Spring中Bean注入源码示例解析

目录

• BeanDefinition和Bean
• BeanDefinition的注入
• BeanDefinitionRegistry接口
• BeanDefinitionRegistry的实现类
• SimpleBeanDefinitionRegistry
• DefaultListableBeanFactory
• Bean的注入
• SingletonBeanRegistry接口
• SingletonBeanRegistry的实现类
• AbstractBeanFactory
• DefaultSingletonBeanRegistry
• 总结

BeanDefinition和Bean

在Spring中Bean的注入涉及两部分：

• BeanDefinition
• Bean

两个对象存在先后顺序，先注入BeanDefinition之后才执行Bean对象的注入。

那么两者有什么关联呢？

BeanDefinition和Bean的关系：可以理解为BeanDefinition是Bean的包装类，类似Java中类和属性的关系。

BeanDefinition是对Bean对象的包装，BeanDefinition中封装了Bean相关的描述，比如bean对象，bean的单例还是原型、bean的父级、懒加载方式等等。

public interface BeanDefinition extends AttributeAccessor, BeanMetadataElement {
	void setScope(@Nullable String scope);
	@Nullable
	String getScope();
	void setLazyInit(boolean lazyInit);
	boolean isLazyInit();
}
//bean的原数据
public interface BeanMetadataElement {
	@Nullable
	Object getSource();
}

在BeanDefinition中继承了BeanMetadataElement，该类是Bean的原数据，该类中实例化了Bean对象为Object。因此在BeanDefinition中调用getSource就可以获取到Bean对象。

分清楚BeanDefinition和Bean之后再看BeanDefinition的注入，因为只有注入了BeanDefinition才会注入后续的Bean。

BeanDefinition的注入

BeanDefinitionRegistry接口

BeanDefinitin的注入通过接口BeanDefinitionRegistry抽象了各种对BeanDefinition的操作，例如

• BeanDefinitionRegistry#registerBeanDefinition（注入到beanDefinition中）
• BeanDefinitionRegistry#removeBeanDefinition（从beanDefinition容器中移除）
• BeanDefinitionRegistry#getBeanDefinition（获取BeanDefinition）

public interface BeanDefinitionRegistry extends AliasRegistry {
	void registerBeanDefinition(String beanName, BeanDefinition beanDefinition)
			throws BeanDefinitionStoreException;
	void removeBeanDefinition(String beanName) throws NoSuchBeanDefinitionException;
	BeanDefinition getBeanDefinition(String beanName) throws NoSuchBeanDefinitionException;
	boolean containsBeanDefinition(String beanName);
	String[] getBeanDefinitionNames();
	int getBeanDefinitionCount();
	boolean isBeanNameInUse(String beanName);
}

该接口是操作BeanDefinition的基础接口，该类是一个接口类型并不是一个抽象类，因此该接口中不会定义任何BeanDefinition的容器，BeanDefinition的容器只在各实现类中定义并使用。任何需要操作BeanDefinition的实现都需要实现该接口。

BeanDefinitionRegistry的实现类

找到操作BeanDefinition的接口就可以通过接口查看接口的实现类，进而找到BeanDefinition的应用。

BeanDefinition常见的几个实现类有：

• DefaultListableBeanFactory
• SimpleBeanDefinitionRegistry

SimpleBeanDefinitionRegistry

public class SimpleBeanDefinitionRegistry extends SimpleAliasRegistry implements BeanDefinitionRegistry {
	/**beanDefinition的容器，通过ConcurrentHashMap实现. */
	private final Map<String, BeanDefinition> beanDefinitionMap = new ConcurrentHashMap<>(64);
	//该类的实现方法只是实现了最简单的beanDefinition的注入，在Java开发环境中并不会使用该实现类
	@Override
	public void registerBeanDefinition(String beanName, BeanDefinition beanDefinition)
			throws BeanDefinitionStoreException {
		Assert.hasText(beanName, "'beanName' must not be empty");
		Assert.notNull(beanDefinition, "BeanDefinition must not be null");
		this.beanDefinitionMap.put(beanName, beanDefinition);
	}
}

