Spring之BeanFactory详解

Posted 2021-08-10 敲代码的小小酥

前言

顾名思义，BeanFactory是生产Bean的工厂。Bean就是Spring容器管理的对象。下面我们分析一下BeanFactory及其子类的源码，对我们深入理解Spring的初始化和原理非常有帮助

一、BeanFactory分析

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 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
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 *      https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
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 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
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package org.springframework.beans.factory;

import org.springframework.beans.BeansException;
import org.springframework.core.ResolvableType;
import org.springframework.lang.Nullable;

/**
 * The root interface for accessing a Spring bean container.
 *
 * <p>This is the basic client view of a bean container;
 * further interfaces such as {@link ListableBeanFactory} and
 * {@link org.springframework.beans.factory.config.ConfigurableBeanFactory}
 * are available for specific purposes.
 *
 * <p>This interface is implemented by objects that hold a number of bean definitions,
 * each uniquely identified by a String name. Depending on the bean definition,
 * the factory will return either an independent instance of a contained object
 * (the Prototype design pattern), or a single shared instance (a superior
 * alternative to the Singleton design pattern, in which the instance is a
 * singleton in the scope of the factory). Which type of instance will be returned
 * depends on the bean factory configuration: the API is the same. Since Spring
 * 2.0, further scopes are available depending on the concrete application
 * context (e.g. "request" and "session" scopes in a web environment).
 *
 * <p>The point of this approach is that the BeanFactory is a central registry
 * of application components, and centralizes configuration of application
 * components (no more do individual objects need to read properties files,
 * for example). See chapters 4 and 11 of "Expert One-on-One J2EE Design and
 * Development" for a discussion of the benefits of this approach.
 *
 * <p>Note that it is generally better to rely on Dependency Injection
 * ("push" configuration) to configure application objects through setters
 * or constructors, rather than use any form of "pull" configuration like a
 * BeanFactory lookup. Spring's Dependency Injection functionality is
 * implemented using this BeanFactory interface and its subinterfaces.
 *
 * <p>Normally a BeanFactory will load bean definitions stored in a configuration
 * source (such as an XML document), and use the {@code org.springframework.beans}
 * package to configure the beans. However, an implementation could simply return
 * Java objects it creates as necessary directly in Java code. There are no
 * constraints on how the definitions could be stored: LDAP, RDBMS, XML,
 * properties file, etc. Implementations are encouraged to support references
 * amongst beans (Dependency Injection).
 *
 * <p>In contrast to the methods in {@link ListableBeanFactory}, all of the
 * operations in this interface will also check parent factories if this is a
 * {@link HierarchicalBeanFactory}. If a bean is not found in this factory instance,
 * the immediate parent factory will be asked. Beans in this factory instance
 * are supposed to override beans of the same name in any parent factory.
 *
 * <p>Bean factory implementations should support the standard bean lifecycle interfaces
 * as far as possible. The full set of initialization methods and their standard order is:
 * <ol>
 * <li>BeanNameAware's {@code setBeanName}
 * <li>BeanClassLoaderAware's {@code setBeanClassLoader}
 * <li>BeanFactoryAware's {@code setBeanFactory}
 * <li>EnvironmentAware's {@code setEnvironment}
 * <li>EmbeddedValueResolverAware's {@code setEmbeddedValueResolver}
 * <li>ResourceLoaderAware's {@code setResourceLoader}
 * (only applicable when running in an application context)
 * <li>ApplicationEventPublisherAware's {@code setApplicationEventPublisher}
 * (only applicable when running in an application context)
 * <li>MessageSourceAware's {@code setMessageSource}
 * (only applicable when running in an application context)
 * <li>ApplicationContextAware's {@code setApplicationContext}
 * (only applicable when running in an application context)
 * <li>ServletContextAware's {@code setServletContext}
 * (only applicable when running in a web application context)
 * <li>{@code postProcessBeforeInitialization} methods of BeanPostProcessors
 * <li>InitializingBean's {@code afterPropertiesSet}
 * <li>a custom init-method definition
 * <li>{@code postProcessAfterInitialization} methods of BeanPostProcessors
 * </ol>
 *
 * <p>On shutdown of a bean factory, the following lifecycle methods apply:
 * <ol>
 * <li>{@code postProcessBeforeDestruction} methods of DestructionAwareBeanPostProcessors
 * <li>DisposableBean's {@code destroy}
 * <li>a custom destroy-method definition
 * </ol>
 *
 * @author Rod Johnson
 * @author Juergen Hoeller
 * @author Chris Beams
 * @since 13 April 2001
 * @see BeanNameAware#setBeanName
 * @see BeanClassLoaderAware#setBeanClassLoader
 * @see BeanFactoryAware#setBeanFactory
 * @see org.springframework.context.ResourceLoaderAware#setResourceLoader
 * @see org.springframework.context.ApplicationEventPublisherAware#setApplicationEventPublisher
 * @see org.springframework.context.MessageSourceAware#setMessageSource
 * @see org.springframework.context.ApplicationContextAware#setApplicationContext
 * @see org.springframework.web.context.ServletContextAware#setServletContext
 * @see org.springframework.beans.factory.config.BeanPostProcessor#postProcessBeforeInitialization
 * @see InitializingBean#afterPropertiesSet
 * @see org.springframework.beans.factory.support.RootBeanDefinition#getInitMethodName
 * @see org.springframework.beans.factory.config.BeanPostProcessor#postProcessAfterInitialization
 * @see DisposableBean#destroy
 * @see org.springframework.beans.factory.support.RootBeanDefinition#getDestroyMethodName
 */
public interface BeanFactory {

