Spring 核心之 IOC 容器学习二

基于 Annotation 的 IOC 初始化

Annotation 的前世今生

从 Spring2.0 以后的版本中，Spring 也引入了基于注解(Annotation)方式的配置，注解(Annotation)是 JDK1.5 中引入的一个新特性，用于简化 Bean 的配置，可以取代 XML 配置文件。开发人员对注解(Annotation)的态度也是萝卜青菜各有所爱，个人认为注解可以大大简化配置，提高开发速度，但也给后期维护增加了难度。目前来说 XML 方式发展的相对成熟，方便于统一管理。随着 Spring Boot 的兴起，基于注解的开发甚至实现了零配置。但作为个人的习惯而言，还是倾向于 XML 配置文件和注解(Annotation)相互配合使用。Spring IOC 容器对于类级别的注解和类内部的注解分以下两种处理策略：
1)、类级别的注解：如@Component、@Repository、@Controller、@Service 以及 JavaEE6 的@ManagedBean 和@Named 注解，都是添加在类上面的类级别注解，Spring 容器根据注解的过滤规则扫描读取注解 Bean 定义类，并将其注册到 Spring IOC 容器中。
2)、类内部的注解：如@Autowire、@Value、@Resource 以及 EJB 和 WebService 相关的注解等，都是添加在类内部的字段或者方法上的类内部注解，SpringIOC 容器通过 Bean 后置注解处理器解析Bean 内部的注解。下面将根据这两种处理策略，分别分析 Spring 处理注解相关的源码。

定位 Bean 扫描路径

在 Spring 中 管 理 注 解 Bean 定 义 的 容 器 有 两 个 ： AnnotationConfigApplicationContext 和AnnotationConfigWebApplicationContex。这两个类是专门处理 Spring 注解方式配置的容器，直接依赖于注解作为容器配置信息来源的 IOC 容器。AnnotationConfigWebApplicationContext 是AnnotationConfigApplicationContext 的 Web 版本，两者的用法以及对注解的处理方式几乎没有差别。现在我们以 AnnotationConfigApplicationContext 为例看看它的源码：

public class AnnotationConfigApplicationContext extends GenericApplicationContext implements AnnotationConfigRegistry {

	//保存一个读取注解的Bean定义读取器，并将其设置到容器中
	private final AnnotatedBeanDefinitionReader reader;

	//保存一个扫描指定类路径中注解Bean定义的扫描器，并将其设置到容器中
	private final ClassPathBeanDefinitionScanner scanner;


	/**
	 * Create a new AnnotationConfigApplicationContext that needs to be populated
	 * through {@link #register} calls and then manually {@linkplain #refresh refreshed}.
	 */
	//默认构造函数，初始化一个空容器，容器不包含任何 Bean 信息，需要在稍后通过调用其register()
	//方法注册配置类，并调用refresh()方法刷新容器，触发容器对注解Bean的载入、解析和注册过程
	public AnnotationConfigApplicationContext() {
		this.reader = new AnnotatedBeanDefinitionReader(this);
		this.scanner = new ClassPathBeanDefinitionScanner(this);
	}

	/**
	 * Create a new AnnotationConfigApplicationContext with the given DefaultListableBeanFactory.
	 * @param beanFactory the DefaultListableBeanFactory instance to use for this context
	 */
	public AnnotationConfigApplicationContext(DefaultListableBeanFactory beanFactory) {
		super(beanFactory);
		this.reader = new AnnotatedBeanDefinitionReader(this);
		this.scanner = new ClassPathBeanDefinitionScanner(this);
	}

	/**
	 * Create a new AnnotationConfigApplicationContext, deriving bean definitions
	 * from the given annotated classes and automatically refreshing the context.
	 * @param annotatedClasses one or more annotated classes,
	 * e.g. {@link Configuration @Configuration} classes
	 */
	//最常用的构造函数，通过将涉及到的配置类传递给该构造函数，以实现将相应配置类中的Bean自动注册到容器中
	public AnnotationConfigApplicationContext(Class<?>... annotatedClasses) {
		this();
		register(annotatedClasses);
		refresh();
	}

	/**
	 * Create a new AnnotationConfigApplicationContext, scanning for bean definitions
	 * in the given packages and automatically refreshing the context.
	 * @param basePackages the packages to check for annotated classes
	 */
	//该构造函数会自动扫描以给定的包及其子包下的所有类，并自动识别所有的Spring Bean，将其注册到容器中
	public AnnotationConfigApplicationContext(String... basePackages) {
		this();
		scan(basePackages);
		refresh();
	}


	/**
	 * {@inheritDoc}
	 * <p>Delegates given environment to underlying {@link AnnotatedBeanDefinitionReader}
	 * and {@link ClassPathBeanDefinitionScanner} members.
	 */
	@Override
	public void setEnvironment(ConfigurableEnvironment environment) {
		super.setEnvironment(environment);
		this.reader.setEnvironment(environment);
		this.scanner.setEnvironment(environment);
	}

	/**
	 * Provide a custom {@link BeanNameGenerator} for use with {@link AnnotatedBeanDefinitionReader}
	 * and/or {@link ClassPathBeanDefinitionScanner}, if any.
	 * <p>Default is {@link org.springframework.context.annotation.AnnotationBeanNameGenerator}.
	 * <p>Any call to this method must occur prior to calls to {@link #register(Class...)}
	 * and/or {@link #scan(String...)}.
	 * @see AnnotatedBeanDefinitionReader#setBeanNameGenerator
	 * @see ClassPathBeanDefinitionScanner#setBeanNameGenerator
	 */
	//为容器的注解Bean读取器和注解Bean扫描器设置Bean名称产生器
	public void setBeanNameGenerator(BeanNameGenerator beanNameGenerator) {
		this.reader.setBeanNameGenerator(beanNameGenerator);
		this.scanner.setBeanNameGenerator(beanNameGenerator);
		getBeanFactory().registerSingleton(
				AnnotationConfigUtils.CONFIGURATION_BEAN_NAME_GENERATOR, beanNameGenerator);
	}