该实现类中只做了最简单的注入功能，没有任何的逻辑处理，因此在实际开发过程中Spring并不会使用该类。

DefaultListableBeanFactory

public class DefaultListableBeanFactory extends AbstractAutowireCapableBeanFactory implements ConfigurableListableBeanFactory, BeanDefinitionRegistry, Serializable {
		//beanDefinition的容器
		private final Map<String, BeanDefinition> beanDefinitionMap = new ConcurrentHashMap<>(256);
		//向BeanFactory中注入BeanDefinition
		public void registerBeanDefinition(String beanName, BeanDefinition beanDefinition) throws BeanDefinitionStoreException {
			Assert.hasText(beanName, "Bean name must not be empty");
			Assert.notNull(beanDefinition, "BeanDefinition must not be null");
			if (beanDefinition instanceof AbstractBeanDefinition) {
				try {
					((AbstractBeanDefinition) beanDefinition).validate();
				}
				catch (BeanDefinitionValidationException ex) {
					throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
							"Validation of bean definition failed", ex);
				}
			}
			BeanDefinition existingDefinition = this.beanDefinitionMap.get(beanName);
			//判断beanDefinition容器中是否已存在该beanDefinition
			if (existingDefinition != null) {
				//是否可以覆盖
				if (!isAllowBeanDefinitionOverriding()) {
					throw new BeanDefinitionOverrideException(beanName, beanDefinition, existingDefinition);
				} else if (existingDefinition.getRole() < beanDefinition.getRole()) {
					// e.g. was ROLE_APPLICATION, now overriding with ROLE_SUPPORT or ROLE_INFRASTRUCTURE
					if (logger.isInfoEnabled()) {
						logger.info("Overriding user-defined bean definition for bean '" + beanName +
								"' with a framework-generated bean definition: replacing [" +
								existingDefinition + "] with [" + beanDefinition + "]");
					}
				} else if (!beanDefinition.equals(existingDefinition)) {
					if (logger.isDebugEnabled()) {
						logger.debug("Overriding bean definition for bean '" + beanName +
								"' with a different definition: replacing [" + existingDefinition +
								"] with [" + beanDefinition + "]");
					}
				} else {
					if (logger.isTraceEnabled()) {
						logger.trace("Overriding bean definition for bean '" + beanName +
								"' with an equivalent definition: replacing [" + existingDefinition +
								"] with [" + beanDefinition + "]");
					}
				}
				//覆盖beanDefinition
				this.beanDefinitionMap.put(beanName, beanDefinition);
			} else { //容器中不存在该beanDefinition的处理
				if (hasBeanCreationStarted()) {
					synchronized (this.beanDefinitionMap) { //对beanDefinition加锁
						//注入该beanDefinition
						this.beanDefinitionMap.put(beanName, beanDefinition);
						List<String> updatedDefinitions = new ArrayList<>(this.beanDefinitionNames.size() + 1);
						updatedDefinitions.addAll(this.beanDefinitionNames);
						updatedDefinitions.add(beanName);
						this.beanDefinitionNames = updatedDefinitions;
						if (this.manualSingletonNames.contains(beanName)) {
							Set<String> updatedSingletons = new LinkedHashSet<>(this.manualSingletonNames);
							updatedSingletons.remove(beanName);
							this.manualSingletonNames = updatedSingletons;
						}
					}
				}
				else {
					// Still in startup registration phase
					this.beanDefinitionMap.put(beanName, beanDefinition);
					this.beanDefinitionNames.add(beanName);
					this.manualSingletonNames.remove(beanName);
				}
				this.frozenBeanDefinitionNames = null;
			}
		if (existingDefinition != null || containsSingleton(beanName)) {
			resetBeanDefinition(beanName);
		}
	}
}

DefaultListableBeanFactory是默认使用的注入BeanDefinition的实现类，可以看到该实现类中对注入BeanDefinition做了很多的逻辑判断，日常开发启动容器过程中都会使用该实现类注入BeanDefinition进行后续的Bean注入。