	/**
	 * Used to dereference a {@link FactoryBean} instance and distinguish it from
	 * beans <i>created</i> by the FactoryBean. For example, if the bean named
	 * {@code myJndiObject} is a FactoryBean, getting {@code &myJndiObject}
	 * will return the factory, not the instance returned by the factory.
	 */
	String FACTORY_BEAN_PREFIX = "&";


	/**
	 * Return an instance, which may be shared or independent, of the specified bean.
	 * <p>This method allows a Spring BeanFactory to be used as a replacement for the
	 * Singleton or Prototype design pattern. Callers may retain references to
	 * returned objects in the case of Singleton beans.
	 * <p>Translates aliases back to the corresponding canonical bean name.
	 * <p>Will ask the parent factory if the bean cannot be found in this factory instance.
	 * @param name the name of the bean to retrieve
	 * @return an instance of the bean
	 * @throws NoSuchBeanDefinitionException if there is no bean with the specified name
	 * @throws BeansException if the bean could not be obtained
	 */
	Object getBean(String name) throws BeansException;

	/**
	 * Return an instance, which may be shared or independent, of the specified bean.
	 * <p>Behaves the same as {@link #getBean(String)}, but provides a measure of type
	 * safety by throwing a BeanNotOfRequiredTypeException if the bean is not of the
	 * required type. This means that ClassCastException can't be thrown on casting
	 * the result correctly, as can happen with {@link #getBean(String)}.
	 * <p>Translates aliases back to the corresponding canonical bean name.
	 * <p>Will ask the parent factory if the bean cannot be found in this factory instance.
	 * @param name the name of the bean to retrieve
	 * @param requiredType type the bean must match; can be an interface or superclass
	 * @return an instance of the bean
	 * @throws NoSuchBeanDefinitionException if there is no such bean definition
	 * @throws BeanNotOfRequiredTypeException if the bean is not of the required type
	 * @throws BeansException if the bean could not be created
	 */
	<T> T getBean(String name, Class<T> requiredType) throws BeansException;

	/**
	 * Return an instance, which may be shared or independent, of the specified bean.
	 * <p>Allows for specifying explicit constructor arguments / factory method arguments,
	 * overriding the specified default arguments (if any) in the bean definition.
	 * @param name the name of the bean to retrieve
	 * @param args arguments to use when creating a bean instance using explicit arguments
	 * (only applied when creating a new instance as opposed to retrieving an existing one)
	 * @return an instance of the bean
	 * @throws NoSuchBeanDefinitionException if there is no such bean definition
	 * @throws BeanDefinitionStoreException if arguments have been given but
	 * the affected bean isn't a prototype
	 * @throws BeansException if the bean could not be created
	 * @since 2.5
	 */
	Object getBean(String name, Object... args) throws BeansException;

	/**
	 * Return the bean instance that uniquely matches the given object type, if any.
	 * <p>This method goes into {@link ListableBeanFactory} by-type lookup territory
	 * but may also be translated into a conventional by-name lookup based on the name
	 * of the given type. For more extensive retrieval operations across sets of beans,
	 * use {@link ListableBeanFactory} and/or {@link BeanFactoryUtils}.
	 * @param requiredType type the bean must match; can be an interface or superclass
	 * @return an instance of the single bean matching the required type
	 * @throws NoSuchBeanDefinitionException if no bean of the given type was found
	 * @throws NoUniqueBeanDefinitionException if more than one bean of the given type was found
	 * @throws BeansException if the bean could not be created
	 * @since 3.0
	 * @see ListableBeanFactory
	 */
	<T> T getBean(Class<T> requiredType) throws BeansException;