	/**
	 * Set the {@link ScopeMetadataResolver} to use for detected bean classes.
	 * <p>The default is an {@link AnnotationScopeMetadataResolver}.
	 * <p>Any call to this method must occur prior to calls to {@link #register(Class...)}
	 * and/or {@link #scan(String...)}.
	 */
	//为容器的注解Bean读取器和注解Bean扫描器设置作用范围元信息解析器
	public void setScopeMetadataResolver(ScopeMetadataResolver scopeMetadataResolver) {
		this.reader.setScopeMetadataResolver(scopeMetadataResolver);
		this.scanner.setScopeMetadataResolver(scopeMetadataResolver);
	}


	//---------------------------------------------------------------------
	// Implementation of AnnotationConfigRegistry
	//---------------------------------------------------------------------

	/**
	 * Register one or more annotated classes to be processed.
	 * <p>Note that {@link #refresh()} must be called in order for the context
	 * to fully process the new classes.
	 * @param annotatedClasses one or more annotated classes,
	 * e.g. {@link Configuration @Configuration} classes
	 * @see #scan(String...)
	 * @see #refresh()
	 */
	//为容器注册一个要被处理的注解Bean，新注册的Bean，必须手动调用容器的
	//refresh()方法刷新容器，触发容器对新注册的Bean的处理
	public void register(Class<?>... annotatedClasses) {
		Assert.notEmpty(annotatedClasses, "At least one annotated class must be specified");
		this.reader.register(annotatedClasses);
	}

	/**
	 * Perform a scan within the specified base packages.
	 * <p>Note that {@link #refresh()} must be called in order for the context
	 * to fully process the new classes.
	 * @param basePackages the packages to check for annotated classes
	 * @see #register(Class...)
	 * @see #refresh()
	 */
	//扫描指定包路径及其子包下的注解类，为了使新添加的类被处理，必须手动调用
	//refresh()方法刷新容器
	public void scan(String... basePackages) {
		Assert.notEmpty(basePackages, "At least one base package must be specified");
		this.scanner.scan(basePackages);
	}


	//---------------------------------------------------------------------
	// Convenient methods for registering individual beans
	//---------------------------------------------------------------------

	/**
	 * Register a bean from the given bean class, deriving its metadata from
	 * class-declared annotations, and optionally providing explicit constructor
	 * arguments for consideration in the autowiring process.
	 * <p>The bean name will be generated according to annotated component rules.
	 * @param annotatedClass the class of the bean
	 * @param constructorArguments argument values to be fed into Spring's
	 * constructor resolution algorithm, resolving either all arguments or just
	 * specific ones, with the rest to be resolved through regular autowiring
	 * (may be {@code null} or empty)
	 * @since 5.0
	 */
	public <T> void registerBean(Class<T> annotatedClass, Object... constructorArguments) {
		registerBean(null, annotatedClass, constructorArguments);
	}

	/**
	 * Register a bean from the given bean class, deriving its metadata from
	 * class-declared annotations, and optionally providing explicit constructor
	 * arguments for consideration in the autowiring process.
	 * @param beanName the name of the bean (may be {@code null})
	 * @param annotatedClass the class of the bean
	 * @param constructorArguments argument values to be fed into Spring's
	 * constructor resolution algorithm, resolving either all arguments or just
	 * specific ones, with the rest to be resolved through regular autowiring
	 * (may be {@code null} or empty)
	 * @since 5.0
	 */
	public <T> void registerBean(@Nullable String beanName, Class<T> annotatedClass, Object... constructorArguments) {
		this.reader.doRegisterBean(annotatedClass, null, beanName, null,
				bd -> {
					for (Object arg : constructorArguments) {
						bd.getConstructorArgumentValues().addGenericArgumentValue(arg);
					}
				});
	}

	@Override
	public <T> void registerBean(@Nullable String beanName, Class<T> beanClass, @Nullable Supplier<T> supplier,
			BeanDefinitionCustomizer... customizers) {

		this.reader.doRegisterBean(beanClass, supplier, beanName, null, customizers);
	}

}

通过上面的源码分析，我们可以看啊到 Spring 对注解的处理分为两种方式：
1)、直接将注解 Bean 注册到容器中
可以在初始化容器时注册；也可以在容器创建之后手动调用注册方法向容器注册，然后通过手动刷新容器，使得容器对注册的注解 Bean 进行处理。
2)、通过扫描指定的包及其子包下的所有类
在初始化注解容器时指定要自动扫描的路径，如果容器创建以后向给定路径动态添加了注解 Bean，则需要手动调用容器扫描的方法，然后手动刷新容器，使得容器对所注册的 Bean 进行处理。
接下来，将会对两种处理方式详细分析其实现过程。

读取 Annotation 元数据

当创建注解处理容器时，如果传入的初始参数是具体的注解 Bean 定义类时，注解容器读取并注册。
1)、AnnotationConfigApplicationContext 通过调用注解 Bean 定义读取器
AnnotatedBeanDefinitionReader 的 register()方法向容器注册指定的注解 Bean，注解 Bean 定义读取器向容器注册注解 Bean 的源码如下：

/**
	 * Register one or more annotated classes to be processed.
	 * <p>Calls to {@code register} are idempotent; adding the same
	 * annotated class more than once has no additional effect.
	 * @param annotatedClasses one or more annotated classes,
	 * e.g. {@link Configuration @Configuration} classes
	 */
	//注册多个注解Bean定义类
	public void register(Class<?>... annotatedClasses) {
		for (Class<?> annotatedClass : annotatedClasses) {
			registerBean(annotatedClass);
		}
	}

	/**
	 * Register a bean from the given bean class, deriving its metadata from
	 * class-declared annotations.
	 * @param annotatedClass the class of the bean
	 */
	//注册一个注解Bean定义类
	public void registerBean(Class<?> annotatedClass) {
		doRegisterBean(annotatedClass, null, null, null);
	}

