Spring IoC是Spring Framework的核心。
控制反轉和依賴注入：
早在2004年，Martin Fowler就提出了“哪些方面的控制被反轉了？”，後來得出的結論是：依賴對象的獲得被反轉了。基于這個結論，他為控制反轉創造了一個更好的名字：依賴注入。
不過我們平時會同時說到控制反轉和依賴注入兩個概念，控制反轉指的就是把對象的獲得交給容器，這樣就反轉了。依賴注入指的是容器把對象之間的依賴注入進去。
控制反轉的實作有多種方式，可以在對象生成或初始化的時候，直接将資料注入到對象中。也可以把對象的引用注入進去。
Spring IoC：
Spring IoC容器的設計，最關鍵的就是兩個接口，BeanFactory和ApplicationContext。
BeanFactory接口定義了IoC容器基本的功能規範，ApplicationContext在BeanFactory的基礎上，增加了許多面向架構的特性，對應用環境做了很多适配，是容器的進階形态。
在Idea中，通過control+H快捷鍵，可以看到BeanFactory的繼承情況。
網上也可以搜到Spring IoC容器的接口設計圖。
在Spring IoC容器中，有兩條線路，一條是BeanFactory，一條是ApplicationContext。BeanFactory這條線是簡單的IoC容器，ApplicationContext繼承了ListableBeanFactory，具備了簡單IoC容器的功能，同時又繼承了MessageSource、ResourcePatternResolver、ApplicationEventPublisher等接口，擁有了更進階的IoC容器特性。
下面先看下BeanFactory這條線：
BeanFactory算是IoC容器中最頂級的一個接口了，它定義了IoC容器最基本的形式，提供了最基本的功能。比如，接口中定義了getBean方法，用來擷取IoC容器中的一個Bean。
下面是BeanFactory的源碼：
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} is a FactoryBean, getting {@code &myJndiObject}
* 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} for any match. For example, if the value
* is {@code Object.class}, this method will succeed whatever the class of the
* returned instance.
* @return an instance of the bean
* @throws NoSuchBeanDefinitionException if there is 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.
* {@code 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 no bean of the given type was found
* @throws NoUniqueBeanDefinitionException if more than one bean of the given type was found
* @throws BeansException if the bean could not be created
* @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 when creating a bean instance using explicit arguments
* (only applied when creating a new instance as opposed to retrieving an existing one)
* @return an instance of the bean
* @throws NoSuchBeanDefinitionException if there is 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;
/**
* 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 requiredType type the bean must match; can be an interface or superclass.
* {@code 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}.
* @param args arguments to use when creating a bean instance using explicit arguments
* (only applied when creating a new instance as opposed to retrieving an existing one)
* @return an instance of the bean
* @throws NoSuchBeanDefinitionException if there is 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 4.1
*/
<T> T getBean(Class<T> requiredType, Object... args) throws BeansException;
/**
* Does this bean factory contain a bean definition or externally registered singleton
* instance with the given name?
* <p>If the given name is an alias, it will be translated back to the corresponding
* canonical bean name.
* <p>If this factory is hierarchical, will ask any parent factory if the bean cannot
* be found in this factory instance.
* <p>If a bean definition or singleton instance matching the given name is found,
* this method will return {@code true} whether the named bean definition is concrete
* or abstract, lazy or eager, in scope or not. Therefore, note that a {@code true}
* return value from this method does not necessarily indicate that {@link #getBean}
* will be able to obtain an instance for the same name.
* @param name the name of the bean to query
* @return whether a bean with the given name is present
*/
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} 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} 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 typeToMatch the type to match against (as a {@code ResolvableType})
* @return {@code true} if the bean type matches,
* {@code false} if it doesn't match or cannot be determined yet
* @throws NoSuchBeanDefinitionException if there is no bean with the given name
* @since 4.2
* @see #getBean
* @see #getType
*/
boolean isTypeMatch(String name, ResolvableType typeToMatch) 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 typeToMatch the type to match against (as a {@code Class})
* @return {@code true} if the bean type matches,
* {@code false} 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<?> typeToMatch) 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} 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);
}
實作了BeanFactory接口的類，我們最常見的是XmlBeanFactory：
@Deprecated
@SuppressWarnings({"serial", "all"})
public class XmlBeanFactory extends DefaultListableBeanFactory {
private final XmlBeanDefinitionReader reader = new XmlBeanDefinitionReader(this);
/**
* Create a new XmlBeanFactory with the given resource,
* which must be parsable using DOM.
* @param resource XML resource to load bean definitions from
* @throws BeansException in case of loading or parsing errors
*/
public XmlBeanFactory(Resource resource) throws BeansException {
this(resource, null);
}
/**
* Create a new XmlBeanFactory with the given input stream,
* which must be parsable using DOM.
