深入理解Spring系列之二：BeanDefinition解析

《深入理解Spring系列之一：开篇》中提到在Spring容器启动的过程中，会将Bean解析成Spring内部的BeanDefinition结构，本篇将深入分析这个BeanDefinition的内部结构。直接看BeanDefinition源码，

public interface BeanDefinition extends AttributeAccessor, BeanMetadataElement {
    /**
     * Scope identifier for the standard singleton scope: "singleton".
     * 
Note that extended bean factories might support further scopes.
     */
    String SCOPE_SINGLETON = ConfigurableBeanFactory.SCOPE_SINGLETON;
    /**
     * Scope identifier for the standard prototype scope: "prototype".
     * 
Note that extended bean factories might support further scopes.
     */
    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;
    /**
     * Return the name of the parent definition of this bean definition, if any.
     */
    String getParentName();
    /**
     * Set the name of the parent definition of this bean definition, if any.
     */
    void setParentName(String parentName);
    /**
     * Return the current bean class name of this bean definition.
     * 
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.
     * Hence, do not consider this to be the definitive bean type at runtime but
     * rather only use it for parsing purposes at the individual bean definition level.
     */
    String getBeanClassName();
    /**
     * Override the bean class name of this bean definition.
     * 
The class name can be modified during bean factory post-processing,
     * typically replacing the original class name with a parsed variant of it.
     */
    void setBeanClassName(String beanClassName);
    /**
     * Return the factory bean name, if any.
     */
    String getFactoryBeanName();
    /**
     * Specify the factory bean to use, if any.
     */
    void setFactoryBeanName(String factoryBeanName);
    /**
     * Return a factory method, if any.
     */
    String getFactoryMethodName();
    /**
     * 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.
     * @param factoryMethodName static factory method name,
     * or {@code null} if normal constructor creation should be used
     */
    void setFactoryMethodName(String factoryMethodName);
    /**
     * Return the name of the current target scope for this bean,
     * or {@code null} if not known yet.
     */
    String getScope();
    /**
     * Override the target scope of this bean, specifying a new scope name.
     */
    void setScope(String scope);
    /**
     * Return whether this bean should be lazily initialized, i.e. not
     * eagerly instantiated on startup. Only applicable to a singleton bean.
     */
    boolean isLazyInit();
    /**
     * Set whether this bean should be lazily initialized.
     * 
If {@code false}, the bean will get instantiated on startup by bean
     * factories that perform eager initialization of singletons.
     */
    void setLazyInit(boolean lazyInit);
    /**
     * Return the bean names that this bean depends on.
     */
    String[] getDependsOn();
    /**
     * 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(String... dependsOn);
    /**
     * Return whether this bean is a candidate for getting autowired into some other bean.
     */
    boolean isAutowireCandidate();
    /**
     * Set whether this bean is a candidate for getting autowired into some other bean.
     */
    void setAutowireCandidate(boolean autowireCandidate);
    /**
     * Return whether this bean is a primary autowire candidate.
     * If this value is true for exactly one bean among multiple
     * matching candidates, it will serve as a tie-breaker.
     */
    boolean isPrimary();
    /**
     * Set whether this bean is a primary autowire candidate.
     * 
If this value is true for exactly one bean among multiple
     * matching candidates, it will serve as a tie-breaker.
     */
    void setPrimary(boolean primary);
    /**
     * Return the constructor argument values for this bean.
     * 
The returned instance can be modified during bean factory post-processing.
     * @return the ConstructorArgumentValues object (never {@code null})
     */
    ConstructorArgumentValues getConstructorArgumentValues();
    /**
     * Return the property values to be applied to a new instance of the bean.
     * 
The returned instance can be modified during bean factory post-processing.
     * @return the MutablePropertyValues object (never {@code null})
     */
    MutablePropertyValues getPropertyValues();
    /**
     * Return whether this a Singleton, with a single, shared instance
     * returned on all calls.
     */
    boolean isSingleton();
    /**
     * Return whether this a Prototype, with an independent instance
     * returned for each call.
     */
    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}.
     */
    int getRole();
    /**
     * Return a human-readable description of this bean definition.
     */
    String getDescription();
    /**
     * Return a description of the resource that this bean definition
     * came from (for the purpose of showing context in case of errors).
     */
    String getResourceDescription();
    /**
     * Return the originating BeanDefinition, or {@code null} if none.
     * Allows for retrieving the decorated bean definition, if any.
     * 
Note that this method returns the immediate originator. Iterate through the
     * originator chain to find the original BeanDefinition as defined by the user.
     */
    BeanDefinition getOriginatingBeanDefinition();
}

可以看到上面的很多属性和方法都很熟悉，例如类名、scope、属性、构造函数参数列表、依赖的bean、是否是单例类、是否是懒加载等，其实就是将Bean的定义信息存储到这个BeanDefinition相应的属性中，后面对Bean的操作就直接对BeanDefinition进行，例如拿到这个BeanDefinition后，可以根据里面的类名、构造函数、构造函数参数，使用反射进行对象创建。BeanDefinition是一个接口，是一个抽象的定义，实际使用的是其实现类，如ChildBeanDefinition、RootBeanDefinition、GenericBeanDefinition等。BeanDefinition继承了AttributeAccessor，说明它具有处理属性的能力；BeanDefinition继承了BeanMetadataElement，说明它可以持有Bean元数据元素，作用是可以持有XML文件的一个bean标签对应的Object。

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