	/**
	 * Register a bean from the given bean class, deriving its metadata from
	 * class-declared annotations, using the given supplier for obtaining a new
	 * instance (possibly declared as a lambda expression or method reference).
	 * @param annotatedClass the class of the bean
	 * @param instanceSupplier a callback for creating an instance of the bean
	 * (may be {@code null})
	 * @since 5.0
	 */
	public <T> void registerBean(Class<T> annotatedClass, @Nullable Supplier<T> instanceSupplier) {
		doRegisterBean(annotatedClass, instanceSupplier, null, null);
	}

	/**
	 * Register a bean from the given bean class, deriving its metadata from
	 * class-declared annotations, using the given supplier for obtaining a new
	 * instance (possibly declared as a lambda expression or method reference).
	 * @param annotatedClass the class of the bean
	 * @param name an explicit name for the bean
	 * @param instanceSupplier a callback for creating an instance of the bean
	 * (may be {@code null})
	 * @since 5.0
	 */
	public <T> void registerBean(Class<T> annotatedClass, String name, @Nullable Supplier<T> instanceSupplier) {
		doRegisterBean(annotatedClass, instanceSupplier, name, null);
	}

	/**
	 * Register a bean from the given bean class, deriving its metadata from
	 * class-declared annotations.
	 * @param annotatedClass the class of the bean
	 * @param qualifiers specific qualifier annotations to consider,
	 * in addition to qualifiers at the bean class level
	 */
	//Bean定义读取器注册注解Bean定义的入口方法
	@SuppressWarnings("unchecked")
	public void registerBean(Class<?> annotatedClass, Class<? extends Annotation>... qualifiers) {
		doRegisterBean(annotatedClass, null, null, qualifiers);
	}

	/**
	 * Register a bean from the given bean class, deriving its metadata from
	 * class-declared annotations.
	 * @param annotatedClass the class of the bean
	 * @param name an explicit name for the bean
	 * @param qualifiers specific qualifier annotations to consider,
	 * in addition to qualifiers at the bean class level
	 */
	//Bean定义读取器向容器注册注解Bean定义类
	@SuppressWarnings("unchecked")
	public void registerBean(Class<?> annotatedClass, String name, Class<? extends Annotation>... qualifiers) {
		doRegisterBean(annotatedClass, null, name, qualifiers);
	}

	/**
	 * Register a bean from the given bean class, deriving its metadata from
	 * class-declared annotations.
	 * @param annotatedClass the class of the bean
	 * @param instanceSupplier a callback for creating an instance of the bean
	 * (may be {@code null})
	 * @param name an explicit name for the bean
	 * @param qualifiers specific qualifier annotations to consider, if any,
	 * in addition to qualifiers at the bean class level
	 * @param definitionCustomizers one or more callbacks for customizing the
	 * factory's {@link BeanDefinition}, e.g. setting a lazy-init or primary flag
	 * @since 5.0
	 */
	//Bean定义读取器向容器注册注解Bean定义类
	<T> void doRegisterBean(Class<T> annotatedClass, @Nullable Supplier<T> instanceSupplier, @Nullable String name,
			@Nullable Class<? extends Annotation>[] qualifiers, BeanDefinitionCustomizer... definitionCustomizers) {

		//根据指定的注解Bean定义类，创建Spring容器中对注解Bean的封装的数据结构
		AnnotatedGenericBeanDefinition abd = new AnnotatedGenericBeanDefinition(annotatedClass);
		if (this.conditionEvaluator.shouldSkip(abd.getMetadata())) {
			return;
		}

		abd.setInstanceSupplier(instanceSupplier);
		//解析注解Bean定义的作用域，若@Scope("prototype")，则Bean为原型类型；
		//若@Scope("singleton")，则Bean为单态类型
		ScopeMetadata scopeMetadata = this.scopeMetadataResolver.resolveScopeMetadata(abd);
		//为注解Bean定义设置作用域
		abd.setScope(scopeMetadata.getScopeName());
		//为注解Bean定义生成Bean名称
		String beanName = (name != null ? name : this.beanNameGenerator.generateBeanName(abd, this.registry));

		//处理注解Bean定义中的通用注解
		AnnotationConfigUtils.processCommonDefinitionAnnotations(abd);
		//如果在向容器注册注解Bean定义时，使用了额外的限定符注解，则解析限定符注解。
		//主要是配置的关于autowiring自动依赖注入装配的限定条件，即@Qualifier注解
		//Spring自动依赖注入装配默认是按类型装配，如果使用@Qualifier则按名称
		if (qualifiers != null) {
			for (Class<? extends Annotation> qualifier : qualifiers) {
				//如果配置了@Primary注解，设置该Bean为autowiring自动依赖注入装//配时的首选
				if (Primary.class == qualifier) {
					abd.setPrimary(true);
				}
				//如果配置了@Lazy注解，则设置该Bean为非延迟初始化，如果没有配置，
				//则该Bean为预实例化
				else if (Lazy.class == qualifier) {
					abd.setLazyInit(true);
				}
				//如果使用了除@Primary和@Lazy以外的其他注解，则为该Bean添加一
				//个autowiring自动依赖注入装配限定符，该Bean在进autowiring
				//自动依赖注入装配时，根据名称装配限定符指定的Bean
				else {
					abd.addQualifier(new AutowireCandidateQualifier(qualifier));
				}
			}
		}
		for (BeanDefinitionCustomizer customizer : definitionCustomizers) {
			customizer.customize(abd);
		}

		//创建一个指定Bean名称的Bean定义对象，封装注解Bean定义类数据
		BeanDefinitionHolder definitionHolder = new BeanDefinitionHolder(abd, beanName);
		//根据注解Bean定义类中配置的作用域，创建相应的代理对象
		definitionHolder = AnnotationConfigUtils.applyScopedProxyMode(scopeMetadata, definitionHolder, this.registry);
		//向IOC容器注册注解Bean类定义对象
		BeanDefinitionReaderUtils.registerBeanDefinition(definitionHolder, this.registry);
	}

