Spring Framework5.x

BeanDefinition对象

Bean定义主要对象包括如下几个属性：

Property	描述
Class	bean实例
Name	Bean名称
Scope	Bean类型（单例、原型..)
构造函数参数	Dependency Injection
Properties	Dependency Injection
Autowirging mode	自动注入模型
Lazy initizlization	懒加载模型
Iniitialization method	初始化方法
Destruction method	销毁方法

bean定义所有属性大概有上百个。就以最常用的RootBeanDefinition为例，这个类是bean定义最核心的实现类。类中定义非常多属性，这些属性都是bean的定义信息。

/** Map with String keys and Object values. */
private final Map<String, Object> attributes = new LinkedHashMap<>();

@Nullable
private Object source;

/**
     * Constant for the default scope name: {@code ""}, equivalent to singleton
     * status unless overridden from a parent bean definition (if applicable).
     * 默认bean作用域
     */
public static final String SCOPE_DEFAULT = "";

/**
     * Constant that indicates no external autowiring at all.
     * 自动注入模式
     * @see #setAutowireMode
     */
public static final int AUTOWIRE_NO = AutowireCapableBeanFactory.AUTOWIRE_NO;

/**
     * Constant that indicates autowiring bean properties by name.
     * 自动注入通过bean名字
     * @see #setAutowireMode
     */
public static final int AUTOWIRE_BY_NAME = AutowireCapableBeanFactory.AUTOWIRE_BY_NAME;

/**
     * Constant that indicates autowiring bean properties by type.
     * 自动注入通过bean类型
     * @see #setAutowireMode
     */
public static final int AUTOWIRE_BY_TYPE = AutowireCapableBeanFactory.AUTOWIRE_BY_TYPE;

/**
     * Constant that indicates autowiring a constructor.
     * 自动注入通过构造函数
     * @see #setAutowireMode
     */
public static final int AUTOWIRE_CONSTRUCTOR = AutowireCapableBeanFactory.AUTOWIRE_CONSTRUCTOR;

/**
     * Constant that indicates determining an appropriate autowire strategy
     * through introspection of the bean class.
     * @see #setAutowireMode
     * @deprecated as of Spring 3.0: If you are using mixed autowiring strategies,
     * use annotation-based autowiring for clearer demarcation of autowiring needs.
     */
@Deprecated
public static final int AUTOWIRE_AUTODETECT = AutowireCapableBeanFactory.AUTOWIRE_AUTODETECT;

/**
     * Constant that indicates no dependency check at all.
     * @see #setDependencyCheck
     */
public static final int DEPENDENCY_CHECK_NONE = 0;

/**
     * Constant that indicates dependency checking for object references.
     * @see #setDependencyCheck
     */
public static final int DEPENDENCY_CHECK_OBJECTS = 1;

/**
     * Constant that indicates dependency checking for "simple" properties.
     * @see #setDependencyCheck
     * @see org.springframework.beans.BeanUtils#isSimpleProperty
     */
public static final int DEPENDENCY_CHECK_SIMPLE = 2;

/**
     * Constant that indicates dependency checking for all properties
     * (object references as well as "simple" properties).
     * @see #setDependencyCheck
     */
public static final int DEPENDENCY_CHECK_ALL = 3;

/**
     * Constant that indicates the container should attempt to infer the
     * {@link #setDestroyMethodName destroy method name} for a bean as opposed to
     * explicit specification of a method name. The value {@value} is specifically
     * designed to include characters otherwise illegal in a method name, ensuring
     * no possibility of collisions with legitimately named methods having the same
     * name.
     * <p>Currently, the method names detected during destroy method inference
     * are "close" and "shutdown", if present on the specific bean class.
     */
public static final String INFER_METHOD = "(inferred)";


@Nullable
private volatile Object beanClass;

@Nullable
private String scope = SCOPE_DEFAULT;

private boolean abstractFlag = false;

@Nullable
private Boolean lazyInit;

private int autowireMode = AUTOWIRE_NO;

private int dependencyCheck = DEPENDENCY_CHECK_NONE;

@Nullable
private String[] dependsOn;

private boolean autowireCandidate = true;

private boolean primary = false;

private final Map<String, AutowireCandidateQualifier> qualifiers = new LinkedHashMap<>();

@Nullable
private Supplier<?> instanceSupplier;

private boolean nonPublicAccessAllowed = true;

private boolean lenientConstructorResolution = true;

