Spring Framework 之 Scope

作用

与@Component和@Bean共同作用，决定生成的Bean的生命周期，即何时创建一个新的bean，要想使用这个功能，那么使用者必须每次都调用BeanFactory#getBean, autrowire的对象不会被自动替换的。

原理

之前在分析beanFactory的时候都分析了，所有的对象都是在getBean的时候根据情况自动创建的。
org.springframework.beans.factory.support.AbstractBeanFactory#doGetBean

protected <T> T doGetBean(
			String name, @Nullable Class<T> requiredType, @Nullable Object[] args, boolean typeCheckOnly)
			throws BeansException {

		String beanName = transformedBeanName(name);
		Object beanInstance;

		// 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 + "'");
				}
			}
			beanInstance = 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);
			}

			StartupStep beanCreation = this.applicationStartup.start("spring.beans.instantiate")
					.tag("beanName", name);
			try {
				if (requiredType != null) {
					beanCreation.tag("beanType", requiredType::toString);
				}
				RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
				checkMergedBeanDefinition(mbd, beanName, args);

				// Guarantee initialization of beans that the current bean depends on.
				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 + "'");
						}
						registerDependentBean(dep, beanName);
						try {
							getBean(dep);
						}
						catch (NoSuchBeanDefinitionException ex) {
							throw new BeanCreationException(mbd.getResourceDescription(), beanName,
									"'" + beanName + "' depends on missing bean '" + dep + "'", ex);
						}
					}
				}

				// Create bean instance.
				if (mbd.isSingleton()) {
					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;
						}
					});
					beanInstance = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
				}

				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);
					}
					beanInstance = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
				}

				else {
					String scopeName = mbd.getScope();
					if (!StringUtils.hasLength(scopeName)) {
						throw new IllegalStateException("No scope name defined for bean '" + beanName + "'");
					}
					Scope scope = this.scopes.get(scopeName);
					if (scope == null) {
						throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'");
					}
					try {
						Object scopedInstance = scope.get(beanName, () -> {
							beforePrototypeCreation(beanName);
							try {
								return createBean(beanName, mbd, args);
							}
							finally {
								afterPrototypeCreation(beanName);
							}
						});
						beanInstance = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
					}
					catch (IllegalStateException ex) {
						throw new ScopeNotActiveException(beanName, scopeName, ex);
					}
				}
			}
			catch (BeansException ex) {
				beanCreation.tag("exception", ex.getClass().toString());
				beanCreation.tag("message", String.valueOf(ex.getMessage()));
				cleanupAfterBeanCreationFailure(beanName);
				throw ex;
			}
			finally {
				beanCreation.end();
			}
		}

		return adaptBeanInstance(name, beanInstance, requiredType);
	}

精简如下：

if (mbd.isSingleton()) {
//create singleton bean
}else if (mbd.isPrototype()) {
//create prototype bean
}else {
Scope scope = this.scopes.get(scopeName);
Object scopedInstance = scope.get(beanName, () -> {
	beforePrototypeCreation(beanName);
	try {
		return createBean(beanName, mbd, args);
	}
	finally {
		afterPrototypeCreation(beanName);
	}
});
}

可以看到SINGLETON和PROTOTYPE都是硬编码的，不存在singleton和prototype的对应实现。其他的scope都称之为自定义scope，需要自定义Scope interface的实现并且调用org.springframework.beans.factory.config.ConfigurableBeanFactory#registerScope

Scope interface

public interface Scope {

	/**
	 * Return the object with the given name from the underlying scope,
	 * {@link org.springframework.beans.factory.ObjectFactory#getObject() creating it}
	 * if not found in the underlying storage mechanism.
	 * 
This is the central operation of a Scope, and the only operation
	 * that is absolutely required.
	 * @param name the name of the object to retrieve
	 * @param objectFactory the {@link ObjectFactory} to use to create the scoped
	 * object if it is not present in the underlying storage mechanism
	 * @return the desired object (never {@code null})
	 * @throws IllegalStateException if the underlying scope is not currently active
	 */
	Object get(String name, ObjectFactory<?> objectFactory);