从上面的源码我们可以看出，注册注解 Bean 定义类的基本步骤：
a、需要使用注解元数据解析器解析注解 Bean 中关于作用域的配置
b、使用 AnnotationConfigUtils 的 processCommonDefinitionAnnotations()方法处理注解 Bean 定义类中通用的注解。
c、使用 AnnotationConfigUtils 的 applyScopedProxyMode()方法创建对于作用域的代理对象。
d、通过 BeanDefinitionReaderUtils 向容器注册 Bean。
2)、AnnotationScopeMetadataResolver 解析作用域元数据
AnnotationScopeMetadataResolver 通过 resolveScopeMetadata()方法解析注解 Bean 定义类的作用域元信息，即判断注册的 Bean 是原生类型(prototype)还是单态(singleton)类型，其源码如下：

//解析注解Bean定义类中的作用域元信息
	@Override
	public ScopeMetadata resolveScopeMetadata(BeanDefinition definition) {
		ScopeMetadata metadata = new ScopeMetadata();
		if (definition instanceof AnnotatedBeanDefinition) {
			AnnotatedBeanDefinition annDef = (AnnotatedBeanDefinition) definition;
			//从注解Bean定义类的属性中查找属性为”Scope”的值，即@Scope注解的值
			//annDef.getMetadata().getAnnotationAttributes()方法将Bean
			//中所有的注解和注解的值存放在一个map集合中
			AnnotationAttributes attributes = AnnotationConfigUtils.attributesFor(
					annDef.getMetadata(), this.scopeAnnotationType);
			//将获取到的@Scope注解的值设置到要返回的对象中
			if (attributes != null) {
				metadata.setScopeName(attributes.getString("value"));
				//获取@Scope注解中的proxyMode属性值，在创建代理对象时会用到
				ScopedProxyMode proxyMode = attributes.getEnum("proxyMode");
				//如果@Scope的proxyMode属性为DEFAULT或者NO
				if (proxyMode == ScopedProxyMode.DEFAULT) {
					//设置proxyMode为NO
					proxyMode = this.defaultProxyMode;
				}
				//为返回的元数据设置proxyMode
				metadata.setScopedProxyMode(proxyMode);
			}
		}
		//返回解析的作用域元信息对象
		return metadata;
	}

上述代码中的 annDef.getMetadata().getAnnotationAttributes()方法就是获取对象中指定类型的注解的值。
3)、AnnotationConfigUtils 处理注解 Bean 定义类中的通用注解
AnnotationConfigUtils 类的 processCommonDefinitionAnnotations()在向容器注册 Bean 之前，首先对注解 Bean 定义类中的通用 Spring 注解进行处理，源码如下：

//处理Bean定义中通用注解
	static void processCommonDefinitionAnnotations(AnnotatedBeanDefinition abd, AnnotatedTypeMetadata metadata) {
		AnnotationAttributes lazy = attributesFor(metadata, Lazy.class);
		//如果Bean定义中有@Lazy注解，则将该Bean预实例化属性设置为@lazy注解的值
		if (lazy != null) {
			abd.setLazyInit(lazy.getBoolean("value"));
		}

		else if (abd.getMetadata() != metadata) {
			lazy = attributesFor(abd.getMetadata(), Lazy.class);
			if (lazy != null) {
				abd.setLazyInit(lazy.getBoolean("value"));
			}
		}
		//如果Bean定义中有@Primary注解，则为该Bean设置为autowiring自动依赖注入装配的首选对象
		if (metadata.isAnnotated(Primary.class.getName())) {
			abd.setPrimary(true);
		}
		//如果Bean定义中有@ DependsOn注解，则为该Bean设置所依赖的Bean名称，
		//容器将确保在实例化该Bean之前首先实例化所依赖的Bean
		AnnotationAttributes dependsOn = attributesFor(metadata, DependsOn.class);
		if (dependsOn != null) {
			abd.setDependsOn(dependsOn.getStringArray("value"));
		}

		if (abd instanceof AbstractBeanDefinition) {
			AbstractBeanDefinition absBd = (AbstractBeanDefinition) abd;
			AnnotationAttributes role = attributesFor(metadata, Role.class);
			if (role != null) {
				absBd.setRole(role.getNumber("value").intValue());
			}
			AnnotationAttributes description = attributesFor(metadata, Description.class);
			if (description != null) {
				absBd.setDescription(description.getString("value"));
			}
		}
	}

4)、AnnotationConfigUtils 根据注解 Bean 定义类中配置的作用域为其应用相应的代理策略
AnnotationConfigUtils 类的 applyScopedProxyMode()方法根据注解 Bean 定义类中配置的作用域@Scope 注解的值，为 Bean 定义应用相应的代理模式，主要是在 Spring 面向切面编程(AOP)中使用。源码如下：

//根据作用域为Bean应用引用的代码模式
	static BeanDefinitionHolder applyScopedProxyMode(
			ScopeMetadata metadata, BeanDefinitionHolder definition, BeanDefinitionRegistry registry) {

		//获取注解Bean定义类中@Scope注解的proxyMode属性值
		ScopedProxyMode scopedProxyMode = metadata.getScopedProxyMode();
		//如果配置的@Scope注解的proxyMode属性值为NO，则不应用代理模式
		if (scopedProxyMode.equals(ScopedProxyMode.NO)) {
			return definition;
		}
		//获取配置的@Scope注解的proxyMode属性值，如果为TARGET_CLASS
		//则返回true，如果为INTERFACES，则返回false
		boolean proxyTargetClass = scopedProxyMode.equals(ScopedProxyMode.TARGET_CLASS);
		//为注册的Bean创建相应模式的代理对象
		return ScopedProxyCreator.createScopedProxy(definition, registry, proxyTargetClass);
	}

这段为 Bean 引用创建相应模式的代理，这里不做深入的分析。
5)、BeanDefinitionReaderUtils 向容器注册 Bean
BeanDefinitionReaderUtils 主要是校验 BeanDefinition 信息，然后将 Bean 添加到容器中一个管理BeanDefinition 的 HashMap 中。

