spring源码学习之路---IOC初探（二）

作者：zuoxiaolong8810（左潇龙），转载请注明出处。

上一章当中我没有提及具体的搭建环境的步骤，一个是不得不承认有点懒，另外一个我觉得如果上章所述的那些环境都还不会搭建的话，研究spring的源码还有些过早。

如果你有兴趣的话，相信已经搭建好了学习研究的环境，接下来就可以进入正题了。

网上也有很多关于spring源码学习的文章以及帖子，讲的也都不错，但是有些可能高估了读者的能力，该深入的地方反倒一句带过，我现在也是在一步一步研究，和大家的进度一样，所以可能在我的角度来和各位探讨，更加容易。

首先我们来说一下IOC，IOC是spring最核心的理念，包括AOP也要屈居第二，那么IOC到底是什么呢，四个字，控制反转。

网上有不少是这么解释IOC的，说IOC是将对象的创建和依赖关系交给容器，这句话我相信不少人都知道，在我个人的理解，IOC就是让我们的开发变的更简单了。

为什么这么说呢，光说没意思，直接上代码。

 public class Person {

	public void work(){
		System.out.println("I am working");
	}
}

上面这个是Person类，如果我们还有一个Company公司类，公司要开张需要人来工作，所以我们可能需要这样。

public class Company {

	public Person person;
	
	public Company(Person person){
		this.person = person;
	}
	
	public void open(){
		person.work();
		System.out.println("I am opening");
	}
}

好了，这样可以了，虽说和现实有些区别，毕竟没有一个人的公司，但是就是这么个意思。必须要有人在公司里，公司才能开张。

有了spring上述情况我们是怎么写的呢？Person类不变，Company就可以简单些了。

public class Company {

	@Autowired
	public Person person;
	
	public void open(){
		person.work();
		System.out.println("I am opening");
	}
}

OK了，使用注解后，spring里的写法是这样的，是不是简单很多？或许你可能会说，这才减少了多少代码，但是事实上是，真正的项目中，不可能有这么简单的依赖关系，或许是2层，3层甚至N层。

当然，可能我们有时候用的XML，XML和注解的区别就在于这里，注解可以快速的完成依赖的注入，但是缺点也很明显，那就是比如我公司里不需要人了，我需要的是机器，那么我还要手动改代码，将Person换成机器（这里应该是英文，英语不好，懒得查了，只记得念“磨洗”），而如果是XML配置，那么我们只需要改下配置文件就可以。维护起来会方便很多，当然XML的缺点也很明显，那就是依赖关系复杂的时候，XML文件会比较臃肿，所以我们一般的做法是将XML分离开来。

说到这里，有些扯远了，但是我觉得以上可以足够说明IOC的好处，知道了IOC的好处，我们自然就要知道怎么来实现IOC了。

或许看了spring的源码，第一感觉是很蒙，包太多，我也很蒙，但是研究东西就是得沉下心来，先来关注一下这个接口，BeanFactory，附上代码。

/*
 * Copyright 2002-2010 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;

import org.springframework.beans.BeansException;

