spring拓展点之ObjectFactory

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前言

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// 注入所有HttpMessageConverters

@Autowired
private ObjectFactory<HttpMessageConverters> messageConverters;
  • 为什么需要objectFactory包装？

接口定义

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public interface ObjectFactory<T> {

	/**
	 * Return an instance (possibly shared or independent)
	 * of the object managed by this factory.
	 * @return an instance of the bean (should never be {@code null})
	 * @throws BeansException in case of creation errors
	 */
	T getObject() throws BeansException;

}

常见实现类

  1. RequestObjectFactory
  2. ResponseObjectFactory
  3. SessionObjectFactory
  4. WebRequestObjectFactory
  5. TargetBeanObjectFactory
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public abstract class WebApplicationContextUtils {
    /**
	 * Factory that exposes the current request object on demand.
	 */
	@SuppressWarnings("serial")
	private static class RequestObjectFactory implements ObjectFactory<ServletRequest>, Serializable {

		@Override
		public ServletRequest getObject() {
			return currentRequestAttributes().getRequest();
		}

		@Override
		public String toString() {
			return "Current HttpServletRequest";
		}
	}


	/**
	 * Factory that exposes the current response object on demand.
	 */
	@SuppressWarnings("serial")
	private static class ResponseObjectFactory implements ObjectFactory<ServletResponse>, Serializable {

		@Override
		public ServletResponse getObject() {
			ServletResponse response = currentRequestAttributes().getResponse();
			if (response == null) {
				throw new IllegalStateException("Current servlet response not available - " +
						"consider using RequestContextFilter instead of RequestContextListener");
			}
			return response;
		}

		@Override
		public String toString() {
			return "Current HttpServletResponse";
		}
	}


	/**
	 * Factory that exposes the current session object on demand.
	 */
	@SuppressWarnings("serial")
	private static class SessionObjectFactory implements ObjectFactory<HttpSession>, Serializable {

		@Override
		public HttpSession getObject() {
			return currentRequestAttributes().getRequest().getSession();
		}

		@Override
		public String toString() {
			return "Current HttpSession";
		}
	}


	/**
	 * Factory that exposes the current WebRequest object on demand.
	 */
	@SuppressWarnings("serial")
	private static class WebRequestObjectFactory implements ObjectFactory<WebRequest>, Serializable {

		@Override
		public WebRequest getObject() {
			ServletRequestAttributes requestAttr = currentRequestAttributes();
			return new ServletWebRequest(requestAttr.getRequest(), requestAttr.getResponse());
		}

		@Override
		public String toString() {
			return "Current ServletWebRequest";
		}
	}

}

测试用例

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/**
 * 控制层基类
 */
public abstract class BaseController {

    @Autowired
    protected HttpServletRequest request;

    @Autowired
    protected HttpServletResponse response;

    @Autowired
    protected HttpSession session;
}

// 子类
@Controller
public class SystemController extends BaseController {
    ........
    ............
}

咋一看，是不是觉得存在线程安全问题? 在SpringMvc中Controller不是单例的吗？那request，response和session对象岂不是所有线程共享的，后面进来的请求岂不是会覆盖前面请求的这些对象。但事实上，官方已经给出证明example，这种写法是正确的，不会存在线程安全问题，下面来分析一下，Spring是如何实现的。

ps: ResponseObjectFactory 是从Spring 4版本以上才开始出现的。

实现原理

XmlWebApplicationContext是Spring提供的容器的一种实现，多运用于Web项目中。继承于AbstractRefreshableWebApplicationContext，而AbstractRefreshableWebApplicationContext重写了AbstractApplicationContext中的postProcessBeanFactory方法，向beanFactory容器中添加依赖解析映射关系。

AbstractApplicationContext：

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public abstract class AbstractApplicationContext extends DefaultResourceLoader
		implements ConfigurableApplicationContext, DisposableBean {
    @Override
	public void refresh() throws BeansException, IllegalStateException {
		synchronized (this.startupShutdownMonitor) {
			// Prepare this context for refreshing.
			prepareRefresh();

			// Tell the subclass to refresh the internal bean factory.
			// 加载Bean信息，并添加到容器中
			ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