扫描指定包并解析为 BeanDefinition

当创建注解处理容器时，如果传入的初始参数是注解 Bean 定义类所在的包时，注解容器将扫描给定的包及其子包，将扫描到的注解 Bean 定义载入并注册。
1)、ClassPathBeanDefinitionScanner 扫描给定的包及其子包
AnnotationConfigApplicationContext 通 过 调 用 类 路 径 Bean 定 义 扫 描 器ClassPathBeanDefinitionScanner 扫描给定包及其子包下的所有类，主要源码如下：

public class ClassPathBeanDefinitionScanner extends ClassPathScanningCandidateComponentProvider {
	/**
	 * Create a new {@code ClassPathBeanDefinitionScanner} for the given bean factory.
	 * @param registry the {@code BeanFactory} to load bean definitions into, in the form
	 * of a {@code BeanDefinitionRegistry}
	 */
	//创建一个类路径Bean定义扫描器
	public ClassPathBeanDefinitionScanner(BeanDefinitionRegistry registry) {
		this(registry, true);
	}
/**
	 * Create a new {@code ClassPathBeanDefinitionScanner} for the given bean factory.
	 * <p>If the passed-in bean factory does not only implement the
	 * {@code BeanDefinitionRegistry} interface but also the {@code ResourceLoader}
	 * interface, it will be used as default {@code ResourceLoader} as well. This will
	 * usually be the case for {@link org.springframework.context.ApplicationContext}
	 * implementations.
	 * <p>If given a plain {@code BeanDefinitionRegistry}, the default {@code ResourceLoader}
	 * will be a {@link org.springframework.core.io.support.PathMatchingResourcePatternResolver}.
	 * <p>If the passed-in bean factory also implements {@link EnvironmentCapable} its
	 * environment will be used by this reader.  Otherwise, the reader will initialize and
	 * use a {@link org.springframework.core.env.StandardEnvironment}. All
	 * {@code ApplicationContext} implementations are {@code EnvironmentCapable}, while
	 * normal {@code BeanFactory} implementations are not.
	 * @param registry the {@code BeanFactory} to load bean definitions into, in the form
	 * of a {@code BeanDefinitionRegistry}
	 * @param useDefaultFilters whether to include the default filters for the
	 * {@link org.springframework.stereotype.Component @Component},
	 * {@link org.springframework.stereotype.Repository @Repository},
	 * {@link org.springframework.stereotype.Service @Service}, and
	 * {@link org.springframework.stereotype.Controller @Controller} stereotype annotations
	 * @see #setResourceLoader
	 * @see #setEnvironment
	 */
	//为容器创建一个类路径Bean定义扫描器，并指定是否使用默认的扫描过滤规则。
	//即Spring默认扫描配置：@Component、@Repository、@Service、@Controller
	//注解的Bean，同时也支持JavaEE6的@ManagedBean和JSR-330的@Named注解
	public ClassPathBeanDefinitionScanner(BeanDefinitionRegistry registry, boolean useDefaultFilters) {
		this(registry, useDefaultFilters, getOrCreateEnvironment(registry));
	}

	/**
	 * Create a new {@code ClassPathBeanDefinitionScanner} for the given bean factory and
	 * using the given {@link Environment} when evaluating bean definition profile metadata.
	 * <p>If the passed-in bean factory does not only implement the {@code
	 * BeanDefinitionRegistry} interface but also the {@link ResourceLoader} interface, it
	 * will be used as default {@code ResourceLoader} as well. This will usually be the
	 * case for {@link org.springframework.context.ApplicationContext} implementations.
	 * <p>If given a plain {@code BeanDefinitionRegistry}, the default {@code ResourceLoader}
	 * will be a {@link org.springframework.core.io.support.PathMatchingResourcePatternResolver}.
	 * @param registry the {@code BeanFactory} to load bean definitions into, in the form
	 * of a {@code BeanDefinitionRegistry}
	 * @param useDefaultFilters whether to include the default filters for the
	 * {@link org.springframework.stereotype.Component @Component},
	 * {@link org.springframework.stereotype.Repository @Repository},
	 * {@link org.springframework.stereotype.Service @Service}, and
	 * {@link org.springframework.stereotype.Controller @Controller} stereotype annotations
	 * @param environment the Spring {@link Environment} to use when evaluating bean
	 * definition profile metadata
	 * @since 3.1
	 * @see #setResourceLoader
	 */
	public ClassPathBeanDefinitionScanner(BeanDefinitionRegistry registry, boolean useDefaultFilters,
			Environment environment) {

		this(registry, useDefaultFilters, environment,
				(registry instanceof ResourceLoader ? (ResourceLoader) registry : null));
	}

	/**
	 * Create a new {@code ClassPathBeanDefinitionScanner} for the given bean factory and
	 * using the given {@link Environment} when evaluating bean definition profile metadata.
	 * @param registry the {@code BeanFactory} to load bean definitions into, in the form
	 * of a {@code BeanDefinitionRegistry}
	 * @param useDefaultFilters whether to include the default filters for the
	 * {@link org.springframework.stereotype.Component @Component},
	 * {@link org.springframework.stereotype.Repository @Repository},
	 * {@link org.springframework.stereotype.Service @Service}, and
	 * {@link org.springframework.stereotype.Controller @Controller} stereotype annotations
	 * @param environment the Spring {@link Environment} to use when evaluating bean
	 * definition profile metadata
	 * @param resourceLoader the {@link ResourceLoader} to use
	 * @since 4.3.6
	 */
	public ClassPathBeanDefinitionScanner(BeanDefinitionRegistry registry, boolean useDefaultFilters,
			Environment environment, @Nullable ResourceLoader resourceLoader) {

		Assert.notNull(registry, "BeanDefinitionRegistry must not be null");
		//为容器设置加载Bean定义的注册器
		this.registry = registry;

		if (useDefaultFilters) {
			registerDefaultFilters();
		}
		setEnvironment(environment);
		//为容器设置资源加载器
		setResourceLoader(resourceLoader);
	}
/**
	 * Perform a scan within the specified base packages.
	 * @param basePackages the packages to check for annotated classes
	 * @return number of beans registered
	 */
	//调用类路径Bean定义扫描器入口方法
	public int scan(String... basePackages) {
		//获取容器中已经注册的Bean个数
		int beanCountAtScanStart = this.registry.getBeanDefinitionCount();

