核心接口

BeanFactory是Spring最核心接口，它是Bean的容器，所有Bean的创建和注入几乎都与它有关。很多后缀是“Context”的类都继承或扩展了它，比如ClassPathXmlApplicationContext。最核心的方法是getBean，当然getBean有很多种重载形式，这里不一一列出。

public interface BeanFactory{

   Object getBean(String name) throws BeansException;

    <T> T getBean(String name, Class<T> requiredType) throws BeansException;

    <T> T getBean(Class<T> requiredType) throws BeansException;

}

Bean的属性定义

BeanDefinition是Bean的配置信息。无论何种XML还是JavaCode还是其他任何形式定义的Bean属性，Spring都会把他们解析成一个BeanDefinition。每一个Bean对应一个BeanDefinition。后续Bean的创建都依赖于BeanDefinition，因为需要从BeanDefinition获取创建时需要的必要属性。

public interface BeanDefinition extends AttributeAccessor,BeanMetadataElement{ String getScope();

    ConstructorArgumentValues getConstructorArgumentValues();

    MutablePropertyValues getPropertyValues();

    boolean isSingleton();

    boolean isPrototype();

    ...

}

Bean定义的注册中心

Spring把解析后Bean配置BeanDefinition注册到BeanDefinitionRegistry。可以想象它为一个map，可以随时根据Bean的名字取出BeanDefinition。

public interface BeanDefinitionRegistry extends AliasRegistry{ //Bean的注册中心 void registerBeanDefinition(String beanName, BeanDefinition beanDefinition)

            throws BeanDefinitionStoreException;

    BeanDefinition getBeanDefinition(String beanName) throws NoSuchBeanDefinitionException;

    ...

}

单例Bean的注册中心

Spring默认Bean的创建方式为单例模式，这也是我们最常用的模式。SingletonBeanRegistry是单例Bean注册中心，其实就是一个单例Bean的容器。Spring把创建好的单例Bean缓存起来，提高性能。DefaultSingletonBeanRegistry是SingletonBeanRegistry默认实现方法。从下面的源码可以看出，大都是Map和Set的集合，缓存了Bean的名字和Bean的实例。

public class DefaultSingletonBeanRegistry extends SimpleAliasRegistry 
 implements SingletonBeanRegistry{   
 /** Cache of singleton objects: bean name --> bean instance */
 private final Map<String, Object> singletonObjects = new 
 ConcurrentHashMap<String, Object>(64);

    /** Cache of singleton factories: bean name --> ObjectFactory */    private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<String, ObjectFactory<?>>(16);

    /** Cache of early singleton objects: bean name --> bean instance */    private final Map<String, Object> earlySingletonObjects = new HashMap<String, Object>(16);

    /** Set of registered singletons, containing the bean names in registration order */    private final Set<String> registeredSingletons = new LinkedHashSet<String>(64);

    /** Names of beans that are currently in creation */    private final Set<String> singletonsCurrentlyInCreation =

            Collections.newSetFromMap(new ConcurrentHashMap<String, Boolean>(16));

    /** Names of beans currently excluded from in creation checks */    private final Set<String> inCreationCheckExclusions =

            Collections.newSetFromMap(new ConcurrentHashMap<String, Boolean>(16));

    /** List of suppressed Exceptions, available for associating related causes */    private Set<Exception> suppressedExceptions;

    /** Flag that indicates whether we're currently within destroySingletons */    private boolean singletonsCurrentlyInDestruction = false;

    /** Disposable bean instances: bean name --> disposable instance */    private final Map<String, Object> disposableBeans = new LinkedHashMap<String, Object>();

    /** Map between containing bean names: bean name --> Set of bean names that the bean contains */    private final Map<String, Set<String>> containedBeanMap = new ConcurrentHashMap<String, Set<String>>(16);

    /** Map between dependent bean names: bean name --> Set of dependent bean names */    private final Map<String, Set<String>> dependentBeanMap = new ConcurrentHashMap<String, Set<String>>(64);

    /** Map between depending bean names: bean name --> Set of bean names for the bean's dependencies */    private final Map<String, Set<String>> dependenciesForBeanMap = new ConcurrentHashMap<String, Set<String>>(64);

}

Bean的生成

DefaultListableBeanFactory是Spring的默认容器。核心方法是getBean方法，最终调用的都是AbstractBeanFactory的doGetBean方法。

首先是从缓存获取Bean，如果缓存存在并且Bean是FactoryBean的实例，走FactoryBean获取Bean的逻辑，否则直接返回缓存中的Bean。