什么是BeanDefinition
在Java中，一切皆对象。在JDK中使用java.lang.Class来描述类这个对象。
在Spring中，存在bean这样一个概念，那Spring又是怎么抽象bean这个概念，用什么类来描述bean这个对象呢？Spring使用BeanDefinition来描述bean。
BeanDefinition
BeanDefinition继承了AttributeAccessor和BeanMetadataElement接口。在Spring中充斥着大量的各种接口，每种接口都拥有不同的能力，某个类实现了某个接口，也就相应的拥有了某种能力。
AttributeAccessor
顾名思义，这是一个属性访问者，它提供了访问属性的能力。
BeanMetadataElement
BeanMetadataElement中只有一个方法，用来获取元数据元素的配置源对象：
public interface BeanMetadataElement { @Nullable Object getSource();}BeanDefinition
BeanDefinition接口是Spring对bean的抽象。
我们可以从源码中可以看出，Spring是怎么描述一个bean：
public interface BeanDefinition extends AttributeAccessor, BeanMetadataElement { /** * Scope identifier for the standard singleton scope: "singleton". * <p>Note that extended bean factories might support further scopes. * * @see #setScope */ String SCOPE_SINGLETON = ConfigurableBeanFactory.SCOPE_SINGLETON; /** * Scope identifier for the standard prototype scope: "prototype". * <p>Note that extended bean factories might support further scopes. * * @see #setScope */ String SCOPE_PROTOTYPE = ConfigurableBeanFactory.SCOPE_PROTOTYPE; /** * Role hint indicating that a {@code BeanDefinition} is a major part * of the application. Typically corresponds to a user-defined bean. */ int ROLE_APPLICATION = 0; /** * Role hint indicating that a {@code BeanDefinition} is a supporting * part of some larger configuration, typically an outer * {@link org.springframework.beans.factory.parsing.ComponentDefinition}. * {@code SUPPORT} beans are considered important enough to be aware * of when looking more closely at a particular * {@link org.springframework.beans.factory.parsing.ComponentDefinition}, * but not when looking at the overall configuration of an application. */ int ROLE_SUPPORT = 1; /** * Role hint indicating that a {@code BeanDefinition} is providing an * entirely background role and has no relevance to the end-user. This hint is * used when registering beans that are completely part of the internal workings * of a {@link org.springframework.beans.factory.parsing.ComponentDefinition}. */ int ROLE_INFRASTRUCTURE = 2; // Modifiable attributes /** * Set the name of the parent definition of this bean definition, if any. */ void setParentName(@Nullable String parentName); /** * Return the name of the parent definition of this bean definition, if any. */ @Nullable String getParentName(); /** * Specify the bean class name of this bean definition. * <p>The class name can be modified during bean factory post-processing, * typically replacing the original class name with a parsed variant of it. * * @see #setParentName * @see #setFactoryBeanName * @see #setFactoryMethodName */ void setBeanClassName(@Nullable String beanClassName); /** * Return the current bean class name of this bean definition. * <p>Note that this does not have to be the actual class name used at runtime, in * case of a child definition overriding/inheriting the class name from its parent. * Also, this may just be the class that a factory method is called on, or it may * even be empty in case of a factory bean reference that a method is called on. * Hence, do <i>not</i> consider this to be the definitive bean type at runtime but * rather only use it for parsing purposes at the individual bean definition level. * * @see #getParentName() * @see #getFactoryBeanName() * @see #getFactoryMethodName() */ @Nullable String getBeanClassName(); /** * Override the target scope of this bean, specifying a new scope name. * * @see #SCOPE_SINGLETON * @see #SCOPE_PROTOTYPE */ void setScope(@Nullable String scope); /** * Return the name of the current target scope for this bean, * or {@code null} if not known yet. */ @Nullable String getScope(); /** * Set whether this bean should be lazily initialized. * <p>If {@code false}, the bean will get instantiated on startup by bean * factories that perform eager initialization of singletons. */ void setLazyInit(boolean lazyInit); /** * Return whether this bean should be lazily initialized, i.e. not * eagerly instantiated on startup. Only applicable to a singleton bean. */ boolean isLazyInit(); /** * Set the names of the beans that this bean depends on being initialized. * The bean factory will guarantee that these beans get initialized first. */ void setDependsOn(@Nullable String... dependsOn); /** * Return the bean names that this bean depends on. */ @Nullable String[] getDependsOn(); /** * Set whether this bean is a candidate for getting autowired into some other bean. * <p>Note that this flag is designed to only affect type-based autowiring. * It does not affect explicit references by name, which will get resolved even * if the specified bean is not marked as an autowire candidate. As a consequence, * autowiring by name will nevertheless inject a bean if the name matches. */ void setAutowireCandidate(boolean autowireCandidate); /** * Return whether this bean is a candidate for getting autowired into some other bean. */ boolean isAutowireCandidate(); /** * Set whether this bean is a primary autowire candidate. * <p>If this value is {@code true} for exactly one bean among multiple * matching candidates, it will serve as a tie-breaker. */ void setPrimary(boolean primary); /** * Return whether this bean is a primary autowire candidate. */ boolean isPrimary(); /** * Specify