		//启动扫描器扫描给定包
		doScan(basePackages);

		// Register annotation config processors, if necessary.
		//注册注解配置(Annotation config)处理器
		if (this.includeAnnotationConfig) {
			AnnotationConfigUtils.registerAnnotationConfigProcessors(this.registry);
		}

		//返回注册的Bean个数
		return (this.registry.getBeanDefinitionCount() - beanCountAtScanStart);
	}
	/**
	 * Perform a scan within the specified base packages,
	 * returning the registered bean definitions.
	 * <p>This method does <i>not</i> register an annotation config processor
	 * but rather leaves this up to the caller.
	 * @param basePackages the packages to check for annotated classes
	 * @return set of beans registered if any for tooling registration purposes (never {@code null})
	 */
	//类路径Bean定义扫描器扫描给定包及其子包
	protected Set<BeanDefinitionHolder> doScan(String... basePackages) {
		Assert.notEmpty(basePackages, "At least one base package must be specified");
		//创建一个集合，存放扫描到Bean定义的封装类
		Set<BeanDefinitionHolder> beanDefinitions = new LinkedHashSet<>();
		//遍历扫描所有给定的包
		for (String basePackage : basePackages) {
			//调用父类ClassPathScanningCandidateComponentProvider的方法
			//扫描给定类路径，获取符合条件的Bean定义
			Set<BeanDefinition> candidates = findCandidateComponents(basePackage);
			//遍历扫描到的Bean
			for (BeanDefinition candidate : candidates) {
				//获取Bean定义类中@Scope注解的值，即获取Bean的作用域
				ScopeMetadata scopeMetadata = this.scopeMetadataResolver.resolveScopeMetadata(candidate);
				//为Bean设置注解配置的作用域
				candidate.setScope(scopeMetadata.getScopeName());
				//为Bean生成名称
				String beanName = this.beanNameGenerator.generateBeanName(candidate, this.registry);
				//如果扫描到的Bean不是Spring的注解Bean，则为Bean设置默认值，
				//设置Bean的自动依赖注入装配属性等
				if (candidate instanceof AbstractBeanDefinition) {
					postProcessBeanDefinition((AbstractBeanDefinition) candidate, beanName);
				}
				//如果扫描到的Bean是Spring的注解Bean，则处理其通用的Spring注解
				if (candidate instanceof AnnotatedBeanDefinition) {
					//处理注解Bean中通用的注解，在分析注解Bean定义类读取器时已经分析过
					AnnotationConfigUtils.processCommonDefinitionAnnotations((AnnotatedBeanDefinition) candidate);
				}
				//根据Bean名称检查指定的Bean是否需要在容器中注册，或者在容器中冲突
				if (checkCandidate(beanName, candidate)) {
					BeanDefinitionHolder definitionHolder = new BeanDefinitionHolder(candidate, beanName);
					//根据注解中配置的作用域，为Bean应用相应的代理模式
					definitionHolder =
							AnnotationConfigUtils.applyScopedProxyMode(scopeMetadata, definitionHolder, this.registry);
					beanDefinitions.add(definitionHolder);
					//向容器注册扫描到的Bean
					registerBeanDefinition(definitionHolder, this.registry);
				}
			}
		}
		return beanDefinitions;
	}
}

类路径 Bean 定义扫描器 ClassPathBeanDefinitionScanner 主要通过 findCandidateComponents()方法调用其父类 ClassPathScanningCandidateComponentProvider 类来扫描获取给定包及其子包下的类。
2)、ClassPathScanningCandidateComponentProvider 扫描给定包及其子包的类
ClassPathScanningCandidateComponentProvider 类的 findCandidateComponents()方法具体实现扫描给定类路径包的功能，主要源码如下：

public class ClassPathScanningCandidateComponentProvider implements EnvironmentCapable, ResourceLoaderAware {
	//保存过滤规则要包含的注解，即Spring默认的@Component、@Repository、@Service、
	//@Controller注解的Bean，以及JavaEE6的@ManagedBean和JSR-330的@Named注解
	private final List<TypeFilter> includeFilters = new LinkedList<>();

	//保存过滤规则要排除的注解
	private final List<TypeFilter> excludeFilters = new LinkedList<>();
	/**
	 * Create a ClassPathScanningCandidateComponentProvider with a {@link StandardEnvironment}.
	 * @param useDefaultFilters whether to register the default filters for the
	 * {@link Component @Component}, {@link Repository @Repository},
	 * {@link Service @Service}, and {@link Controller @Controller}
	 * stereotype annotations
	 * @see #registerDefaultFilters()
	 */
	//构造方法，该方法在子类ClassPathBeanDefinitionScanner的构造方法中被调用
	public ClassPathScanningCandidateComponentProvider(boolean useDefaultFilters) {
		this(useDefaultFilters, new StandardEnvironment());
	}

	/**
	 * Create a ClassPathScanningCandidateComponentProvider with the given {@link Environment}.
	 * @param useDefaultFilters whether to register the default filters for the
	 * {@link Component @Component}, {@link Repository @Repository},
	 * {@link Service @Service}, and {@link Controller @Controller}
	 * stereotype annotations
	 * @param environment the Environment to use
	 * @see #registerDefaultFilters()
	 */
	public ClassPathScanningCandidateComponentProvider(boolean useDefaultFilters, Environment environment) {
		//如果使用Spring默认的过滤规则，则向容器注册过滤规则
		if (useDefaultFilters) {
			registerDefaultFilters();
		}
		setEnvironment(environment);
		setResourceLoader(null);
	}

