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：
@Component
public 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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