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