@Nullable
private String factoryBeanName;

@Nullable
private String factoryMethodName;

@Nullable
private ConstructorArgumentValues constructorArgumentValues;

@Nullable
private MutablePropertyValues propertyValues;

private MethodOverrides methodOverrides = new MethodOverrides();

@Nullable
private String initMethodName;

@Nullable
private String destroyMethodName;

private boolean enforceInitMethod = true;

private boolean enforceDestroyMethod = true;

private boolean synthetic = false;

private int role = BeanDefinition.ROLE_APPLICATION;

@Nullable
private String description;

@Nullable
private Resource resource;

@Nullable
private BeanDefinitionHolder decoratedDefinition;

@Nullable
private AnnotatedElement qualifiedElement;

/** Determines if the definition needs to be re-merged. */
volatile boolean stale;

boolean allowCaching = true;

boolean isFactoryMethodUnique = false;

@Nullable
volatile ResolvableType targetType;

/** Package-visible field for caching the determined Class of a given bean definition. */
@Nullable
volatile Class<?> resolvedTargetType;

/** Package-visible field for caching if the bean is a factory bean. */
@Nullable
volatile Boolean isFactoryBean;

/** Package-visible field for caching the return type of a generically typed factory method. */
@Nullable
volatile ResolvableType factoryMethodReturnType;

/** Package-visible field for caching a unique factory method candidate for introspection. */
@Nullable
volatile Method factoryMethodToIntrospect;

/** Common lock for the four constructor fields below. */
final Object constructorArgumentLock = new Object();

/** Package-visible field for caching the resolved constructor or factory method. */
@Nullable
Executable resolvedConstructorOrFactoryMethod;

/** Package-visible field that marks the constructor arguments as resolved. */
boolean constructorArgumentsResolved = false;

/** Package-visible field for caching fully resolved constructor arguments. */
@Nullable
Object[] resolvedConstructorArguments;

/** Package-visible field for caching partly prepared constructor arguments. */
@Nullable
Object[] preparedConstructorArguments;

/** Common lock for the two post-processing fields below. */
final Object postProcessingLock = new Object();

/** Package-visible field that indicates MergedBeanDefinitionPostProcessor having been applied. */
boolean postProcessed = false;

/** Package-visible field that indicates a before-instantiation post-processor having kicked in. */
@Nullable
volatile Boolean beforeInstantiationResolved;

@Nullable
private Set<Member> externallyManagedConfigMembers;

@Nullable
private Set<String> externallyManagedInitMethods;

@Nullable
private Set<String> externallyManagedDestroyMethods;

Spring IoC体系结构

BeanFactory

Spring Bean的创建是典型的工厂模式，这一系列的Bean工厂也就是IoC容器为开发者管理对象间的依赖关系提供了很多的便利和基础服务，在Spring中有许多的实现提供用户选择和使用。最核心的类结构如下：

graph BT

ListableBeanFactory --> BeanFactory
ConfigurableListableBeanFactory --> ListableBeanFactory
DefaultListableBeanFactory -.-> ConfigurableListableBeanFactory

HierarchicalBeanFactory --> BeanFactory
ConfigurableBeanFactory --> HierarchicalBeanFactory
ConfigurableListableBeanFactory --> ConfigurableBeanFactory

AutowireCapableBeanFactory --> BeanFactory
ConfigurableListableBeanFactory --> AutowireCapableBeanFactory

AbstractAutowireCapableBeanFactory -.-> ConfigurableBeanFactory
DefaultListableBeanFactory --> AbstractAutowireCapableBeanFactory

其中BeanFactory作为顶层的接口，它定义了IoC容器的基本功能实现，比如getBean()方法就是这个接口定义的。
它有三个子类（或者是实现类），它们是：ListableBeanFactory、HierarchicalBeanFactory和AutowireCapableBeanFactory。但是从上图可以看到最终的默认实现类是DefaultListableBeanFactory。它实现了所有的接口，这样做的目的是什么呢？
查阅相关的资料可以发现，这些几个接口的实现都是有特定的使用场景的，它们主要是在Spring内部操作对象的传递和转换的过程中，对对象的数据访问所做的限制。比如ListableBeanFactory接口表示这些bean是可列表的；HierarchicalBeanFactory表示这些bean是有继承关系的，也就是说这些bean是可能有父类bean；AutowireCapableBeanFactory接口定义了bean的自动装配规则。这四个接口共同定义了Bean的集合，Bean之间的关系，Bean的行为，从而组成了IoC容器的基本结构。