/**
	 * Register the default filter for {@link Component @Component}.
	 * <p>This will implicitly register all annotations that have the
	 * {@link Component @Component} meta-annotation including the
	 * {@link Repository @Repository}, {@link Service @Service}, and
	 * {@link Controller @Controller} stereotype annotations.
	 * <p>Also supports Java EE 6's {@link javax.annotation.ManagedBean} and
	 * JSR-330's {@link javax.inject.Named} annotations, if available.
	 *
	 */
	//向容器注册过滤规则
	@SuppressWarnings("unchecked")
	protected void registerDefaultFilters() {
		//向要包含的过滤规则中添加@Component注解类，注意Spring中@Repository
		//@Service和@Controller都是Component，因为这些注解都添加了@Component注解
		this.includeFilters.add(new AnnotationTypeFilter(Component.class));
		//获取当前类的类加载器
		ClassLoader cl = ClassPathScanningCandidateComponentProvider.class.getClassLoader();
		try {
			//向要包含的过滤规则添加JavaEE6的@ManagedBean注解
			this.includeFilters.add(new AnnotationTypeFilter(
					((Class<? extends Annotation>) ClassUtils.forName("javax.annotation.ManagedBean", cl)), false));
			logger.debug("JSR-250 'javax.annotation.ManagedBean' found and supported for component scanning");
		}
		catch (ClassNotFoundException ex) {
			// JSR-250 1.1 API (as included in Java EE 6) not available - simply skip.
		}
		try {
			//向要包含的过滤规则添加@Named注解
			this.includeFilters.add(new AnnotationTypeFilter(
					((Class<? extends Annotation>) ClassUtils.forName("javax.inject.Named", cl)), false));
			logger.debug("JSR-330 'javax.inject.Named' annotation found and supported for component scanning");
		}
		catch (ClassNotFoundException ex) {
			// JSR-330 API not available - simply skip.
		}
	}
	/**
	 * Scan the class path for candidate components.
	 * @param basePackage the package to check for annotated classes
	 * @return a corresponding Set of autodetected bean definitions
	 */
	//扫描给定类路径的包
	public Set<BeanDefinition> findCandidateComponents(String basePackage) {
		if (this.componentsIndex != null && indexSupportsIncludeFilters()) {
			return addCandidateComponentsFromIndex(this.componentsIndex, basePackage);
		}
		else {
			return scanCandidateComponents(basePackage);
		}
	}
	private Set<BeanDefinition> addCandidateComponentsFromIndex(CandidateComponentsIndex index, String basePackage) {
		//创建存储扫描到的类的集合
		Set<BeanDefinition> candidates = new LinkedHashSet<>();
		try {
			Set<String> types = new HashSet<>();
			for (TypeFilter filter : this.includeFilters) {
				String stereotype = extractStereotype(filter);
				if (stereotype == null) {
					throw new IllegalArgumentException("Failed to extract stereotype from "+ filter);
				}
				types.addAll(index.getCandidateTypes(basePackage, stereotype));
			}
			boolean traceEnabled = logger.isTraceEnabled();
			boolean debugEnabled = logger.isDebugEnabled();
			for (String type : types) {
				//为指定资源获取元数据读取器，元信息读取器通过汇编(ASM)读//取资源元信息
				MetadataReader metadataReader = getMetadataReaderFactory().getMetadataReader(type);
				//如果扫描到的类符合容器配置的过滤规则
				if (isCandidateComponent(metadataReader)) {
					//通过汇编(ASM)读取资源字节码中的Bean定义元信息
					AnnotatedGenericBeanDefinition sbd = new AnnotatedGenericBeanDefinition(
							metadataReader.getAnnotationMetadata());
					if (isCandidateComponent(sbd)) {
						if (debugEnabled) {
							logger.debug("Using candidate component class from index: " + type);
						}
						candidates.add(sbd);
					}
					else {
						if (debugEnabled) {
							logger.debug("Ignored because not a concrete top-level class: " + type);
						}
					}
				}
				else {
					if (traceEnabled) {
						logger.trace("Ignored because matching an exclude filter: " + type);
					}
				}
			}
		}
		catch (IOException ex) {
			throw new BeanDefinitionStoreException("I/O failure during classpath scanning", ex);
		}
		return candidates;
	}
		/**
	 * Determine whether the given class does not match any exclude filter
	 * and does match at least one include filter.
	 * @param metadataReader the ASM ClassReader for the class
	 * @return whether the class qualifies as a candidate component
	 */
	//判断元信息读取器读取的类是否符合容器定义的注解过滤规则
	protected boolean isCandidateComponent(MetadataReader metadataReader) throws IOException {
		//如果读取的类的注解在排除注解过滤规则中，返回false
		for (TypeFilter tf : this.excludeFilters) {
			if (tf.match(metadataReader, getMetadataReaderFactory())) {
				return false;
			}
		}
		//如果读取的类的注解在包含的注解的过滤规则中，则返回ture
		for (TypeFilter tf : this.includeFilters) {
			if (tf.match(metadataReader, getMetadataReaderFactory())) {
				return isConditionMatch(metadataReader);
			}
		}
		//如果读取的类的注解既不在排除规则，也不在包含规则中，则返回false
		return false;
	}
}

注册注解 BeanDefinition
AnnotationConfigWebApplicationContext 是 AnnotationConfigApplicationContext 的 Web 版，它们对于注解 Bean 的注册和扫描是基本相同的，但是 AnnotationConfigWebApplicationContext对注解 Bean 定义的载入稍有不同，AnnotationConfigWebApplicationContext 注入注解 Bean 定义源码如下：

/**
	 * Register a {@link org.springframework.beans.factory.config.BeanDefinition} for
	 * any classes specified by {@link #register(Class...)} and scan any packages
	 * specified by {@link #scan(String...)}.
	 * <p>For any values specified by {@link #setConfigLocation(String)} or
	 * {@link #setConfigLocations(String[])}, attempt first to load each location as a
	 * class, registering a {@code BeanDefinition} if class loading is successful,
	 * and if class loading fails (i.e. a {@code ClassNotFoundException} is raised),
	 * assume the value is a package and attempt to scan it for annotated classes.
	 * <p>Enables the default set of annotation configuration post processors, such that
	 * {@code @Autowired}, {@code @Required}, and associated annotations can be used.
	 * <p>Configuration class bean definitions are registered with generated bean
	 * definition names unless the {@code value} attribute is provided to the stereotype
	 * annotation.
	 * @param beanFactory the bean factory to load bean definitions into
	 * @see #register(Class...)
	 * @see #scan(String...)
	 * @see #setConfigLocation(String)
	 * @see #setConfigLocations(String[])
	 * @see AnnotatedBeanDefinitionReader
	 * @see ClassPathBeanDefinitionScanner
	 */
	//载入注解Bean定义资源
	@Override
	protected void loadBeanDefinitions(DefaultListableBeanFactory beanFactory) {
		//为容器设置注解Bean定义读取器
		AnnotatedBeanDefinitionReader reader = getAnnotatedBeanDefinitionReader(beanFactory);
		//为容器设置类路径Bean定义扫描器
		ClassPathBeanDefinitionScanner scanner = getClassPathBeanDefinitionScanner(beanFactory);