			// Prepare the bean factory for use in this context.
			prepareBeanFactory(beanFactory);

			try {
				// Allows post-processing of the bean factory in context subclasses.
				// 调用子类重写的方法
				postProcessBeanFactory(beanFactory);

				// Invoke factory processors registered as beans in the context.
				invokeBeanFactoryPostProcessors(beanFactory);

				// Register bean processors that intercept bean creation.
				registerBeanPostProcessors(beanFactory);

				// Initialize message source for this context.
				initMessageSource();

				// Initialize event multicaster for this context.
				initApplicationEventMulticaster();

				// Initialize other special beans in specific context subclasses.
				onRefresh();

				// Check for listener beans and register them.
				registerListeners();

				// Instantiate all remaining (non-lazy-init) singletons.
				finishBeanFactoryInitialization(beanFactory);

				// Last step: publish corresponding event.
				finishRefresh();
			}
			catch (BeansException ex) {
				if (logger.isWarnEnabled()) {
					logger.warn("Exception encountered during context initialization - " +
							"cancelling refresh attempt: " + ex);
				}

				// Destroy already created singletons to avoid dangling resources.
				destroyBeans();

				// Reset 'active' flag.
				cancelRefresh(ex);

				// Propagate exception to caller.
				throw ex;
			}
			finally {
				// Reset common introspection caches in Spring's core, since we
				// might not ever need metadata for singleton beans anymore...
				resetCommonCaches();
			}
		}
	}
	
	/**
	 * Modify the application context's internal bean factory after its standard
	 * initialization. All bean definitions will have been loaded, but no beans
	 * will have been instantiated yet. This allows for registering special
	 * BeanPostProcessors etc in certain ApplicationContext implementations.
	 * @param beanFactory the bean factory used by the application context
	 */
	protected void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) {
	}
	
}

AbstractRefreshableWebApplicationContext是AbstractApplicationContext的子类：

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public abstract class AbstractRefreshableWebApplicationContext extends AbstractRefreshableConfigApplicationContext
		implements ConfigurableWebApplicationContext, ThemeSource {
		
    @Override
	protected void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) {
		beanFactory.addBeanPostProcessor(new ServletContextAwareProcessor(this.servletContext, this.servletConfig));
		beanFactory.ignoreDependencyInterface(ServletContextAware.class);
		beanFactory.ignoreDependencyInterface(ServletConfigAware.class);

// 关键步骤：添加解析映射关系	
WebApplicationContextUtils.registerWebApplicationScopes(beanFactory, this.servletContext);
		WebApplicationContextUtils.registerEnvironmentBeans(beanFactory, this.servletContext, this.servletConfig);
	}
	
}

WebApplicationContextUtils:

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public abstract class WebApplicationContextUtils {

   /**
	 * Register web-specific scopes ("request", "session", "globalSession", "application")
	 * with the given BeanFactory, as used by the WebApplicationContext.
	 * @param beanFactory the BeanFactory to configure
	 * @param sc the ServletContext that we're running within
	 */
	public static void registerWebApplicationScopes(ConfigurableListableBeanFactory beanFactory, ServletContext sc) {
		beanFactory.registerScope(WebApplicationContext.SCOPE_REQUEST, new RequestScope());
		beanFactory.registerScope(WebApplicationContext.SCOPE_SESSION, new SessionScope(false));
		beanFactory.registerScope(WebApplicationContext.SCOPE_GLOBAL_SESSION, new SessionScope(true));
		if (sc != null) {
			ServletContextScope appScope = new ServletContextScope(sc);
			beanFactory.registerScope(WebApplicationContext.SCOPE_APPLICATION, appScope);
			// Register as ServletContext attribute, for ContextCleanupListener to detect it.
			sc.setAttribute(ServletContextScope.class.getName(), appScope);
		}

        // 向容器中添加依赖解析映射关系
		beanFactory.registerResolvableDependency(ServletRequest.class, new RequestObjectFactory());
		beanFactory.registerResolvableDependency(ServletResponse.class, new ResponseObjectFactory());
		beanFactory.registerResolvableDependency(HttpSession.class, new SessionObjectFactory());
		beanFactory.registerResolvableDependency(WebRequest.class, new WebRequestObjectFactory());
		if (jsfPresent) {
			FacesDependencyRegistrar.registerFacesDependencies(beanFactory);
		}
	}
	