	/**
	 * Return an instance, which may be shared or independent, of the specified bean.
	 * <p>Allows for specifying explicit constructor arguments / factory method arguments,
	 * overriding the specified default arguments (if any) in the bean definition.
	 * <p>This method goes into {@link ListableBeanFactory} by-type lookup territory
	 * but may also be translated into a conventional by-name lookup based on the name
	 * of the given type. For more extensive retrieval operations across sets of beans,
	 * use {@link ListableBeanFactory} and/or {@link BeanFactoryUtils}.
	 * @param requiredType type the bean must match; can be an interface or superclass
	 * @param args arguments to use when creating a bean instance using explicit arguments
	 * (only applied when creating a new instance as opposed to retrieving an existing one)
	 * @return an instance of the bean
	 * @throws NoSuchBeanDefinitionException if there is no such bean definition
	 * @throws BeanDefinitionStoreException if arguments have been given but
	 * the affected bean isn't a prototype
	 * @throws BeansException if the bean could not be created
	 * @since 4.1
	 */
	<T> T getBean(Class<T> requiredType, Object... args) throws BeansException;

	/**
	 * Return a provider for the specified bean, allowing for lazy on-demand retrieval
	 * of instances, including availability and uniqueness options.
	 * @param requiredType type the bean must match; can be an interface or superclass
	 * @return a corresponding provider handle
	 * @since 5.1
	 * @see #getBeanProvider(ResolvableType)
	 */
	<T> ObjectProvider<T> getBeanProvider(Class<T> requiredType);

	/**
	 * Return a provider for the specified bean, allowing for lazy on-demand retrieval
	 * of instances, including availability and uniqueness options.
	 * @param requiredType type the bean must match; can be a generic type declaration.
	 * Note that collection types are not supported here, in contrast to reflective
	 * injection points. For programmatically retrieving a list of beans matching a
	 * specific type, specify the actual bean type as an argument here and subsequently
	 * use {@link ObjectProvider#orderedStream()} or its lazy streaming/iteration options.
	 * @return a corresponding provider handle
	 * @since 5.1
	 * @see ObjectProvider#iterator()
	 * @see ObjectProvider#stream()
	 * @see ObjectProvider#orderedStream()
	 */
	<T> ObjectProvider<T> getBeanProvider(ResolvableType requiredType);

	/**
	 * Does this bean factory contain a bean definition or externally registered singleton
	 * instance with the given name?
	 * <p>If the given name is an alias, it will be translated back to the corresponding
	 * canonical bean name.
	 * <p>If this factory is hierarchical, will ask any parent factory if the bean cannot
	 * be found in this factory instance.
	 * <p>If a bean definition or singleton instance matching the given name is found,
	 * this method will return {@code true} whether the named bean definition is concrete
	 * or abstract, lazy or eager, in scope or not. Therefore, note that a {@code true}
	 * return value from this method does not necessarily indicate that {@link #getBean}
	 * will be able to obtain an instance for the same name.
	 * @param name the name of the bean to query
	 * @return whether a bean with the given name is present
	 */
	boolean containsBean(String name);

	/**
	 * Is this bean a shared singleton? That is, will {@link #getBean} always
	 * return the same instance?
	 * <p>Note: This method returning {@code false} does not clearly indicate
	 * independent instances. It indicates non-singleton instances, which may correspond
	 * to a scoped bean as well. Use the {@link #isPrototype} operation to explicitly
	 * check for independent instances.
	 * <p>Translates aliases back to the corresponding canonical bean name.
	 * <p>Will ask the parent factory if the bean cannot be found in this factory instance.
	 * @param name the name of the bean to query
	 * @return whether this bean corresponds to a singleton instance
	 * @throws NoSuchBeanDefinitionException if there is no bean with the given name
	 * @see #getBean
	 * @see #isPrototype
	 */
	boolean isSingleton(String name) throws NoSuchBeanDefinitionException;