/**
 * The root interface for accessing a Spring bean container.
 * This is the basic client view of a bean container;
 * further interfaces such as {@link ListableBeanFactory} and
 * {@link org.springframework.beans.factory.config.ConfigurableBeanFactory}
 * are available for specific purposes.
 *
 * <p>This interface is implemented by objects that hold a number of bean definitions,
 * each uniquely identified by a String name. Depending on the bean definition,
 * the factory will return either an independent instance of a contained object
 * (the Prototype design pattern), or a single shared instance (a superior
 * alternative to the Singleton design pattern, in which the instance is a
 * singleton in the scope of the factory). Which type of instance will be returned
 * depends on the bean factory configuration: the API is the same. Since Spring
 * 2.0, further scopes are available depending on the concrete application
 * context (e.g. "request" and "session" scopes in a web environment).
 *
 * <p>The point of this approach is that the BeanFactory is a central registry
 * of application components, and centralizes configuration of application
 * components (no more do individual objects need to read properties files,
 * for example). See chapters 4 and 11 of "Expert One-on-One J2EE Design and
 * Development" for a discussion of the benefits of this approach.
 *
 * <p>Note that it is generally better to rely on Dependency Injection
 * ("push" configuration) to configure application objects through setters
 * or constructors, rather than use any form of "pull" configuration like a
 * BeanFactory lookup. Spring's Dependency Injection functionality is
 * implemented using this BeanFactory interface and its subinterfaces.
 *
 * <p>Normally a BeanFactory will load bean definitions stored in a configuration
 * source (such as an XML document), and use the <code>org.springframework.beans</code>
 * package to configure the beans. However, an implementation could simply return
 * Java objects it creates as necessary directly in Java code. There are no
 * constraints on how the definitions could be stored: LDAP, RDBMS, XML,
 * properties file, etc. Implementations are encouraged to support references
 * amongst beans (Dependency Injection).
 *
 * <p>In contrast to the methods in {@link ListableBeanFactory}, all of the
 * operations in this interface will also check parent factories if this is a
 * {@link HierarchicalBeanFactory}. If a bean is not found in this factory instance,
 * the immediate parent factory will be asked. Beans in this factory instance
 * are supposed to override beans of the same name in any parent factory.
 *
 * <p>Bean factory implementations should support the standard bean lifecycle interfaces
 * as far as possible. The full set of initialization methods and their standard order is:<br>
 * 1. BeanNameAware's <code>setBeanName</code><br>
 * 2. BeanClassLoaderAware's <code>setBeanClassLoader</code><br>
 * 3. BeanFactoryAware's <code>setBeanFactory</code><br>
 * 4. ResourceLoaderAware's <code>setResourceLoader</code>
 * (only applicable when running in an application context)<br>
 * 5. ApplicationEventPublisherAware's <code>setApplicationEventPublisher</code>
 * (only applicable when running in an application context)<br>
 * 6. MessageSourceAware's <code>setMessageSource</code>
 * (only applicable when running in an application context)<br>
 * 7. ApplicationContextAware's <code>setApplicationContext</code>
 * (only applicable when running in an application context)<br>
 * 8. ServletContextAware's <code>setServletContext</code>
 * (only applicable when running in a web application context)<br>
 * 9. <code>postProcessBeforeInitialization</code> methods of BeanPostProcessors<br>
 * 10. InitializingBean's <code>afterPropertiesSet</code><br>
 * 11. a custom init-method definition<br>
 * 12. <code>postProcessAfterInitialization</code> methods of BeanPostProcessors
 *
 * <p>On shutdown of a bean factory, the following lifecycle methods apply:<br>
 * 1. DisposableBean's <code>destroy</code><br>
 * 2. a custom destroy-method definition
 *
 * @author Rod Johnson
 * @author Juergen Hoeller
 * @since 13 April 2001
 * @see BeanNameAware#setBeanName
 * @see BeanClassLoaderAware#setBeanClassLoader
 * @see BeanFactoryAware#setBeanFactory
 * @see org.springframework.context.ResourceLoaderAware#setResourceLoader
 * @see org.springframework.context.ApplicationEventPublisherAware#setApplicationEventPublisher
 * @see org.springframework.context.MessageSourceAware#setMessageSource
 * @see org.springframework.context.ApplicationContextAware#setApplicationContext
 * @see org.springframework.web.context.ServletContextAware#setServletContext
 * @see org.springframework.beans.factory.config.BeanPostProcessor#postProcessBeforeInitialization
 * @see InitializingBean#afterPropertiesSet
 * @see org.springframework.beans.factory.support.RootBeanDefinition#getInitMethodName
 * @see org.springframework.beans.factory.config.BeanPostProcessor#postProcessAfterInitialization
 * @see DisposableBean#destroy
 * @see org.springframework.beans.factory.support.RootBeanDefinition#getDestroyMethodName
 */
public interface BeanFactory {

	/**
	 * Used to dereference a {@link FactoryBean} instance and distinguish it from
	 * beans <i>created</i> by the FactoryBean. For example, if the bean named
	 * <code>myJndiObject</code> is a FactoryBean, getting <code>&myJndiObject</code>
	 * will return the factory, not the instance returned by the factory.
	 */
	String FACTORY_BEAN_PREFIX = "&";


