Spring中Bean的Scope周期的详解

Bean的使用范围有几种？

@Target({ElementType.TYPE, ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface Scope {

	/**
	 * Alias for {@link #scopeName}.
	 * @see #scopeName
	 */
	@AliasFor("scopeName")
	String value() default "";

	/**
	 * Specifies the name of the scope to use for the annotated component/bean.
	 * <p>Defaults to an empty string ({@code ""}) which implies
	 * {@link ConfigurableBeanFactory#SCOPE_SINGLETON SCOPE_SINGLETON}.
	 * @since 4.2
	 * @see ConfigurableBeanFactory#SCOPE_PROTOTYPE
	 * @see ConfigurableBeanFactory#SCOPE_SINGLETON
	 * @see org.springframework.web.context.WebApplicationContext#SCOPE_REQUEST
	 * @see org.springframework.web.context.WebApplicationContext#SCOPE_SESSION
	 * @see #value
	 */
	@AliasFor("value")
	String scopeName() default "";

	/**
	 * Specifies whether a component should be configured as a scoped proxy
	 * and if so, whether the proxy should be interface-based or subclass-based.
	 * <p>Defaults to {@link ScopedProxyMode#DEFAULT}, which typically indicates
	 * that no scoped proxy should be created unless a different default
	 * has been configured at the component-scan instruction level.
	 * <p>Analogous to {@code <aop:scoped-proxy/>} support in Spring XML.
	 * @see ScopedProxyMode
	 */
	ScopedProxyMode proxyMode() default ScopedProxyMode.DEFAULT;

}

通过Spring的注解Scope可以知道，对于Bean的使用方位有四种——singleton、prototype、session、request，那么这几种Bean的使用范围是如何实现的呢？首先要看Spring中的一个重要的接口——BeanDefinition

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;
	...
	/**
	 * Override the target scope of this bean, specifying a new scope name.
	 * @see #SCOPE_SINGLETON
	 * @see #SCOPE_PROTOTYPE
	 */
	void setScope(@Nullable String scope);

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

这里可以看到有一个getScope以及setScope，Spring就是通过这个获知Bean的使用范围是四种中的哪一种。

那么现在问题来了，Spring在得知Scope后，获取Bean的行为有什么不同呢？这个时候需要看AbstractBeanFactory的doGetBean方法了

/**
 * Return an instance, which may be shared or independent, of the specified bean.
 * @param name the name of the bean to retrieve
 * @param requiredType the required type 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)
 * @param typeCheckOnly whether the instance is obtained for a type check,
 * not for actual use
 * @return an instance of the bean
 * @throws BeansException if the bean could not be created
 */
@SuppressWarnings("unchecked")
protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,
			@Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {

	final String beanName = transformedBeanName(name);
	Object bean;

	// Eagerly check singleton cache for manually registered singletons.
	Object sharedInstance = getSingleton(beanName);
	if (sharedInstance != null && args == null) {
		if (logger.isTraceEnabled()) {
			if (isSingletonCurrentlyInCreation(beanName)) {
				logger.trace("Returning eagerly cached instance of singleton bean '" + beanName +
							"' that is not fully initialized yet - a consequence of a circular reference");
			}
			else {
				logger.trace("Returning cached instance of singleton bean '" + beanName + "'");
			}
		}
		bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
	}

	else {
		// Fail if we're already creating this bean instance:
		// We're assumably within a circular reference.
		if (isPrototypeCurrentlyInCreation(beanName)) {
			throw new BeanCurrentlyInCreationException(beanName);
		}

		// Check if bean definition exists in this factory.
		BeanFactory parentBeanFactory = getParentBeanFactory();
		if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
		// Not found -> check parent.
			String nameToLookup = originalBeanName(name);
			if (parentBeanFactory instanceof AbstractBeanFactory) {
				return ((AbstractBeanFactory) parentBeanFactory).doGetBean(nameToLookup, requiredType, args, typeCheckOnly);
			}
			else if (args != null) {
				// Delegation to parent with explicit args.
				return (T) parentBeanFactory.getBean(nameToLookup, args);
			}
			else if (requiredType != null) {
				// No args -> delegate to standard getBean method.
				return parentBeanFactory.getBean(nameToLookup, requiredType);
			}
			else {
				return (T) parentBeanFactory.getBean(nameToLookup);
			}
		}

		if (!typeCheckOnly) {
			markBeanAsCreated(beanName);
		}

		try {
      // 获取 beanName 所对应的 BeanDefinition
			final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
      // 检查是否是抽象类，如果是则直接抛出异常
			checkMergedBeanDefinition(mbd, beanName, args);