the factory bean to use, if any. * This the name of the bean to call the specified factory method on. * * @see #setFactoryMethodName */ void setFactoryBeanName(@Nullable String factoryBeanName); /** * Return the factory bean name, if any. */ @Nullable String getFactoryBeanName(); /** * Specify a factory method, if any. This method will be invoked with * constructor arguments, or with no arguments if none are specified. * The method will be invoked on the specified factory bean, if any, * or otherwise as a static method on the local bean class. * * @see #setFactoryBeanName * @see #setBeanClassName */ void setFactoryMethodName(@Nullable String factoryMethodName); /** * Return a factory method, if any. */ @Nullable String getFactoryMethodName(); /** * Return the constructor argument values for this bean. * <p>The returned instance can be modified during bean factory post-processing. * * @return the ConstructorArgumentValues object (never {@code null}) */ ConstructorArgumentValues getConstructorArgumentValues(); /** * Return if there are constructor argument values defined for this bean. * * @since 5.0.2 */ default boolean hasConstructorArgumentValues() { return !getConstructorArgumentValues().isEmpty(); } /** * Return the property values to be applied to a new instance of the bean. * <p>The returned instance can be modified during bean factory post-processing. * * @return the MutablePropertyValues object (never {@code null}) */ MutablePropertyValues getPropertyValues(); /** * Return if there are property values values defined for this bean. * * @since 5.0.2 */ default boolean hasPropertyValues() { return !getPropertyValues().isEmpty(); } /** * Set the name of the initializer method. * * @since 5.1 */ void setInitMethodName(@Nullable String initMethodName); /** * Return the name of the initializer method. * * @since 5.1 */ @Nullable String getInitMethodName(); /** * Set the name of the destroy method. * * @since 5.1 */ void setDestroyMethodName(@Nullable String destroyMethodName); /** * Return the name of the destroy method. * * @since 5.1 */ @Nullable String getDestroyMethodName(); /** * Set the role hint for this {@code BeanDefinition}. The role hint * provides the frameworks as well as tools with an indication of * the role and importance of a particular {@code BeanDefinition}. * * @see #ROLE_APPLICATION * @see #ROLE_SUPPORT * @see #ROLE_INFRASTRUCTURE * @since 5.1 */ void setRole(int role); /** * Get the role hint for this {@code BeanDefinition}. The role hint * provides the frameworks as well as tools with an indication of * the role and importance of a particular {@code BeanDefinition}. * * @see #ROLE_APPLICATION * @see #ROLE_SUPPORT * @see #ROLE_INFRASTRUCTURE */ int getRole(); /** * Set a human-readable description of this bean definition. * * @since 5.1 */ void setDescription(@Nullable String description); /** * Return a human-readable description of this bean definition. */ @Nullable String getDescription(); // Read-only attributes /** * Return whether this a <b>Singleton</b>, with a single, shared instance * returned on all calls. * * @see #SCOPE_SINGLETON */ boolean isSingleton(); /** * Return whether this a <b>Prototype</b>, with an independent instance * returned for each call. * * @see #SCOPE_PROTOTYPE * @since 3.0 */ boolean isPrototype(); /** * Return whether this bean is "abstract", that is, not meant to be instantiated. */ boolean isAbstract(); /** * Return a description of the resource that this bean definition * came from (for the purpose of showing context in case of errors). */ @Nullable String getResourceDescription(); /** * Return the originating BeanDefinition, or {@code null} if none. * Allows for retrieving the decorated bean definition, if any. * <p>Note that this method returns the immediate originator. Iterate through the * originator chain to find the original BeanDefinition as defined by the user. */ @Nullable BeanDefinition getOriginatingBeanDefinition();}AnnotatedBeanDefinition
AnnotatedBeanDefinition 继承了BeanDefinition，拓展了BeanDefinition接口的能力：
BeanDefinition常见实现类
- ChildBeanDefinition
- RootBeanDefinition
- GenericBeanDefinition
- AnnotatedGenericBeanDefinition
- ScannedGenericBeanDefinition
BeanDefinitionBuilder
BeanDefinitionBuilder是Builder模式的应用。通过这个类我们可以方便的构建BeanDefinition的实例对象。举个例子：
@Componentpublic class MyBeanDefinitionRegistryPostProcessor implements BeanDefinitionRegistryPostProcessor { @Override public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) throws BeansException { BeanDefinition beanDefinition = BeanDefinitionBuilder.genericBeanDefinition(OrderService.class) //这里的属性名是根据setter方法 .addPropertyReference("dao", "orderDao") .setInitMethodName("init") .setScope(BeanDefinition.SCOPE_SINGLETON) .getBeanDefinition(); registry.registerBeanDefinition("orderService", beanDefinition); } @Override public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException { //do nothing }}关于BeanDefinitionRegistryPostProcessor，这里先简单介绍一下，详细的我们后面再说。
BeanDefinitionRegistryPostProcessor继承了BeanFactoryPostProcessor接口，同时又增加了一个新的方法BeanDefinitionRegistryPostProcessor#postProcessBeanDefinitionRegistry，该方法允许程序员通过代码编码的方式手动往程序中注册BeanDefinition。
实例中的具体程序已上传到github：https://github.com/shenjianen...
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