	/**
	 * Return an instance, which may be shared or independent, of the specified bean.
	 * <p>This method allows a Spring BeanFactory to be used as a replacement for the
	 * Singleton or Prototype design pattern. Callers may retain references to
	 * returned objects in the case of Singleton beans.
	 * <p>Translates aliases back to the corresponding canonical bean name.
	 * Will ask the parent factory if the bean cannot be found in this factory instance.
	 * @param name the name of the bean to retrieve
	 * @return an instance of the bean
	 * @throws NoSuchBeanDefinitionException if there is no bean definition
	 * with the specified name
	 * @throws BeansException if the bean could not be obtained
	 */
	Object getBean(String name) throws BeansException;

	/**
	 * Return an instance, which may be shared or independent, of the specified bean.
	 * <p>Behaves the same as {@link #getBean(String)}, but provides a measure of type
	 * safety by throwing a BeanNotOfRequiredTypeException if the bean is not of the
	 * required type. This means that ClassCastException can't be thrown on casting
	 * the result correctly, as can happen with {@link #getBean(String)}.
	 * <p>Translates aliases back to the corresponding canonical bean name.
	 * Will ask the parent factory if the bean cannot be found in this factory instance.
	 * @param name the name of the bean to retrieve
	 * @param requiredType type the bean must match. Can be an interface or superclass
	 * of the actual class, or <code>null</code> for any match. For example, if the value
	 * is <code>Object.class</code>, this method will succeed whatever the class of the
	 * returned instance.
	 * @return an instance of the bean
	 * @throws NoSuchBeanDefinitionException if there's no such bean definition
	 * @throws BeanNotOfRequiredTypeException if the bean is not of the required type
	 * @throws BeansException if the bean could not be created
	 */
	<T> T getBean(String name, Class<T> requiredType) throws BeansException;

	/**
	 * Return the bean instance that uniquely matches the given object type, if any.
	 * @param requiredType type the bean must match; can be an interface or superclass.
	 * {@literal null} is disallowed.
	 * <p>This method goes into {@link ListableBeanFactory} by-type lookup territory
	 * but may also be translated into a conventional by-name lookup based on the name
	 * of the given type. For more extensive retrieval operations across sets of beans,
	 * use {@link ListableBeanFactory} and/or {@link BeanFactoryUtils}.
	 * @return an instance of the single bean matching the required type
	 * @throws NoSuchBeanDefinitionException if there is not exactly one matching bean found
	 * @since 3.0
	 * @see ListableBeanFactory
	 */
	<T> T getBean(Class<T> requiredType) throws BeansException;

	/**
	 * Return an instance, which may be shared or independent, of the specified bean.
	 * <p>Allows for specifying explicit constructor arguments / factory method arguments,
	 * overriding the specified default arguments (if any) in the bean definition.
	 * @param name the name of the bean to retrieve
	 * @param args arguments to use if creating a prototype using explicit arguments to a
	 * static factory method. It is invalid to use a non-null args value in any other case.
	 * @return an instance of the bean
	 * @throws NoSuchBeanDefinitionException if there's no such bean definition
	 * @throws BeanDefinitionStoreException if arguments have been given but
	 * the affected bean isn't a prototype
	 * @throws BeansException if the bean could not be created
	 * @since 2.5
	 */
	Object getBean(String name, Object... args) throws BeansException;

	/**
	 * Does this bean factory contain a bean with the given name? More specifically,
	 * is {@link #getBean} able to obtain a bean instance for the given name?
	 * <p>Translates aliases back to the corresponding canonical bean name.
	 * Will ask the parent factory if the bean cannot be found in this factory instance.
	 * @param name the name of the bean to query
	 * @return whether a bean with the given name is defined
	 */
	boolean containsBean(String name);