* @param resource XML resource to load bean definitions from
* @param parentBeanFactory parent bean factory
* @throws BeansException in case of loading or parsing errors
*/
public XmlBeanFactory(Resource resource, BeanFactory parentBeanFactory) throws BeansException {
super(parentBeanFactory);
this.reader.loadBeanDefinitions(resource);
}
}
從代碼中可以看到，XmlBeanFactory類繼承了DefaultListableBeanFactory ，而DefaultListableBeanFactory類可以看成一個預設的功能完整的IoC容器。XmlBeanFactory在繼承了DefaultListableBeanFactory的基礎上，定義了一個XmlBeanDefinitionReader，然後通過reader.loadBeanDefinitions方法，來加載xml檔案中定義的Bean。可以看到XmlBeanFactory的構造函數需要傳入一個Resource類型的參數，這個參數就是指定我們xml檔案的來源的。Resource是一個接口，我們可以傳入具體的某個實作類來指定xml檔案路徑。
接下來再看下ApplicationContext：
ApplicationContext是一個進階形态的IoC容器，和BeanFactory相比，由于ApplicationContext繼承了MessageSource、ResourcePatternResolver、ApplicationEventPublisher等接口，使得ApplicationContext可以擁有BeanFactory不具備的特性，比如支援不同的資訊源，通路資源，支援應用事件，提供附加服務，是以在開發應用中，推薦使用ApplicationContext作為IoC容器的基本形式。
public interface ApplicationContext extends EnvironmentCapable, ListableBeanFactory, HierarchicalBeanFactory,
MessageSource, ApplicationEventPublisher, ResourcePatternResolver {
/**
* Return the unique id of this application context.
* @return the unique id of the context, or {@code null} if none
*/
String getId();
/**
* Return a name for the deployed application that this context belongs to.
* @return a name for the deployed application, or the empty String by default
*/
String getApplicationName();
/**
* Return a friendly name for this context.
* @return a display name for this context (never {@code null})
*/
String getDisplayName();
/**
* Return the timestamp when this context was first loaded.
* @return the timestamp (ms) when this context was first loaded
*/
long getStartupDate();
/**
* Return the parent context, or {@code null} if there is no parent
* and this is the root of the context hierarchy.
* @return the parent context, or {@code null} if there is no parent
*/
ApplicationContext getParent();
/**
* Expose AutowireCapableBeanFactory functionality for this context.
* <p>This is not typically used by application code, except for the purpose of
* initializing bean instances that live outside of the application context,
* applying the Spring bean lifecycle (fully or partly) to them.
* <p>Alternatively, the internal BeanFactory exposed by the
* {@link ConfigurableApplicationContext} interface offers access to the
* {@link AutowireCapableBeanFactory} interface too. The present method mainly
* serves as a convenient, specific facility on the ApplicationContext interface.
* <p><b>NOTE: As of 4.2, this method will consistently throw IllegalStateException
* after the application context has been closed.</b> In current Spring Framework
* versions, only refreshable application contexts behave that way; as of 4.2,
* all application context implementations will be required to comply.
* @return the AutowireCapableBeanFactory for this context
* @throws IllegalStateException if the context does not support the
* {@link AutowireCapableBeanFactory} interface, or does not hold an
* autowire-capable bean factory yet (e.g. if {@code refresh()} has
* never been called), or if the context has been closed already
* @see ConfigurableApplicationContext#refresh()
* @see ConfigurableApplicationContext#getBeanFactory()
*/
AutowireCapableBeanFactory getAutowireCapableBeanFactory() throws IllegalStateException;
}
實作了ApplicationContext的類中，比較常見的是FileSystemXmlApplicationContext：
FileSystemXmlApplicationContext繼承了AbstractXmlApplicationContext，主要的功能都是在AbstractXmlApplicationContext中實作的，可以看到在FileSystemXmlApplicationContext的構造方法中，有這麼一個方法：
/**
* Create a new FileSystemXmlApplicationContext with the given parent,
* loading the definitions from the given XML files.
* @param configLocations array of file paths
* @param refresh whether to automatically refresh the context,
* loading all bean definitions and creating all singletons.
* Alternatively, call refresh manually after further configuring the context.
* @param parent the parent context
* @throws BeansException if context creation failed
* @see #refresh()
*/
public FileSystemXmlApplicationContext(String[] configLocations, boolean refresh, ApplicationContext parent)
throws BeansException {
super(parent);
setConfigLocations(configLocations);
if (refresh) {
refresh();
}
}
如果有人想直接使用FileSystemXmlApplicationContext的話，這個構造方法就會調用refresh()，來啟動IoC容器。
IoC容器的初始化過程分為三個階段：
1.Resource定位
2.BeanDefinition載入
3.在IoC容器中注冊BeanDefinition
Resource的定位指的是我們定義的那些Bean的配置檔案，比如定義在classpath下的一些xml。IoC容器首先得能夠定位到這些檔案，才能從中讀取配置。
BeanDefinition的載入，就是把我們定義的Bean，轉換成IoC容器内部的資料結構（因為作為一個容器來說，肯定不會和我們業務自己定義的那些Bean耦合，容器會自己定義一種類型，可以用來表示所有的Bean），也就是BeanDefinition的類型。通過這個BeanDefinition，IoC容器就可以友善的進行管理。
IoC容器注冊BeanDefinition：把Bean解析成BeanDefinition之後，還要向IoC容器注冊，這樣IoC收到了這個注冊資訊，就會把這個BeanDefinition維護起來（通過HashMap方式），這樣在使用的時候，就可以直接找到這個BeanDefinition了。
這裡先對IoC容器進行了大體的介紹，介紹了BeanFactory和ApplicationContext這兩條線路，并且說明了IoC容器初始化的過程，接下來會對初始化過程的這三個階段詳細的分析。
參考資料：
1.《Spring技術内幕》 計文柯 著