			// Guarantee initialization of beans that the current bean depends on.
      // 解决依赖问题，获取当前 Bean 依赖的其他 Bean 的名字信息
			String[] dependsOn = mbd.getDependsOn();
			if (dependsOn != null) {
				for (String dep : dependsOn) {
					if (isDependent(beanName, dep)) {
						throw new BeanCreationException(mbd.getResourceDescription(), beanName,
									"Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
					}
          // 对依赖的 Bean 进行注册操作
					registerDependentBean(dep, beanName);
					try {
            // 进行依赖 Bean 的加载
						getBean(dep);
					}
					catch (NoSuchBeanDefinitionException ex) {
						throw new BeanCreationException(mbd.getResourceDescription(), beanName,
										"'" + beanName + "' depends on missing bean '" + dep + "'", ex);
					}
				}
			}

			// Create bean instance.
      // 如果是单例模式的
			if (mbd.isSingleton()) {
        // 创建单例模式的 Bean，并且只有单例模式下会存在提早暴露 Bean 已解决循环依赖的问题
        // 具体代码在 AbstractAutowireCapableBeanFactory.doCreateBean(...)
				sharedInstance = getSingleton(beanName, () -> {
					try {
						return createBean(beanName, mbd, args);
					}
					catch (BeansException ex) {
						// Explicitly remove instance from singleton cache: It might have been put there
						// eagerly by the creation process, to allow for circular reference resolution.
						// Also remove any beans that received a temporary reference to the bean.
						destroySingleton(beanName);
						throw ex;
					}
				});
        // 获取 Bean
				bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
			}

      // 原型模式，每次 getBean 都会创建一个新的 Bean 出来
			else if (mbd.isPrototype()) {
				// It's a prototype -> create a new instance.
				Object prototypeInstance = null;
				try {
					beforePrototypeCreation(beanName);
					prototypeInstance = createBean(beanName, mbd, args);
				}
				finally {
					afterPrototypeCreation(beanName);
				}
				bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
			}

      // 如果是其他的 Scope，则使用对应对的 Scope 进行相应 Bean 处理
			else {
        // 获取 Bean 的 Scope 范围
				String scopeName = mbd.getScope();
        // 根据 ScopeName 获取对应 Scope 的处理{AbstractRequestAttributesScope}
				final Scope scope = this.scopes.get(scopeName);
				if (scope == null) {
					throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'");
				}
        // 根据 Scope 获取对应 Scope 范围的 Bean
				try {
					Object scopedInstance = scope.get(beanName, () -> {
						beforePrototypeCreation(beanName);
						try {
							return createBean(beanName, mbd, args);
						}
						finally {
							afterPrototypeCreation(beanName);
						}
					});
					bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
				}
				catch (IllegalStateException ex) {
					throw new BeanCreationException(beanName,
									"Scope '" + scopeName + "' is not active for the current thread; consider " +
									"defining a scoped proxy for this bean if you intend to refer to it from a singleton", ex);
				}
			}
		}
    // 如果在这期间创建 Bean 失败了，则将相应的缓存数据进行清除
		catch (BeansException ex) {
			cleanupAfterBeanCreationFailure(beanName);
			throw ex;
		}
	}

	// Check if required type matches the type of the actual bean instance.
	if (requiredType != null && !requiredType.isInstance(bean)) {
		try {
      // 对Bean进行类型适配
			T convertedBean = getTypeConverter().convertIfNecessary(bean, requiredType);
			if (convertedBean == null) {
				throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
			}
			return convertedBean;
		}
    // 类型适配失败
		catch (TypeMismatchException ex) {
			if (logger.isTraceEnabled()) {
				logger.trace("Failed to convert bean '" + name + "' to required type '" +
							ClassUtils.getQualifiedName(requiredType) + "'", ex);
			}
			throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
		}
	}
	return (T) bean;
}

