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". * <p>Note that extended bean factories might support further scopes. */ String SCOPE_SINGLETON = ConfigurableBeanFactory.SCOPE_SINGLETON; /** * Scope identifier for the standard prototype scope: "prototype". * <p>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. * <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. * 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. */ String getBeanClassName(); /** * Override 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. */ 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. * <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 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. * <p>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. * <p>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. * <p>The returned instance can be modified during bean factory post-processing. * @return the MutablePropertyValues object (never {@code null}) */ MutablePropertyValues getPropertyValues(); /** * Return whether this a <b>Singleton</b>, with a single, shared instance * returned on all calls. */ boolean isSingleton(); /** * Return whether this a <b>Prototype</b>, 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. * <p>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。