}

当Spring容器启动的时候，会调用refresh方法，从而触发postProcessBeanFactory方法的执行。向beanFactory中添加依赖解析映射关系，当调用beanFactory.getBean方法获取实例时，注入依赖Bean则会使用到该依赖解析映射关系。

DefaultListableBeanFactory:

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public class DefaultListableBeanFactory extends AbstractAutowireCapableBeanFactory
		implements ConfigurableListableBeanFactory, BeanDefinitionRegistry, Serializable {
    
    /** Map from dependency type to corresponding autowired value */
	private final Map<Class<?>, Object> resolvableDependencies = new ConcurrentHashMap<Class<?>, Object>(16);
	
	@Override
	public void registerResolvableDependency(Class<?> dependencyType, Object autowiredValue) {
		Assert.notNull(dependencyType, "Dependency type must not be null");
		if (autowiredValue != null) {
			if (!(autowiredValue instanceof ObjectFactory || dependencyType.isInstance(autowiredValue))) {
				throw new IllegalArgumentException("Value [" + autowiredValue +
						"] does not implement specified dependency type [" + dependencyType.getName() + "]");
			}
			this.resolvableDependencies.put(dependencyType, autowiredValue);
		}
	}
	
	/**
	 * Find bean instances that match the required type.
	 * Called during autowiring for the specified bean.
	 * @param beanName the name of the bean that is about to be wired
	 * @param requiredType the actual type of bean to look for
	 * (may be an array component type or collection element type)
	 * @param descriptor the descriptor of the dependency to resolve
	 * @return a Map of candidate names and candidate instances that match
	 * the required type (never {@code null})
	 * @throws BeansException in case of errors
	 * @see #autowireByType
	 * @see #autowireConstructor
	 */
	protected Map<String, Object> findAutowireCandidates(
			String beanName, Class<?> requiredType, DependencyDescriptor descriptor) {

		String[] candidateNames = BeanFactoryUtils.beanNamesForTypeIncludingAncestors(
				this, requiredType, true, descriptor.isEager());
		Map<String, Object> result = new LinkedHashMap<String, Object>(candidateNames.length);
		for (Class<?> autowiringType : this.resolvableDependencies.keySet()) {
		    // 判断当前依赖Bean类型requiredType是否是autowiringType及其子类
			if (autowiringType.isAssignableFrom(requiredType)) {
			    // 如果是则使用映射关系值，如：RequestObjectFactory等
				Object autowiringValue = this.resolvableDependencies.get(autowiringType);
				// 关键步骤：生成JDK动态代理
				autowiringValue = AutowireUtils.resolveAutowiringValue(autowiringValue, requiredType);
				if (requiredType.isInstance(autowiringValue)) {
					result.put(ObjectUtils.identityToString(autowiringValue), autowiringValue);
					break;
				}
			}
		}
		for (String candidate : candidateNames) {
			if (!isSelfReference(beanName, candidate) && isAutowireCandidate(candidate, descriptor)) {
				addCandidateEntry(result, candidate, descriptor, requiredType);
			}
		}
		if (result.isEmpty() && !indicatesMultipleBeans(requiredType)) {
			// Consider fallback matches if the first pass failed to find anything...
			DependencyDescriptor fallbackDescriptor = descriptor.forFallbackMatch();
			for (String candidate : candidateNames) {
				if (!isSelfReference(beanName, candidate) && isAutowireCandidate(candidate, fallbackDescriptor)) {
					addCandidateEntry(result, candidate, descriptor, requiredType);
				}
			}
			if (result.isEmpty()) {
				// Consider self references as a final pass...
				// but in the case of a dependency collection, not the very same bean itself.
				for (String candidate : candidateNames) {
					if (isSelfReference(beanName, candidate) &&
							(!(descriptor instanceof MultiElementDescriptor) || !beanName.equals(candidate)) &&
							isAutowireCandidate(candidate, fallbackDescriptor)) {
						addCandidateEntry(result, candidate, descriptor, requiredType);
					}
				}
			}
		}
		return result;
	}
		
