Bean的概述

Spring IoC容器管理一个或多个bean。这些bean是使用您提供给容器的配置元数据创建的，例如，以XML 定义的形式 。
在容器本身中，这些bean定义被表示为BeanDefinition 对象，其中包含以下元数据（以及其他信息）：

一、下面来看一下BeanDefinition

1.1

详细看一下BeanDefinition接口里面讲的有那些东西
首先来看一下，用的比较多的Spring表示范围Scope中的prototype与singleton区别

1.1.1 singleton作用域

当一个bean的作用域设置为singleton, 那么Spring IOC容器中只会存在一个共享的bean实例，并且所有对bean的请求，只要id与该bean定义相匹配，则只会返回bean的同一实例。换言之，当把一个bean定义设置为singleton作用域时，Spring IOC容器只会创建该bean定义的唯一实例。这个单一实例会被存储到单例缓存（singleton cache）中，并且所有针对该bean的后续请求和引用都将返回被缓存的对象实例。
这里要注意的是singleton作用域和GOF设计模式中的单例是完全不同的，单例设计模式表示一个ClassLoader中只有一个class存在，而这里的singleton则表示一个容器对应一个bean，也就是说当一个bean被标识为singleton时候，spring的IOC容器中只会存在一个该bean。

1.1.1 prototype作用域

prototype作用域部署的bean，每一次请求（将其注入到另一个bean中，或者以程序的方式调用容器的getBean()方法）都会产生一个新的bean实例，相当与一个new的操作，对于prototype作用域的bean，有一点非常重要，那就是Spring不能对一个prototype bean的整个生命周期负责，容器在初始化、配置、装饰或者是装配完一个prototype实例后，将它交给客户端，随后就对该prototype实例不闻不问了。
不管何种作用域，容器都会调用所有对象的初始化生命周期回调方法，而对prototype而言，任何配置好的析构生命周期回调方法都将不会被调用。清除prototype作用域的对象并释放任何prototype bean所持有的昂贵资源，都是客户端代码的职责。（让Spring容器释放被singleton作用域bean占用资源的一种可行方式是，通过使用bean的后置处理器，该处理器持有要被清除的bean的引用。）

1.1.1.1 关于scope=”prototype”没写的问题

项目中对一个表的增删该操作是用一个action，这个action拥有add,update,delete,save等方法， 添加和修改是共用一个页面，当页面得到id时代表进行的修改操作，反之是添加操作。如果在配置spring的bean时，忘了写scope=”prototype” ，所以每次添加时都显示最后一次访问过的记录。scope=”prototype” 会在该类型的对象被请求时创建一个新的action对象，如果没有配置scope=”prototype”则添加的时候不会新建一个action，它仍然会保留上次访问的过记录的信息 ，所以，webwork的Action不是线程安全的，要求在多线程环境下必须是一个线程对应一个独立的实例，则不能使用singleton。所以，我们在Spring配置Webwork Action Bean时，需要加上属性scope=”prototype”或singleton=”false”。
singleton模式指的是对某个对象的完全共享，包括代码空间和数据空间，说白了，如果一个类是singleton的，假如这个类有成员变量，那么这个成员变量的值是各个线程共享的（有点类似于static的样子了），当线程A往给变量赋了一个值以后，线程B就能读出这个值。
因此，对于前台Action，肯定不能使用singleton的模式，必须是一个线程请求对应一个独立的实例。推而广之，只要是带数据成员变量的类，为了防止多个线程混用数据，就不能使用singleton。对于我们用到的Service、Dao，之所以用了singleton，就是因为他们没有用到数据成员变量，如果谁的Service需要数据成员变量，请设置singleton=false。 有状态的bean都使用Prototype作用域，而对无状态的bean则应该使用singleton作用域。

二、看一下BeanDefinition的源码

/*
 * Copyright 2002-2017 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.beans.factory.config;

import org.springframework.beans.BeanMetadataElement;
import org.springframework.beans.MutablePropertyValues;
import org.springframework.core.AttributeAccessor;
import org.springframework.lang.Nullable;

/**
 * A BeanDefinition describes a bean instance, which has property values,
 * constructor argument values, and further information supplied by
 * concrete implementations.
 * 一个bean的定义就是描述一个bean的实例，这个bean有属性值，构造参数的值
 * 这个bean的实现会提供更多的信息
 *
 * <p>This is just a minimal interface: The main intention is to allow a
 * {@link BeanFactoryPostProcessor} such as {@link PropertyPlaceholderConfigurer}
 * to introspect and modify property values and other bean metadata.
 * 这个BeanDefinition仅仅只是一个最简洁的接口，它主要的目的是为了允许BeanFactoryPostProcessor和PropertyPlaceholderConfigurer
 * 去反省和定义属性值和一些其他的元数据
 *
 * @author Juergen Hoeller
 * @author Rob Harrop
 * @since 19.03.2004
 * @see ConfigurableListableBeanFactory#getBeanDefinition
 * 通过ConfigurableListableBeanFactory可以获取到bean的属性定义对应的值
 * @see org.springframework.beans.factory.support.RootBeanDefinition
 * @see org.springframework.beans.factory.support.ChildBeanDefinition
 */
public interface BeanDefinition extends AttributeAccessor, BeanMetadataElement {

