。 transformedBeanName 除了在处理别名之后，这里会有以下几种情况：

第一种情况，直接往容器中注册Bean。

@Bean
public UserBean userBean() {
	return new UserBean("shen", 111);
}
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在这种情况下， name 变量如果为 userBean ，那么就是要从容器中获取 UserBean 对象。

调用 transformedBeanName 方法返回 beanName 对象的取值 userBean 。

然后根据 beanName 去容器中获取相应的 Bean ，而获取到的就是 UserBean 对象。

最后调用 getObjectForBeanInstance 方法，返回的还是 UserBean 对象。

第二种情况，通过 FactoryBean 往容器中注册Bean

@Bean
public FactoryBean userBean() {
	return new FactoryBean<UserBean>() {
		@Override
		public UserBean getObject() throws Exception {
			return new UserBean("shen", 111);
		}

		@Override
		public Class<?> getObjectType() {
			return UserBean.class;
		}
	};
}
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这里又可以分为两种情况：

第一种情况：

如果 name 变量如果为 userBean ，那么也要从容器中获取 UserBean 对象。

调用 transformedBeanName 方法返回 beanName 对象的取值为 userBean 。

然后根据 beanName 去容器中获取相应的 Bean ，而获取到的是 FactoryBean 对象。

最后调用 getObjectForBeanInstance 方法，发现是从容器中获取 UserBean 对象，于是调用 FactoryBean#getObject 返回 UserBean 对象。

第二种情况：

name 变量如果为 &userBean ，那么就是要从容器中获取 FactoryBean 对象本身。

调用 transformedBeanName 方法返回 beanName 对象的取值为 userBean 。

然后根据 beanName 去容器中获取相应的 Bean ，而获取到的是 FactoryBean 对象，

最后调用 getObjectForBeanInstance 方法，就是要获取 FactoryBean 对象，于是方法返回 FactoryBean 对象。

@DependsOn

通过源码可以知道，Spring在创建Bean之前，首先会创建当前Bean所有依赖的Bean。

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 + "'");
		}
		// 如果当前Bean依赖其他Bean,把被依赖Bean注册给当前Bean
		registerDependentBean(dep, beanName);
		try {
			// 先去创建所依赖的Bean
			getBean(dep);
		} catch (NoSuchBeanDefinitionException ex) {
			throw new BeanCreationException(mbd.getResourceDescription(), beanName,
					"'" + beanName + "' depends on missing bean '" + dep + "'", ex);
		}
	}
}
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那什么是当前Bean所有依赖的Bean呢？也就是说， String[] dependsOn = mbd.getDependsOn(); 什么情况下这个数组中会有值呢？

在Spring中有这个一个注解： @DependsOn 。这个注解一般用的很少(碰巧最近项目中用到了这个注解，哈哈哈)。来看一下Spring文档中是怎么描述的：

Beans on which the current bean depends. Any beans specified are guaranteed to be created by the container before this bean. Used infrequently in cases where a bean does not explicitly depend on another through properties or constructor arguments, but rather depends on the side effects of another bean's initialization. A depends-on declaration can specify both an initialization-time dependency and, in the case of singleton beans only, a corresponding destruction-time dependency. Dependent beans that define a depends-on relationship with a given bean are destroyed first, prior to the given bean itself being destroyed. Thus, a depends-on declaration can also control shutdown order.

机器翻译：当前bean所依赖的bean。任何指定的bean都保证在此bean之前由容器创建。当一个bean不是通过属性或构造函数参数显式依赖于另一个bean，而是依赖于另一个bean初始化的副作用时，很少使用。依赖项声明既可以指定初始化时间依赖项，也可以指定(在只有单例bean的情况下)对应的销毁时间依赖项。在销毁给定bean之前，首先销毁定义与给定bean的依赖关系的依赖bean。因此，依赖声明也可以控制关机顺序。

举个例子

@Service
public class OrderService {

	public OrderService() {
		System.out.println("OrderService create");
	}


	@PreDestroy
	public void destroy() {
		System.out.println("OrderService destroy");
	}
}
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@DependsOn("orderService")
@Service
public class UserService {

	public UserService() {
		System.out.println("UserService create");
	}


	@PreDestroy
	public void destroy() {
		System.out.println("UserService destroy");
	}
}
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@Configuration
@ComponentScan
public class DependsOnMain {
	public static void main(String[] args) {
		AnnotationConfigApplicationContext context =
				new AnnotationConfigApplicationContext(DependsOnMain.class);
		context.close();
	}
}
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//允许结果
OrderService create
UserService create
UserService destroy
OrderService destroy
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createBean(beanName, mbd, args);

在Spring中存在者多种scope，Spring会根据不用的scope选用不同的初始化方式。但是，不管怎么样，Spring在底层创建Bean的时候都是通过调用 AbstractAutowireCapableBeanFactory#createBean 方法来创建对象的。

那Spring究竟是怎么创建Bean的呢？ AbstractAutowireCapableBeanFactory#createBean 方法内部到底做了什么事情呢？循环依赖该怎么解决呢？

未完待续...

源码注释GITHUB地址： github.com/shenjianeng…

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