	/**
	 * Remove the object with the given {@code name} from the underlying scope.
	 * 
Returns {@code null} if no object was found; otherwise
	 * returns the removed {@code Object}.
	 * 
Note that an implementation should also remove a registered destruction
	 * callback for the specified object, if any. It does, however, not
	 * need to execute a registered destruction callback in this case,
	 * since the object will be destroyed by the caller (if appropriate).
	 * 
Note: This is an optional operation. Implementations may throw
	 * {@link UnsupportedOperationException} if they do not support explicitly
	 * removing an object.
	 * @param name the name of the object to remove
	 * @return the removed object, or {@code null} if no object was present
	 * @throws IllegalStateException if the underlying scope is not currently active
	 * @see #registerDestructionCallback
	 */
	@Nullable
	Object remove(String name);

	/**
	 * Register a callback to be executed on destruction of the specified
	 * object in the scope (or at destruction of the entire scope, if the
	 * scope does not destroy individual objects but rather only terminates
	 * in its entirety).
	 * 
Note: This is an optional operation. This method will only
	 * be called for scoped beans with actual destruction configuration
	 * (DisposableBean, destroy-method, DestructionAwareBeanPostProcessor).
	 * Implementations should do their best to execute a given callback
	 * at the appropriate time. If such a callback is not supported by the
	 * underlying runtime environment at all, the callback must be
	 * ignored and a corresponding warning should be logged.
	 * 
Note that 'destruction' refers to automatic destruction of
	 * the object as part of the scope's own lifecycle, not to the individual
	 * scoped object having been explicitly removed by the application.
	 * If a scoped object gets removed via this facade's {@link #remove(String)}
	 * method, any registered destruction callback should be removed as well,
	 * assuming that the removed object will be reused or manually destroyed.
	 * @param name the name of the object to execute the destruction callback for
	 * @param callback the destruction callback to be executed.
	 * Note that the passed-in Runnable will never throw an exception,
	 * so it can safely be executed without an enclosing try-catch block.
	 * Furthermore, the Runnable will usually be serializable, provided
	 * that its target object is serializable as well.
	 * @throws IllegalStateException if the underlying scope is not currently active
	 * @see org.springframework.beans.factory.DisposableBean
	 * @see org.springframework.beans.factory.support.AbstractBeanDefinition#getDestroyMethodName()
	 * @see DestructionAwareBeanPostProcessor
	 */
	void registerDestructionCallback(String name, Runnable callback);

	/**
	 * Resolve the contextual object for the given key, if any.
	 * E.g. the HttpServletRequest object for key "request".
	 * @param key the contextual key
	 * @return the corresponding object, or {@code null} if none found
	 * @throws IllegalStateException if the underlying scope is not currently active
	 */
	@Nullable
	Object resolveContextualObject(String key);

	/**
	 * Return the conversation ID for the current underlying scope, if any.
	 * 
The exact meaning of the conversation ID depends on the underlying
	 * storage mechanism. In the case of session-scoped objects, the
	 * conversation ID would typically be equal to (or derived from) the
	 * {@link javax.servlet.http.HttpSession#getId() session ID}; in the
	 * case of a custom conversation that sits within the overall session,
	 * the specific ID for the current conversation would be appropriate.
	 * 
Note: This is an optional operation. It is perfectly valid to
	 * return {@code null} in an implementation of this method if the
	 * underlying storage mechanism has no obvious candidate for such an ID.
	 * @return the conversation ID, or {@code null} if there is no
	 * conversation ID for the current scope
	 * @throws IllegalStateException if the underlying scope is not currently active
	 */
	@Nullable
	String getConversationId();

}

我们只关心Object get(String name, ObjectFactory objectFactory);, 其他的回调都没有用到过。
get方法总是在getBean的时候被调用，并且objectFactory总是会创建一个新的bean（类似prototype类型）。通常Scope的实现会自己实现一个缓存，用来决定是返回新的还是旧的bean

例子 ServletContextScope

这个scope在webApplication的整个生命周期有效
在webServer初始化的时候会注册这个生命周期
org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext#registerApplicationScope

	private void registerApplicationScope(ServletContext servletContext) {
		ServletContextScope appScope = new ServletContextScope(servletContext);
		getBeanFactory().registerScope(WebApplicationContext.SCOPE_APPLICATION, appScope);
		// Register as ServletContext attribute, for ContextCleanupListener to detect it.
		servletContext.setAttribute(ServletContextScope.class.getName(), appScope);
	}

public class ServletContextScope implements Scope, DisposableBean {

	private final ServletContext servletContext;

	private final Map<String, Runnable> destructionCallbacks = new LinkedHashMap<>();