Bean的注入

BeanDefinition注入后后续执行Bean的注入，bean注入的方式也是将bean注入到bean的容器中，因此在Spring中BeanDefinition和Bean都是通过容器化的方式操作的，那么在Bean的注入中也通过对应的接口定义对Bean的操作，该接口就是SingletonBeanRegistry。

SingletonBeanRegistry接口

该接口实现了对Bean的操作：

• SingletonBeanRegistry#registerSingleton（将bean注入到容器中）
• SingletonBeanRegistry#getSingleton（从容器中获取bean）

public interface SingletonBeanRegistry {
	//注入bean
	void registerSingleton(String beanName, Object singletonObject);
	//获取bean
	@Nullable
	Object getSingleton(String beanName);
	//是否存在bean
	boolean containsSingleton(String beanName);
	String[] getSingletonNames();
	int getSingletonCount();
	Object getSingletonMutex();
}

SingletonBeanRegistry的实现类

SingletonBeanRegistry的实现类有多个，分别为：

• AbstractBeanFactory
• DefaultListableBeanFactory
• DefaultSingletonBeanRegistry
• FactoryBeanRegistrySupport
• AbstractAutowireCapableBeanFactory

下面说两个有代表性的：AbstractBeanFactory和DefaultSingletonBeanRegistry

AbstractBeanFactory

该实现类是一个抽象类，即该类是bean的工厂类，其中主要实现的是可以复用的具体逻辑，该抽象类中包含获取bean、beanPostProcessor、初始化bean、销毁bean等对bean的操作抽象类。

public abstract class AbstractBeanFactory extends FactoryBeanRegistrySupport implements ConfigurableBeanFactory {
	//获取bean的方式平时使用ApplicationContext就是最终调用该方法
	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, @Nullable Class<T> requiredType, @Nullable Object... args)
			throws BeansException {
		return doGetBean(name, requiredType, args, false);
	}
	/**
	* 从容器中获取bean的主要逻辑
	*/
	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);
		Object bean;
		// Eagerly check singleton cache for manually registered singletons.
		Object sharedInstance = getSingleton(beanName);
		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 + "'");
				}
			}
			bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
		}
		else {
			//省略部分代码
			try {
				//先获取beanDefinition
				final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
				checkMergedBeanDefinition(mbd, beanName, args);
				// 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);
						}
					}
				}
				// 单例模式下获取bean的方式
				if (mbd.isSingleton()) {
					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()) {//原型模式下的bean的获取
					// It's a prototype -> create a new instance.
					Object prototypeInstance = null;
					try {
						beforePrototypeCreation(beanName);
						//原型模式每次都需要创建bean
						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;
			}
		}
		//省略部分代码
		return (T) bean;
	}
	//将实现beanPostProcessor的实现类加入到链表中用于对bean的前置或者后置处理
	@Override
	public void addBeanPostProcessor(BeanPostProcessor beanPostProcessor) {
		Assert.notNull(beanPostProcessor, "BeanPostProcessor must not be null");
		// Remove from old position, if any
		this.beanPostProcessors.remove(beanPostProcessor);
		// Track whether it is instantiation/destruction aware
		if (beanPostProcessor instanceof InstantiationAwareBeanPostProcessor) {
			this.hasInstantiationAwareBeanPostProcessors = true;
		}
		if (beanPostProcessor instanceof DestructionAwareBeanPostProcessor) {
			this.hasDestructionAwareBeanPostProcessors = true;
		}
		// Add to end of list
		this.beanPostProcessors.add(beanPostProcessor);
	}
	//销毁bean的方法
	@Override
	public void destroyBean(String beanName, Object beanInstance) {
		destroyBean(beanName, beanInstance, getMergedLocalBeanDefinition(beanName));
	}
	protected void destroyBean(String beanName, Object bean, RootBeanDefinition mbd) {
		new DisposableBeanAdapter(bean, beanName, mbd, getBeanPostProcessors(), getAccessControlContext()).destroy();
	}
}

