1 概述
Bean的定义主要是由BeanDefinition来描述的。BeanDefinition是Spring中包装bean的数据结构，她描述了一个bean实例属性、构造参数。当然针对她的具体实现类提供了更加丰富的功能。这仅仅是一个基本的接口，这个接口的主要目的是允许修改和获取bean的属性和元数据。
2 类关系
针对BeanDefinition，我们首先来看一下UML类图。
从上图我们可以看出BeanDefinition是一个接口，并且继承了AttributeAccessor和BeanMetadataElement这两个接口。我们来看一下这两个接口做了什么。
（1）AttributeAccessor
这个接口主要定义了一些对任意对象的元数据的访问的函数，具体的源码如下。
public interface AttributeAccessor {
//设置指定名称的属性为指定的内容，当然如果这个内容为null就相当于将这个属性进行了删除。
void setAttribute(String name, @Nullable Object value);
//获取指定名称的属性值
@Nullable
Object getAttribute(String name);
//移除指定名称的属性
@Nullable
Object removeAttribute(String name);
//判断对象是否有对应名称的属性
boolean hasAttribute(String name);
//获取所有属性名称的数组
String[] attributeNames();
}（2）BeanMetadataElement
返回元数据元素的配置源对象，这个类就仅仅包含了一个getResource函数。
3 属性
(1) SCOPE_SINGLETON
这是一个bean为单列时候的一个标识符。
(2) SCOPE_PROTOTYPE
这是一个bean为非单列时候的标识服。这里针对单列和非单列简单说明一下。
单列是在整个程序运行的过程当中仅仅只有一个对应的对象被创建，比如我们常见的配置类，这在整个程序运行的过程中就只有一个。
非单列，是在每次使用类对应的bean的时候都重新创建对应的对象，当然如果类包含状态的时候我们就考虑使用非单列。
(3) ROLE_APPLICATION
表示这个bean是应用程序的主要部分。
(4) ROLE_SUPPORT
表示BeanDefinition是某些大型配置的支持部分的角色提示，通常是一个外部ComponentDefinition。当查看某个特定的ComponentDefinition时，认为bean非常重要，以便在查看应用程序的整体配置时能够意识到这一点。
(5) ROLE_INFRASTRUCTURE
表明一个BeanDefinition是提供一个完全背景的角色，并且与最终用户没有关系。这个提示用于注册完全是ComponentDefinition内部工作的一部分的bean。
4 函数
真针对函数部分的学习，我们直接查看源码部分吧，这些函数其实主要是对一些基础属性的访问器和修改器。
/**
* 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();
}
// 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.
* @since 3.0
* @see #SCOPE_PROTOTYPE
*/
boolean isPrototype();
/**
* Return whether this bean is "abstract", that is, not meant to be instantiated.
*/
boolean isAbstract();
/**
* 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();
/**
* Return a human-readable description of this bean definition.
*/
@Nullable
String getDescription();
/**
* 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();利用注释我们可以理解每个函数的具体作用，我们这里就不进行一一解释了。
通过上面的分析，我们对BeanDefinition有了个深入的认识，接下来我们会对BeanDefinition的子类进行详细分析，欢迎交流。