	/**
	 * Create a new Scope wrapper for the given ServletContext.
	 * @param servletContext the ServletContext to wrap
	 */
	public ServletContextScope(ServletContext servletContext) {
		Assert.notNull(servletContext, "ServletContext must not be null");
		this.servletContext = servletContext;
	}


	@Override
	public Object get(String name, ObjectFactory<?> objectFactory) {
		Object scopedObject = this.servletContext.getAttribute(name);
		if (scopedObject == null) {
			scopedObject = objectFactory.getObject();
			this.servletContext.setAttribute(name, scopedObject);
		}
		return scopedObject;
	}

	@Override
	@Nullable
	public Object remove(String name) {
		Object scopedObject = this.servletContext.getAttribute(name);
		if (scopedObject != null) {
			synchronized (this.destructionCallbacks) {
				this.destructionCallbacks.remove(name);
			}
			this.servletContext.removeAttribute(name);
			return scopedObject;
		}
		else {
			return null;
		}
	}

	@Override
	public void registerDestructionCallback(String name, Runnable callback) {
		synchronized (this.destructionCallbacks) {
			this.destructionCallbacks.put(name, callback);
		}
	}

	@Override
	@Nullable
	public Object resolveContextualObject(String key) {
		return null;
	}

	@Override
	@Nullable
	public String getConversationId() {
		return null;
	}


	/**
	 * Invoke all registered destruction callbacks.
	 * To be called on ServletContext shutdown.
	 * @see org.springframework.web.context.ContextCleanupListener
	 */
	@Override
	public void destroy() {
		synchronized (this.destructionCallbacks) {
			for (Runnable runnable : this.destructionCallbacks.values()) {
				runnable.run();
			}
			this.destructionCallbacks.clear();
		}
	}

}

其实就是把servletContext当作自己的缓存了， Request和Session的Scope类似

自定义Scope

1， 可以在SpringApplication初始化之前调用beanFactory.registerScope
2， 可以实现org.springframework.beans.factory.config.CustomScopeConfigurer

public class CustomScopeConfigurer implements BeanFactoryPostProcessor, BeanClassLoaderAware, Ordered {

	@Nullable
	private Map<String, Object> scopes;

	private int order = Ordered.LOWEST_PRECEDENCE;

	@Nullable
	private ClassLoader beanClassLoader = ClassUtils.getDefaultClassLoader();


	/**
	 * Specify the custom scopes that are to be registered.
	 * 
The keys indicate the scope names (of type String); each value
	 * is expected to be the corresponding custom {@link Scope} instance
	 * or class name.
	 */
	public void setScopes(Map<String, Object> scopes) {
		this.scopes = scopes;
	}

	/**
	 * Add the given scope to this configurer's map of scopes.
	 * @param scopeName the name of the scope
	 * @param scope the scope implementation
	 * @since 4.1.1
	 */
	public void addScope(String scopeName, Scope scope) {
		if (this.scopes == null) {
			this.scopes = new LinkedHashMap<>(1);
		}
		this.scopes.put(scopeName, scope);
	}


	public void setOrder(int order) {
		this.order = order;
	}

	@Override
	public int getOrder() {
		return this.order;
	}

	@Override
	public void setBeanClassLoader(@Nullable ClassLoader beanClassLoader) {
		this.beanClassLoader = beanClassLoader;
	}


	@Override
	public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {
		if (this.scopes != null) {
			this.scopes.forEach((scopeKey, value) -> {
				if (value instanceof Scope) {
					beanFactory.registerScope(scopeKey, (Scope) value);
				}
				else if (value instanceof Class) {
					Class<?> scopeClass = (Class<?>) value;
					Assert.isAssignable(Scope.class, scopeClass, "Invalid scope class");
					beanFactory.registerScope(scopeKey, (Scope) BeanUtils.instantiateClass(scopeClass));
				}
				else if (value instanceof String) {
					Class<?> scopeClass = ClassUtils.resolveClassName((String) value, this.beanClassLoader);
					Assert.isAssignable(Scope.class, scopeClass, "Invalid scope class");
					beanFactory.registerScope(scopeKey, (Scope) BeanUtils.instantiateClass(scopeClass));
				}
				else {
					throw new IllegalArgumentException("Mapped value [" + value + "] for scope key [" +
							scopeKey + "] is not an instance of required type [" + Scope.class.getName() +
							"] or a corresponding Class or String value indicating a Scope implementation");
				}
			});
		}
	}

}

可以看到就是利用的BeanFactoryPostProcessor实现的beanFactory.registerScope