}

AutowireUtils:

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abstract class AutowireUtils {
   
   /**
	 * Resolve the given autowiring value against the given required type,
	 * e.g. an {@link ObjectFactory} value to its actual object result.
	 * @param autowiringValue the value to resolve
	 * @param requiredType the type to assign the result to
	 * @return the resolved value
	 */
	public static Object resolveAutowiringValue(Object autowiringValue, Class<?> requiredType) {
		if (autowiringValue instanceof ObjectFactory && !requiredType.isInstance(autowiringValue)) {
			ObjectFactory<?> factory = (ObjectFactory<?>) autowiringValue;
			if (autowiringValue instanceof Serializable && requiredType.isInterface()) {
			    // 如果是接口则生成基于JDK的动态代理
				autowiringValue = Proxy.newProxyInstance(requiredType.getClassLoader(),
						new Class<?>[] {requiredType}, new ObjectFactoryDelegatingInvocationHandler(factory));
			}
			else {
			    // 不是接口则返回factory.getObject()对象
				return factory.getObject();
			}
		}
		// 返回原始值
		return autowiringValue;
	}
	
	/**
	 * Reflective InvocationHandler for lazy access to the current target object.
	 */
	@SuppressWarnings("serial")
	private static class ObjectFactoryDelegatingInvocationHandler implements InvocationHandler, Serializable {

		private final ObjectFactory<?> objectFactory;

		public ObjectFactoryDelegatingInvocationHandler(ObjectFactory<?> objectFactory) {
			this.objectFactory = objectFactory;
		}

		@Override
		public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
			String methodName = method.getName();
			if (methodName.equals("equals")) {
				// Only consider equal when proxies are identical.
				return (proxy == args[0]);
			}
			else if (methodName.equals("hashCode")) {
				// Use hashCode of proxy.
				return System.identityHashCode(proxy);
			}
			else if (methodName.equals("toString")) {
				return this.objectFactory.toString();
			}
			try {
			    // 通过反射调用，如RequestObjectFactory等中的方法
				return method.invoke(this.objectFactory.getObject(), args);
			}
			catch (InvocationTargetException ex) {
				throw ex.getTargetException();
			}
		}
	}

}

我们在回到WebApplicationContextUtils类中的RequestObjectFactory，看下其具体的实现。

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public abstract class WebApplicationContextUtils {

	/**
	 * Factory that exposes the current request object on demand.
	 */
	@SuppressWarnings("serial")
	private static class RequestObjectFactory implements ObjectFactory<ServletRequest>, Serializable {

		@Override
		public ServletRequest getObject() {
			return currentRequestAttributes().getRequest();
		}

		@Override
		public String toString() {
			return "Current HttpServletRequest";
		}
	}
	
    /**
     * RequestContextHolder 从当前请求上下文中获取真实的Request，Response，Session等对象
	 * Return the current RequestAttributes instance as ServletRequestAttributes.
	 * @see RequestContextHolder#currentRequestAttributes()
	 */
	private static ServletRequestAttributes currentRequestAttributes() {
		RequestAttributes requestAttr = RequestContextHolder.currentRequestAttributes();
		if (!(requestAttr instanceof ServletRequestAttributes)) {
			throw new IllegalStateException("Current request is not a servlet request");
		}
		return (ServletRequestAttributes) requestAttr;
	}

}

请求线程上下文管理器RequestContextHolder：

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public abstract class RequestContextHolder  {
	private static final ThreadLocal<RequestAttributes> requestAttributesHolder =
			new NamedThreadLocal<RequestAttributes>("Request attributes");

	private static final ThreadLocal<RequestAttributes> inheritableRequestAttributesHolder =
			new NamedInheritableThreadLocal<RequestAttributes>("Request context");
			