上面的doGetBean就是获取Bean时的最核心的操作，相较于singleton以及prototype两种Scope，Bean的使用范围为request以及session，或许是比较难以理解的，不知道Spring是如何做到将Bean的使用范围给控制在request以及session的。

Bean的使用范围为 Request以及Sesison是如何实现的

上面说到了，在处理scope为非singleton以及prototype的Bean时，Spring是从Scope中获取Bean的，那这个Scope的实现是什么呢？

public abstract class RequestContextHolder  {

	private static final boolean jsfPresent =
			ClassUtils.isPresent("javax.faces.context.FacesContext", RequestContextHolder.class.getClassLoader());

  // 仅仅当前线程可以得到数据
	private static final ThreadLocal<RequestAttributes> requestAttributesHolder =
			new NamedThreadLocal<>("Request attributes");

  // 父子线程都可以得到数据的
	private static final ThreadLocal<RequestAttributes> inheritableRequestAttributesHolder =
			new NamedInheritableThreadLocal<>("Request context");
  
  ...
  @Nullable
	public static RequestAttributes getRequestAttributes() {
    // 获取 RequestAttributes 对象，如果本线程没有，那么从父线程中获取
		RequestAttributes attributes = requestAttributesHolder.get();
		if (attributes == null) {
			attributes = inheritableRequestAttributesHolder.get();
		}
		return attributes;
	}
  ...
}

public abstract class AbstractRequestAttributesScope implements Scope {

	@Override
	public Object get(String name, ObjectFactory<?> objectFactory) {
    // 获取 RequestAttributes 对象，注意，这个由 ThreadLocal 实现的，由于每一个 Request 对应
    // 着一个线程，因此可以很好地把不同 Request 的 RequestAttributes 隔离开来
		RequestAttributes attributes = RequestContextHolder.currentRequestAttributes();
    // 从 RequestAttributes 中获取对应 Scope 的 Bean
		Object scopedObject = attributes.getAttribute(name, getScope());
		if (scopedObject == null) {
			scopedObject = objectFactory.getObject();
			attributes.setAttribute(name, scopedObject, getScope());
			// Retrieve object again, registering it for implicit session attribute updates.
			// As a bonus, we also allow for potential decoration at the getAttribute level.
			Object retrievedObject = attributes.getAttribute(name, getScope());
			if (retrievedObject != null) {
				// Only proceed with retrieved object if still present (the expected case).
				// If it disappeared concurrently, we return our locally created instance.
				scopedObject = retrievedObject;
			}
		}
		return scopedObject;
	}
  ...
}

public class ServletRequestAttributes extends AbstractRequestAttributes {
  
  ...
  
  // 获取 Bean
	@Override
	public Object getAttribute(String name, int scope) {
    // 如果是 Request 范围的，先判断当前请求是否是活的
		if (scope == SCOPE_REQUEST) {
			if (!isRequestActive()) {
				throw new IllegalStateException(
						"Cannot ask for request attribute - request is not active anymore!");
			}
			return this.request.getAttribute(name);
		}
    // 如果是 Session 范围内的，直接从 session 中获取，如果没有直接将 Bean 放入 ConcurrentHashMap中
		else {
			HttpSession session = getSession(false);
			if (session != null) {
				try {
					Object value = session.getAttribute(name);
					if (value != null) {
						this.sessionAttributesToUpdate.put(name, value);
					}
					return value;
				}
				catch (IllegalStateException ex) {
					// Session invalidated - shouldn't usually happen.
				}
			}
			return null;
		}
	}
  ...
}

至此，Spring的Bean的Scope的实现就差不多了，通过Request以及Session的attributes(本质是一个Map)，将Bean存入attributes中，实现Bean的使用范围和Request以及Session的生命周期进行绑定

参考博文

Spring源码解读(2)IOC容器bean的实例化