	/**
	 * Is this bean a prototype? That is, will {@link #getBean} always return
	 * independent instances?
	 * <p>Note: This method returning {@code false} does not clearly indicate
	 * a singleton object. It indicates non-independent instances, which may correspond
	 * to a scoped bean as well. Use the {@link #isSingleton} operation to explicitly
	 * check for a shared singleton instance.
	 * <p>Translates aliases back to the corresponding canonical bean name.
	 * <p>Will ask the parent factory if the bean cannot be found in this factory instance.
	 * @param name the name of the bean to query
	 * @return whether this bean will always deliver independent instances
	 * @throws NoSuchBeanDefinitionException if there is no bean with the given name
	 * @since 2.0.3
	 * @see #getBean
	 * @see #isSingleton
	 */
	boolean isPrototype(String name) throws NoSuchBeanDefinitionException;

	/**
	 * Check whether the bean with the given name matches the specified type.
	 * More specifically, check whether a {@link #getBean} call for the given name
	 * would return an object that is assignable to the specified target type.
	 * <p>Translates aliases back to the corresponding canonical bean name.
	 * <p>Will ask the parent factory if the bean cannot be found in this factory instance.
	 * @param name the name of the bean to query
	 * @param typeToMatch the type to match against (as a {@code ResolvableType})
	 * @return {@code true} if the bean type matches,
	 * {@code false} if it doesn't match or cannot be determined yet
	 * @throws NoSuchBeanDefinitionException if there is no bean with the given name
	 * @since 4.2
	 * @see #getBean
	 * @see #getType
	 */
	boolean isTypeMatch(String name, ResolvableType typeToMatch) throws NoSuchBeanDefinitionException;

	/**
	 * Check whether the bean with the given name matches the specified type.
	 * More specifically, check whether a {@link #getBean} call for the given name
	 * would return an object that is assignable to the specified target type.
	 * <p>Translates aliases back to the corresponding canonical bean name.
	 * <p>Will ask the parent factory if the bean cannot be found in this factory instance.
	 * @param name the name of the bean to query
	 * @param typeToMatch the type to match against (as a {@code Class})
	 * @return {@code true} if the bean type matches,
	 * {@code false} if it doesn't match or cannot be determined yet
	 * @throws NoSuchBeanDefinitionException if there is no bean with the given name
	 * @since 2.0.1
	 * @see #getBean
	 * @see #getType
	 */
	boolean isTypeMatch(String name, Class<?> typeToMatch) throws NoSuchBeanDefinitionException;

	/**
	 * Determine the type of the bean with the given name. More specifically,
	 * determine the type of object that {@link #getBean} would return for the given name.
	 * <p>For a {@link FactoryBean}, return the type of object that the FactoryBean creates,
	 * as exposed by {@link FactoryBean#getObjectType()}. This may lead to the initialization
	 * of a previously uninitialized {@code FactoryBean} (see {@link #getType(String, boolean)}).
	 * <p>Translates aliases back to the corresponding canonical bean name.
	 * <p>Will ask the parent factory if the bean cannot be found in this factory instance.
	 * @param name the name of the bean to query
	 * @return the type of the bean, or {@code null} if not determinable
	 * @throws NoSuchBeanDefinitionException if there is no bean with the given name
	 * @since 1.1.2
	 * @see #getBean
	 * @see #isTypeMatch
	 */
	@Nullable
	Class<?> getType(String name) throws NoSuchBeanDefinitionException;

	/**
	 * Determine the type of the bean with the given name. More specifically,
	 * determine the type of object that {@link #getBean} would return for the given name.
	 * <p>For a {@link FactoryBean}, return the type of object that the FactoryBean creates,
	 * as exposed by {@link FactoryBean#getObjectType()}. Depending on the
	 * {@code allowFactoryBeanInit} flag, this may lead to the initialization of a previously
	 * uninitialized {@code FactoryBean} if no early type information is available.
	 * <p>Translates aliases back to the corresponding canonical bean name.
	 * <p>Will ask the parent factory if the bean cannot be found in this factory instance.
	 * @param name the name of the bean to query
	 * @param allowFactoryBeanInit whether a {@code FactoryBean} may get initialized
	 * just for the purpose of determining its object type
	 * @return the type of the bean, or {@code null} if not determinable
	 * @throws NoSuchBeanDefinitionException if there is no bean with the given name
	 * @since 5.2
	 * @see #getBean
	 * @see #isTypeMatch
	 */
	@Nullable
	Class<?> getType(String name, boolean allowFactoryBeanInit) throws NoSuchBeanDefinitionException;

	/**
	 * Return the aliases for the given bean name, if any.
	 * <p>All of those aliases point to the same bean when used in a {@link #getBean} call.
	 * <p>If the given name is an alias, the corresponding original bean name
	 * and other aliases (if any) will be returned, with the original bean name
	 * being the first element in the array.
	 * <p>Will ask the parent factory if the bean cannot be found in this factory instance.
	 * @param name the bean name to check for aliases
	 * @return the aliases, or an empty array if none
	 * @see #getBean
	 */
	String[] getAliases(String name);