	/**
	 * Bind the given RequestAttributes to the current thread.
	 * @param attributes the RequestAttributes to expose,
	 * or {@code null} to reset the thread-bound context
	 * @param inheritable whether to expose the RequestAttributes as inheritable
	 * for child threads (using an {@link InheritableThreadLocal})
	 */
	public static void setRequestAttributes(RequestAttributes attributes, boolean inheritable) {
		if (attributes == null) {
			resetRequestAttributes();
		}
		else {
			if (inheritable) {
				inheritableRequestAttributesHolder.set(attributes);
				requestAttributesHolder.remove();
			}
			else {
				requestAttributesHolder.set(attributes);
				inheritableRequestAttributesHolder.remove();
			}
		}
	}

	/**
	 * Return the RequestAttributes currently bound to the thread.
	 * @return the RequestAttributes currently bound to the thread,
	 * or {@code null} if none bound
	 */
	public static RequestAttributes getRequestAttributes() {
		RequestAttributes attributes = requestAttributesHolder.get();
		if (attributes == null) {
			attributes = inheritableRequestAttributesHolder.get();
		}
		return attributes;
	}

	/**
	 * Return the RequestAttributes currently bound to the thread.
	 * <p>Exposes the previously bound RequestAttributes instance, if any.
	 * Falls back to the current JSF FacesContext, if any.
	 * @return the RequestAttributes currently bound to the thread
	 * @throws IllegalStateException if no RequestAttributes object
	 * is bound to the current thread
	 * @see #setRequestAttributes
	 * @see ServletRequestAttributes
	 * @see FacesRequestAttributes
	 * @see javax.faces.context.FacesContext#getCurrentInstance()
	 */
	public static RequestAttributes currentRequestAttributes() throws IllegalStateException {
		RequestAttributes attributes = getRequestAttributes();
		if (attributes == null) {
			if (jsfPresent) {
				attributes = FacesRequestAttributesFactory.getFacesRequestAttributes();
			}
			if (attributes == null) {
				throw new IllegalStateException("No thread-bound request found: " +
						"Are you referring to request attributes outside of an actual web request, " +
						"or processing a request outside of the originally receiving thread? " +
						"If you are actually operating within a web request and still receive this message, " +
						"your code is probably running outside of DispatcherServlet/DispatcherPortlet: " +
						"In this case, use RequestContextListener or RequestContextFilter to expose the current request.");
			}
		}
		return attributes;
	}

}

当请求进来时，设置当前请求的上下文信息，RequestContextFilter类。RequestContextFilter：

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public class RequestContextFilter extends OncePerRequestFilter {
   @Override
	protected void doFilterInternal(
			HttpServletRequest request, HttpServletResponse response, FilterChain filterChain)
			throws ServletException, IOException {

		ServletRequestAttributes attributes = new ServletRequestAttributes(request, response);
		initContextHolders(request, attributes);

		try {
			filterChain.doFilter(request, response);
		}
		finally {
			resetContextHolders();
			if (logger.isDebugEnabled()) {
				logger.debug("Cleared thread-bound request context: " + request);
			}
			attributes.requestCompleted();
		}
	}

    // 设置请求上下文信息
	private void initContextHolders(HttpServletRequest request, ServletRequestAttributes requestAttributes) {
		LocaleContextHolder.setLocale(request.getLocale(), this.threadContextInheritable);
		RequestContextHolder.setRequestAttributes(requestAttributes, this.threadContextInheritable);
		if (logger.isDebugEnabled()) {
			logger.debug("Bound request context to thread: " + request);
		}
	}

	private void resetContextHolders() {
		LocaleContextHolder.resetLocaleContext();
		RequestContextHolder.resetRequestAttributes();
	}
	
}

至此，怎个流程算是串起来了，之所以不会存在线程安全问题，那是因为注入的是实现了HttpServletRequest， HttpServletResponse接口的动态代理对象，然后在这个动态代理对象中执行invoke时，获取当前线程上下文所关联的实际request，response等对象，再反射执行其中的方法。整体设计非常巧妙，设计思想值得学习和借鉴。

后记

objectFactory有实现类：org.springframework.beans.factory.support.DefaultListableBeanFactory.DependencyObjectProvider

通过它，可以实现bean的懒加载。即，第一次调用getObject时，解析所有spring管理的泛型bean，返回。