DefaultSingletonBeanRegistry

该实现类通过名字也可以推断出该实现类是对单例bean的注入，该类中实现了bean的所有容器，平时开发过程中Spring容器启动就是使用了该实现类中的容器，该实现类是对bean最终注入的实现，涉及的四个容器主要有：

• singletonObjects
• singletonFactories
• earlySingletonObjects
• registeredSingletons

四个容器其中三个的核心作用是解决循环依赖，这里不展开说。

找一条主线梳理下Bean注入的过程，当BeanDefinition注入成功后，后续执行bean的注入，以AbstractAutowireCapableBeanFactory的实现为例，该实现类中会对bean的注入，整个注入的链路如下：

• 获取资源数据：CommonAnnotationBeanPostProcessor#autowireResource
• 通过beanName解析bean：AbstractAutowireCapableBeanFactory#resolveBeanByName
• 调用抽象类中获取bean：AbstractBeanFactory#getBean
• 执行抽象类中获取bean的核心逻辑：AbstractBeanFactory#doGetBean
• bean不存在时创建bean：AbstractAutowireCapableBeanFactory#createBean
• 获取单例bean：efaultSingletonBeanRegistry#getSingleton
• 单例bean不存在将类注入到容器中：DefaultSingletonBeanRegistry#addSingleton

public class DefaultSingletonBeanRegistry extends SimpleAliasRegistry implements SingletonBeanRegistry {
	//单例bean的容器，开发过程中获取的bean都是从该容器中获取的：即所谓的一级缓存
	private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);
	//单例bean的beanFactory的容器：即所谓的三级缓存
	private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16);
	//提前初始化的bean的容器，该容器的bean并没有任何属性值只是提前将bean注入到该容器中用于后续的判断
	//即所谓的二级缓存
	private final Map<String, Object> earlySingletonObjects = new HashMap<>(16);
	//已注册的单例bean的beanName列表
	private final Set<String> registeredSingletons = new LinkedHashSet<>(256);
	//注入bean的方法
	public void registerSingleton(String beanName, Object singletonObject) throws IllegalStateException {
		Assert.notNull(beanName, "Bean name must not be null");
		Assert.notNull(singletonObject, "Singleton object must not be null");
		synchronized (this.singletonObjects) {
			Object oldObject = this.singletonObjects.get(beanName);
			if (oldObject != null) {
				throw new IllegalStateException("Could not register object [" + singletonObject +
												"] under bean name '" + beanName + "': there is already object [" + oldObject + "] bound");
			}
			addSingleton(beanName, singletonObject);
		}
	}
	/**
	* 将bean注入容器中，操作四个容器主要用于bean的循环依赖问题
	*/
	protected void addSingleton(String beanName, Object singletonObject) {
		synchronized (this.singletonObjects) {
			this.singletonObjects.put(beanName, singletonObject);
			this.singletonFactories.remove(beanName);
			this.earlySingletonObjects.remove(beanName);
			this.registeredSingletons.add(beanName);
		}
	}
}

总结

以上就是对bean注入从源码的梳理，该梳理主要从bean注入前的准备（beanDefinition的注入）和bean注入的实现来完成，从整个梳理中学到最多的主要有两点：

• 抽象的思想：在BeanDefinition的实现和Bean的实现逻辑中，都完成高度抽象并且遵循单一原则和高度抽象，代码的可扩展性和复用性很高，我们作为开发者也可以通过Spring暴露的各种Registry接口实现对Bean的操作。
• 容器化思想：从BeanDefinition和Bean的注入来看，整个的实现思路都是基于容器化思想来实现的，将开发中需要关心的对象都实例化到容器中，开发者在使用时只要从容器中获取即可，并且获取bean的方式也很灵活，开发者在使用和扩展性上更便利。

对于源码的学习自己理解来说并不是背其中的代码，而是学习大佬实现代码的思想，为什么可以抽象出扩展性这么高的代码，并且从源头梳理下来都能很轻松的按照主线一步一步梳理到目的地，这也是每个开发者需要学习的地方，提高自己代码的可读性和可扩展性，能够掌握面向对象的核心：封装、继承、多态。

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