		//获取容器的Bean名称生成器
		BeanNameGenerator beanNameGenerator = getBeanNameGenerator();
		//为注解Bean定义读取器和类路径扫描器设置Bean名称生成器
		if (beanNameGenerator != null) {
			reader.setBeanNameGenerator(beanNameGenerator);
			scanner.setBeanNameGenerator(beanNameGenerator);
			beanFactory.registerSingleton(AnnotationConfigUtils.CONFIGURATION_BEAN_NAME_GENERATOR, beanNameGenerator);
		}

		//获取容器的作用域元信息解析器
		ScopeMetadataResolver scopeMetadataResolver = getScopeMetadataResolver();
		//为注解Bean定义读取器和类路径扫描器设置作用域元信息解析器
		if (scopeMetadataResolver != null) {
			reader.setScopeMetadataResolver(scopeMetadataResolver);
			scanner.setScopeMetadataResolver(scopeMetadataResolver);
		}

		if (!this.annotatedClasses.isEmpty()) {
			if (logger.isInfoEnabled()) {
				logger.info("Registering annotated classes: [" +
						StringUtils.collectionToCommaDelimitedString(this.annotatedClasses) + "]");
			}
			reader.register(this.annotatedClasses.toArray(new Class<?>[this.annotatedClasses.size()]));
		}

		if (!this.basePackages.isEmpty()) {
			if (logger.isInfoEnabled()) {
				logger.info("Scanning base packages: [" +
						StringUtils.collectionToCommaDelimitedString(this.basePackages) + "]");
			}
			scanner.scan(this.basePackages.toArray(new String[this.basePackages.size()]));
		}

		//获取容器定义的Bean定义资源路径
		String[] configLocations = getConfigLocations();
		//如果定位的Bean定义资源路径不为空
		if (configLocations != null) {
			for (String configLocation : configLocations) {
				try {
					//使用当前容器的类加载器加载定位路径的字节码类文件
					Class<?> clazz = ClassUtils.forName(configLocation, getClassLoader());
					if (logger.isInfoEnabled()) {
						logger.info("Successfully resolved class for [" + configLocation + "]");
					}
					reader.register(clazz);
				}
				catch (ClassNotFoundException ex) {
					if (logger.isDebugEnabled()) {
						logger.debug("Could not load class for config location [" + configLocation +
								"] - trying package scan. " + ex);
					}
					//如果容器类加载器加载定义路径的Bean定义资源失败
					//则启用容器类路径扫描器扫描给定路径包及其子包中的类
					int count = scanner.scan(configLocation);
					if (logger.isInfoEnabled()) {
						if (count == 0) {
							logger.info("No annotated classes found for specified class/package [" + configLocation + "]");
						}
						else {
							logger.info("Found " + count + " annotated classes in package [" + configLocation + "]");
						}
					}
				}
			}
		}
	}

以上就是解析和注入注解配置资源的全过程分析。

IOC 容器初始化小结

现在通过上面的代码，总结一下 IOC 容器初始化的基本步骤：
1、初始化的入口在容器实现中的 refresh()调用来完成。
2、对 Bean 定义载入 IOC 容器使用的方法是 loadBeanDefinition(), 其中的大致过程如下：通过 ResourceLoader 来完成资源文件位置的定位，DefaultResourceLoader是默认的实现，同时上下文本身就给出了 ResourceLoader 的实现，可以从类路径，文件系统,URL 等方式来定为资源位置。如果是 XmlBeanFactory 作为 IOC 容器，那么需要为它指定 Bean 定义的资源，也 就 是 说 Bean 定 义 文 件 时 通 过 抽 象 成 Resource 来 被 IOC 容 器 处 理 的 ， 容 器 通 过BeanDefinitionReader 来 完 成 定 义 信 息 的 解 析 和 Bean 信 息 的 注 册 , 往 往 使 用 的 是XmlBeanDefinitionReader 来 解 析 Bean 的 XML 定 义 文 件 - 实 际 的 处 理 过 程 是 委 托 给BeanDefinitionParserDelegate 来完成的，从而得到 bean 的定义信息，这些信息在 Spring 中使用BeanDefinition对象来表示-这个名字可以让我们想到loadBeanDefinition(),registerBeanDefinition()这些相关方法。它们都是为处理 BeanDefinitin 服务的，容器解析得到 BeanDefinition 以后，需要把它在 IOC 容器中注册，这由 IOC 实现 BeanDefinitionRegistry 接口来实现。注册过程就是在 IOC 容器内部维护的一个 HashMap 来保存得到的 BeanDefinition 的过程。这个 HashMap 是 IOC 容器持有Bean 信息的场所，以后对 Bean 的操作都是围绕这个 HashMap 来实现的。
然后我们就可以通过 BeanFactory 和 ApplicationContext 来享受到 Spring IOC 的服务了,在使用 IOC容器的时候，我们注意到除了少量粘合代码，绝大多数以正确 IOC 风格编写的应用程序代码完全不用关心如何到达工厂，因为容器将把这些对象与容器管理的其他对象钩在一起。基本的策略是把工厂放到已知的地方，最好是放在对预期使用的上下文有意义的地方，以及代码将实际需要访问工厂的地方。Spring本身提供了对声明式载入web应用程序用法的应用程序上下文,并将其存储在ServletContext中的框架实现。

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