    /**
     * SCOPE范围
     */
    /**
     * Scope identifier for the standard singleton scope: "singleton".
     * scope范围标识符的单例singleton由字符串"singleton"表示。
     * <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".
     * prototype由字符串prototype表示
     * <p>Note that extended bean factories might support further scopes.
     * @see #setScope
     */
    String SCOPE_PROTOTYPE = ConfigurableBeanFactory.SCOPE_PROTOTYPE;


    /**
     * Role角色
     */
    /**
     * Role hint indicating that a {@code BeanDefinition} is a major part
     * of the application. Typically corresponds to a user-defined bean.
     * 角色应用
     */
    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.
     * 角色支持
     */
    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}.
     * 角色的基础构造
     */
    int ROLE_INFRASTRUCTURE = 2;


    // Modifiable attributes

    /**
     * Set the name of the parent definition of this bean definition, if any.
     * 设置父类Bean的定义名称
     */
    void setParentName(@Nullable String parentName);

    /**
     * Return the name of the parent definition of this bean definition, if any.
     * 获取父类Bean的名字
     */
    @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
     * 设定bean的类的名字
     */
    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()
     * 获取bean的类的名字
     */
    @Nullable
    String getBeanClassName();

    /**
     * Override the target scope of this bean, specifying a new scope name.
     * @see #SCOPE_SINGLETON
     * @see #SCOPE_PROTOTYPE
     * 设置范围
     */
    void setScope(@Nullable String scope);

    /**
     * Return the name of the current target scope for this bean,
     * or {@code null} if not known yet.
     * 获取范围
     */
    @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.
     * 设置是否延迟加载bean
     */
    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.
     * 获取这个bean是否需要延迟加载
     */
    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.
     * 设置这个bean,它的依赖的bean需要被初始化
     */
    void setDependsOn(@Nullable String... dependsOn);

    /**
     * Return the bean names that this bean depends on.
     * 返回这个bean所依赖的其他bean的名字
     */
    @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.
     * 设置这个Bean为自动装配的候选bean
     */
    void setAutowireCandidate(boolean autowireCandidate);

    /**
     * Return whether this bean is a candidate for getting autowired into some other bean.
     * 获取这个Bean是否是其他Bean的自动装配的候选人
     */
    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.
     * 设置这个Bean是否为主要需要自动装配的候选bean
     */
    void setPrimary(boolean primary);

    /**
     * Return whether this bean is a primary autowire candidate.
     * 获取这个Bean是否是需要首先自动装配的Bean
     */
    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
     * 设置需要使用的工厂bean
     */
    void setFactoryBeanName(@Nullable String factoryBeanName);

    /**
     * Return the factory bean name, if any.
     * 获取工厂Bean的名字
     */
    @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
     * 设置工厂方法
     */
    void setFactoryMethodName(@Nullable String factoryMethodName);

    /**
     * Return a factory method, if any.
     * 获取工厂方法
     */
    @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})
     * 获取这个Bean的构造方法的参数值
     */
    ConstructorArgumentValues getConstructorArgumentValues();

    /**
     * Return if there are constructor argument values defined for this bean.
     * 获取这个bean是否有构造方法的参数值
     * @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})
     * 获取属性值
     */
    MutablePropertyValues getPropertyValues();

    /**
     * Return if there are property values values defined for this bean.
     * @since 5.0.2
     * 获取这个Bean是否设置了属性值
     */
    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
     * 获取这个Bean是否是单例的
     */
    boolean isSingleton();

    /**
     * Return whether this a <b>Prototype</b>, with an independent instance
     * returned for each call.
     * @since 3.0
     * @see #SCOPE_PROTOTYPE
     * 这个Bean是否是Prototype(
     */
    boolean isPrototype();

    /**
     * Return whether this bean is "abstract", that is, not meant to be instantiated.
     * 这个Bean是否是抽象的
     */
    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
     * 获取Role角色的值
     */
    int getRole();

    /**
     * Return a human-readable description of this bean definition.
     * 获取描述
     */
    @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).
     * 获取资源的描述
     */
    @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.
     * 获取原始的BeanDefinition
     */
    @Nullable
    BeanDefinition getOriginatingBeanDefinition();

}

三、配置Bean的工厂接口

可以在里面，对Bean的作用范围等信息进行配置

四、Bean的定义主要有以下几个方面

表1.bean的定义

注意：
除了包含有关如何创建特定bean的信息的bean定义之外，这些ApplicationContext实现还允许用户注册在容器外部创建的现有对象。这是通过getBeanFactory()返回BeanFactory实现的方法访问ApplicationContext的BeanFactory来完成的DefaultListableBeanFactory。DefaultListableBeanFactory 支持通过方法该登记registerSingleton(..)和 registerBeanDefinition(..)。但是，典型的应用程序只能通过元数据bean定义来定义bean。

Bean元数据和手动提供的单例实例需要尽早注册，以便容器在自动装配和其他自省步骤中正确推理它们。虽然重写现有的元数据和现有的单例实例在某种程度上受到支持，但在运行时注册新的Bean（与实时访问工厂同时）并未得到正式支持，并且可能导致并发访问异常和/或bean容器中的状态不一致。

未完待～～～