	/**
	 * Is this bean a shared singleton? That is, will {@link #getBean} always
	 * return the same instance?
	 * <p>Note: This method returning <code>false</code> does not clearly indicate
	 * independent instances. It indicates non-singleton instances, which may correspond
	 * to a scoped bean as well. Use the {@link #isPrototype} operation to explicitly
	 * check for independent instances.
	 * <p>Translates aliases back to the corresponding canonical bean name.
	 * Will ask the parent factory if the bean cannot be found in this factory instance.
	 * @param name the name of the bean to query
	 * @return whether this bean corresponds to a singleton instance
	 * @throws NoSuchBeanDefinitionException if there is no bean with the given name
	 * @see #getBean
	 * @see #isPrototype
	 */
	boolean isSingleton(String name) throws NoSuchBeanDefinitionException;

	/**
	 * Is this bean a prototype? That is, will {@link #getBean} always return
	 * independent instances?
	 * <p>Note: This method returning <code>false</code> does not clearly indicate
	 * a singleton object. It indicates non-independent instances, which may correspond
	 * to a scoped bean as well. Use the {@link #isSingleton} operation to explicitly
	 * check for a shared singleton instance.
	 * <p>Translates aliases back to the corresponding canonical bean name.
	 * Will ask the parent factory if the bean cannot be found in this factory instance.
	 * @param name the name of the bean to query
	 * @return whether this bean will always deliver independent instances
	 * @throws NoSuchBeanDefinitionException if there is no bean with the given name
	 * @since 2.0.3
	 * @see #getBean
	 * @see #isSingleton
	 */
	boolean isPrototype(String name) throws NoSuchBeanDefinitionException;

	/**
	 * Check whether the bean with the given name matches the specified type.
	 * More specifically, check whether a {@link #getBean} call for the given name
	 * would return an object that is assignable to the specified target type.
	 * <p>Translates aliases back to the corresponding canonical bean name.
	 * Will ask the parent factory if the bean cannot be found in this factory instance.
	 * @param name the name of the bean to query
	 * @param targetType the type to match against
	 * @return <code>true</code> if the bean type matches,
	 * <code>false</code> if it doesn't match or cannot be determined yet
	 * @throws NoSuchBeanDefinitionException if there is no bean with the given name
	 * @since 2.0.1
	 * @see #getBean
	 * @see #getType
	 */
	boolean isTypeMatch(String name, Class targetType) throws NoSuchBeanDefinitionException;

	/**
	 * Determine the type of the bean with the given name. More specifically,
	 * determine the type of object that {@link #getBean} would return for the given name.
	 * <p>For a {@link FactoryBean}, return the type of object that the FactoryBean creates,
	 * as exposed by {@link FactoryBean#getObjectType()}.
	 * <p>Translates aliases back to the corresponding canonical bean name.
	 * Will ask the parent factory if the bean cannot be found in this factory instance.
	 * @param name the name of the bean to query
	 * @return the type of the bean, or <code>null</code> if not determinable
	 * @throws NoSuchBeanDefinitionException if there is no bean with the given name
	 * @since 1.1.2
	 * @see #getBean
	 * @see #isTypeMatch
	 */
	Class<?> getType(String name) throws NoSuchBeanDefinitionException;

	/**
	 * Return the aliases for the given bean name, if any.
	 * All of those aliases point to the same bean when used in a {@link #getBean} call.
	 * <p>If the given name is an alias, the corresponding original bean name
	 * and other aliases (if any) will be returned, with the original bean name
	 * being the first element in the array.
	 * <p>Will ask the parent factory if the bean cannot be found in this factory instance.
	 * @param name the bean name to check for aliases
	 * @return the aliases, or an empty array if none
	 * @see #getBean
	 */
	String[] getAliases(String name);

}

这个便是spring核心的Bean工厂定义，上面的author说是2001年写的，已经历史久远了，这个类是spring中所有bean工厂，也就是俗称的IOC容器的祖宗，各种IOC容器都只是它的实现或者为了满足特别需求的扩展实现，包括我们平时用的最多的ApplicationContext。从上面的方法就可以看出，这些工厂的实现最大的作用就是根据bean的名称亦或类型等等，来返回一个bean的实例。