}

由上面的源码我们可以看到，BeacFactory是一个接口，但是其注释很多。下面我们来翻译有用的注释。

BeanFactory是一个中央注册中心应用程序组件，并集中配置应用程序组件。

由此可知，Spring容器管理的对象，都注册在了BeanFactory中。BeanFactory是一个对象的注册中心。

Note that it is generally better to rely on Dependency Injection(“push” configuration) to configure application objects through setters or constructors,rather than use any form of “pull” configuration like a BeanFactory lookup

注意，依赖依赖依赖注入（“推”配置）通过setter或constructor来配置应用程序对象通常更好，而不是像BeanFactory查找那样使用任何形式的“拉”配置

对于这句话，笔者并不是特别理解。按照 字面意思是依赖注入通过setter方法或者构造方法实现更好，而不是让BeanFactory查找对象。Spring难道不是使用查找的方式进行依赖注入的吗？

Spring’s Dependency Injection functionality is implemented using this BeanFactory interface and its subinterfaces.
Spring的依赖注入功能是使用这个BeanFactory接口及其子接口实现的。

具体如何实现的依赖注入，我们后面再研究，本篇文章重点讲BeacFactory。这里只需要知道依赖注入由BeacFactory及其子类实现。

Bean factory implementations should support the standard bean lifecycle interfaces as far as possible. The full set of initialization methods and their standard order is:
BeanNameAware’s setBeanName
BeanClassLoaderAware’s setBeanClassLoader
BeanFactoryAware’s setBeanFactory
EnvironmentAware’s setEnvironment
EmbeddedValueResolverAware’s setEmbeddedValueResolver
ResourceLoaderAware’s setResourceLoader (only applicable when running in an application context)
ApplicationEventPublisherAware’s setApplicationEventPublisher (only applicable when running in an application context)
MessageSourceAware’s setMessageSource (only applicable when running in an application context)
ApplicationContextAware’s setApplicationContext (only applicable when running in an application context)
ServletContextAware’s setServletContext (only applicable when running in a web application context)
postProcessBeforeInitialization methods of BeanPostProcessors
InitializingBean’s afterPropertiesSet
a custom init-method definition
postProcessAfterInitialization methods of BeanPostProcessors

Bean工厂实现应该尽可能支持标准Bean生命周期接口。全套初始化方法及其标准顺序为：
1.设置BeanName
2.设置Bean的ClassLoader
3.设置BeanFactory
4.设置Environment
5.设置EmbeddedValueResolver
6.设置ResourceLoader（仅在应用程序上下文中运行时适用）
7.设置ApplicationEventPublisher（仅在应用程序上下文中运行时适用）
8.设置MessageSource（仅在应用程序上下文中运行时适用）
9.设置ApplicationContext（仅在应用程序上下文中运行时适用）
10.设置ServletContext（仅在应用程序上下文中运行时适用）
11.执行BeanPostProcessors
12.执行Bean的afterPropertiesSet
13.执行 init-method
14.执行BeanPostProcessors

具体的流程，我们在以后源码分析中一一讲解，这里先知道这个流程的规范。

On shutdown of a bean factory, the following lifecycle methods apply:

1. postProcessBeforeDestruction methods of DestructionAwareBeanPostProcessors
   2.DisposableBean’s destroy
   3.a custom destroy-method definition

BeanFactory的销毁过程有三个步骤，这个我们后面一一解释。继续往下读源码。
下面就是BeanFactory接口的方法了，我们看如下截图:

由上面的截图可知，BeanFactory接口的方法，都是跟Spring容器中管理的bean有关的。所以，使用BeanFactory的重点是初始化FactoryBean。初始化BeanFactory的流程上面已经提到，我们会逐一讲解。

二、DefaultListableBeanFactory类讲解

BeanFactory下面有很多子类，众多的子类都是对BeanFactory功能的扩展。在Spring初始化过程中，用到的是DefaultListableBeanFactory类进行的初始化，我们看该类的关系图:

可以看出，DefaultListableBeanFactory类对BeanFactory的多层子类进行了实现和扩展。且该类还实现了AliasRegistry(别名注册)接口。该类的源码较多，我们在阅读Spring流程的时候，用到哪里了，再细读哪里。总之知道这个类是bean的注册中心即可。