public interface BeanFactory {
    /**
     * 对FactoryBean的转义定义，如果使用beanName获取到的是FactoryBean接口中getObject()方法返回的实例
     * 如果要拿到FactoryBean本身则需要加上&前缀，applicationContext.getBean(&beanName)
     */
    String FACTORY_BEAN_PREFIX = "&";
    /**
     * 根据beanName获取容器中实例化好的bean
     */
    Object getBean(String name) throws BeansException;
    /**
     * 根据bean的名字和Class类型来得到bean实例，增加了类型安全验证机制。
     */
    <T> T getBean(String name, Class<T> requiredType) throws BeansException;
    /**
     * 获取bean实例时可以传递构造方法的参数，args就是构造方法的参数。
     */
    Object getBean(String name, Object... args) throws BeansException;
    /**
     * 根据bean的类型获取bean实例。比如getBean(User.class)
     */
    <T> T getBean(Class<T> requiredType) throws BeansException;
    /**
     * 根据bean的类型和构造函数的参数获取bean实例。比如getBean(User.class, age, name)
     */
    <T> T getBean(Class<T> requiredType, Object... args) throws BeansException;
    /**
     * 获取一个提供器类型的bean，通常是Spring框架内部使用。
     */
    <T> ObjectProvider<T> getBeanProvider(Class<T> requiredType);

    <T> ObjectProvider<T> getBeanProvider(ResolvableType requiredType);
    /**
     * 提供对bean的检索，看看是否在IOC容器有这个名字的bean
     */
    boolean containsBean(String name);

    boolean isSingleton(String name) throws NoSuchBeanDefinitionException;

    boolean isPrototype(String name) throws NoSuchBeanDefinitionException;

    boolean isTypeMatch(String name, Class<?> typeToMatch) throws NoSuchBeanDefinitionException;
    /**
     * 得到bean实例的Class类型
     */
    @Nullable
    Class<?> getType(String name) throws NoSuchBeanDefinitionException;

    @Nullable
    Class<?> getType(String name, boolean allowFactoryBeanInit) throws NoSuchBeanDefinitionException;
    /**
     * 得到bean的别名，如果根据别名检索，那么其原名也会被检索出来    
     */
    String[] getAliases(String name);
}

FactoryBean只定义了IoC容器的基本行为，它并不关心bean是如何定义，怎么加载的。
而具体是定义和加载是在其他的实现类中处理的，比如XmlBeanFactory、ClasspathXmlApplicationContext等。
XmlBeanFactory是容器的最基本实现（但是目前这个类已经声明为过时），其他的加载类都是在此基础上的扩展。

BeanDefinition（bean定义信息，IoC核心数据结构）

Spring容器管理了我们定义的各种bean对象及其相互关系，bean在对象实现是以BeanDefinition类描述的。

graph BT

AttributeAccessorSupport -.-> AttributeAccessor
BeanMatadataAttributeAccessor --> AttributeAccessorSupport
BeanMatadataAttributeAccessor -.-> BeanMatadataElement
AbstractBeanDefinition --> BeanMatadataAttributeAccessor
RootBeanDefinition --> AbstractBeanDefinition
GenericBeanDefinition --> AbstractBeanDefinition
ChildBeanDefinition --> AbstractBeanDefinition

BeanDefinition --> AttributeAccessor
BeanDefinition --> BeanMatadataElement
AbstractBeanDefinition -.-> BeanDefinition

Bean的解析过程非常复杂，功能被分的很细，因为这里需要被扩展的地方非常多，也提供了很多接口给开发者用于修改bean的定义。保证了足够的灵活性，以应对变化。Bean的解析主要是对Spring配置文件或者配置类的解析，解析过程如下：

graph BT
XmlBeanDefinitionReader --> AbstractBeanDefinitionReader
AbstractBeanDefinitionReader -.-> BeanDefinitionReader

XmlBeanDefinitionReader -.-> DefaultBeanDefinitionDocumentReader
DefaultBeanDefinitionDocumentReader -.-> BeanDefinitionDocumentReader

XmlBeanDefinitionReader -.-> BeanDefinitionDocumentReader

IoC 容器初始化、bean实例化（核心内容）

这部分是非常非常复杂，在另外一篇《Spring IoC原理》中详细介绍。