一个工厂如果想拥有这样的功能，那么它一定需要以下几个因素：

1.需要持有各种bean的定义，否则无法正确的完成bean的实例化。

2.需要持有bean之间的依赖关系，否则在bean实例化的过程中也会出现问题。例如上例，如果我们只是各自持有Person和Company，却不知道他们的依赖关系，那么在Company初始化以后，调用open方法时，就会报空指针。这是因为Company其实并没有真正的被正确初始化。

3.以上两种都要依赖于我们所写的依赖关系的定义，暂且认为是XML文件（其实可以是各种各样的），那么我们需要一个工具来完成XML文件的读取。

我目前想到的，只需要满足以上三种条件，便可以创建一个bean工厂，来生产各种bean。当然，spring有更高级的做法，以上只是我们直观的去想如何实现IOC。

其实在上述过程中仍旧有一些问题，比如第一步，我们需要持有bean的定义，如何持有？这是一个问题。我们知道spring的XML配置文件中，有一个属性是lazy-init，这就说明，bean在何时实例化我们是可以控制的。但是这个属性默认为true，也就是说spring容器初始化以后，其实各种bean都还未产生，它们只在需要的时候出现。

所以我们无法直接的创建一个Map<String,Object>来持有这些bean的实例，在这里要注意，我们要储存的是bean的定义，而非实例。

那么接下来，又是一个祖宗级别的接口要出现了，来看BeanDefinition。

/*
 * Copyright 2002-2009 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;

/**
 * A BeanDefinition describes a bean instance, which has property values,
 * constructor argument values, and further information supplied by
 * concrete implementations.
 *
 * <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.
 *
 * @author Juergen Hoeller
 * @author Rob Harrop
 * @since 19.03.2004
 * @see ConfigurableListableBeanFactory#getBeanDefinition
 * @see org.springframework.beans.factory.support.RootBeanDefinition
 * @see org.springframework.beans.factory.support.ChildBeanDefinition
 */
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</code> 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</code> is a supporting
	 * part of some larger configuration, typically an outer
	 * {@link org.springframework.beans.factory.parsing.ComponentDefinition}.
	 * <code>SUPPORT</code> 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</code> 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 specifed factory bean, if any,
	 * or as static method on the local bean class else.
	 * @param factoryMethodName static factory method name,
	 * or <code>null</code> if normal constructor creation should be used
	 * @see #getBeanClassName()
	 */
	void setFactoryMethodName(String factoryMethodName);

	/**
	 * Return the name of the current target scope for this bean,
	 * or <code>null</code> if not known yet.
	 */
	String getScope();

	/**
	 * Override the target scope of this bean, specifying a new scope name.
	 * @see #SCOPE_SINGLETON
	 * @see #SCOPE_PROTOTYPE
	 */
	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</code>, 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</code>)
	 */
	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</code>)
	 */
	MutablePropertyValues getPropertyValues();


	/**
	 * 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.
	 * @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</code>. The role hint
	 * provides tools with an indication of the importance of a particular
	 * <code>BeanDefinition</code>.
	 * @see #ROLE_APPLICATION
	 * @see #ROLE_INFRASTRUCTURE
	 * @see #ROLE_SUPPORT
	 */
	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</code> 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();

}

顾名思义，这个便是spring中的bean定义接口，所以其实我们工厂里持有的bean定义，就是一堆这个玩意，或者是他的实现类和子接口。这个接口并非直接的祖宗接口，他所继承的两个接口一个是core下面的AttributeAccessor，继承这个接口就以为这我们的bean定义接口同样具有处理属性的能力，而另外一个是beans下面的BeanMetadataElement，字面翻译这个接口就是bean的元数据元素，它可以获得bean的配置定义的一个元素。在XML文件中来说，就是会持有一个bean标签。

仔细观看，能发现beanDefinition中有两个方法分别是String[] getDependsOn()和void setDependsOn(String[] dependsOn)，这两个方法就是获取依赖的beanName和设置依赖的beanName，这样就好办了，只要我们有一个BeanDefinition，就可以完全的产生一个完整的bean实例。

今天就先到这里吧，我也是边看边写的，与其说跟我一起学，其实这个系列的应该叫源码学习笔记，更多的还是看到哪里，写到哪里，谈不上跟我一起学。

如果文中有不周到的地方，希望各位尽管指出。