SpringMVC源码解析-HTTP请求处理和分发

1.HandlerMapping的配置和设计

在初始化完成时，所有的handlerMapping都已经被加载，handlerMapping存储着HTTP请求对应的映射数据，每一个handlerMapping持有从URL请求到Controller的映射。

这里以SimpleUrlHandlerMapping为例来分析它的设计与实现。在SimpleUrlHandlerMapping中，定义了一个map来维持映射关系，即URL请求和控制器对应关系，

是SpringMVC应用可以根据HTTP请求确定对一个对应的Controller，具体来说，这些映射关系是通过接口类HandlerMapping来封装，在HandlerMapping接口中定义了一个getHandler方法，

通过这个方法，可以获得与HTTP请求对应的HandlerExecutionChain,在这个HandlerExecutionChain中，封装具体的Controller对象。

SimpleUrlHandlerMapping中的urlMap:

public class SimpleUrlHandlerMapping extends AbstractUrlHandlerMapping {
       //url和controller映射的map
	private final Map<String, Object> urlMap = new HashMap<String, Object>();

HandlerMapping中的getHandler

public interface HandlerMapping {
        //省略。。。
 
	HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception;
 
}

　接下来，看一下HandlerExecutionChain的实现。

public class HandlerExecutionChain {
 
	private static final Log logger = LogFactory.getLog(HandlerExecutionChain.class);
 
	private final Object handler;//处理对象，也就是controller对象
 
	private HandlerInterceptor[] interceptors;//拦截器
 
	private List<HandlerInterceptor> interceptorList;//拦截器
 
	private int interceptorIndex = -1;
 
	/**
	 * Create a new HandlerExecutionChain.
	 * @param handler the handler object to execute
	 */
	public HandlerExecutionChain(Object handler) {
		this(handler, (HandlerInterceptor[]) null);
	}
 
	/**
	 * Create a new HandlerExecutionChain.
	 * @param handler the handler object to execute
	 * @param interceptors the array of interceptors to apply
	 * (in the given order) before the handler itself executes
	 *///初始化
	public HandlerExecutionChain(Object handler, HandlerInterceptor... interceptors) {
		if (handler instanceof HandlerExecutionChain) {
			HandlerExecutionChain originalChain = (HandlerExecutionChain) handler;
			this.handler = originalChain.getHandler();
			this.interceptorList = new ArrayList<HandlerInterceptor>();
			CollectionUtils.mergeArrayIntoCollection(originalChain.getInterceptors(), this.interceptorList);
			CollectionUtils.mergeArrayIntoCollection(interceptors, this.interceptorList);
		}
		else {
			this.handler = handler;
			this.interceptors = interceptors;
		}
	}
 
	/**
	 * Return the handler object to execute.
	 * @return the handler object
	 *///获取handler，本质就是controller对象
	public Object getHandler() {
		return this.handler;
	}
　　　　//添加拦截器
	public void addInterceptor(HandlerInterceptor interceptor) {
		initInterceptorList().add(interceptor);
	}
       //添加多个拦截器
	public void addInterceptors(HandlerInterceptor... interceptors) {
		if (!ObjectUtils.isEmpty(interceptors)) {
			initInterceptorList().addAll(Arrays.asList(interceptors));
		}
	}
 
	private List<HandlerInterceptor> initInterceptorList() {
		if (this.interceptorList == null) {
			this.interceptorList = new ArrayList<HandlerInterceptor>();
			if (this.interceptors != null) {
				// An interceptor array specified through the constructor
				this.interceptorList.addAll(Arrays.asList(this.interceptors));
			}
		}
		this.interceptors = null;
		return this.interceptorList;
	}
 
	/**
	 * Return the array of interceptors to apply (in the given order).
	 * @return the array of HandlerInterceptors instances (may be {@code null})
	 */
	public HandlerInterceptor[] getInterceptors() {
		if (this.interceptors == null && this.interceptorList != null) {
			this.interceptors = this.interceptorList.toArray(new HandlerInterceptor[this.interceptorList.size()]);
		}
		return this.interceptors;
	}
 
	/**
	 * Apply preHandle methods of registered interceptors.
	 * @return {@code true} if the execution chain should proceed with the
	 * next interceptor or the handler itself. Else, DispatcherServlet assumes
	 * that this interceptor has already dealt with the response itself.
	 *///执行之前调用拦截器
	boolean applyPreHandle(HttpServletRequest request, HttpServletResponse response) throws Exception {
		HandlerInterceptor[] interceptors = getInterceptors();
		if (!ObjectUtils.isEmpty(interceptors)) {
			for (int i = 0; i < interceptors.length; i++) {
				HandlerInterceptor interceptor = interceptors[i];
				if (!interceptor.preHandle(request, response, this.handler)) {
					triggerAfterCompletion(request, response, null);
					return false;
				}
				this.interceptorIndex = i;
			}
		}
		return true;
	}
 
	/**
	 * Apply postHandle methods of registered interceptors.
	 *///执行之后调用拦截器
	void applyPostHandle(HttpServletRequest request, HttpServletResponse response, ModelAndView mv) throws Exception {
		HandlerInterceptor[] interceptors = getInterceptors();
		if (!ObjectUtils.isEmpty(interceptors)) {
			for (int i = interceptors.length - 1; i >= 0; i--) {
				HandlerInterceptor interceptor = interceptors[i];
				interceptor.postHandle(request, response, this.handler, mv);
			}
		}
	}
 
	/**
	 * Trigger afterCompletion callbacks on the mapped HandlerInterceptors.
	 * Will just invoke afterCompletion for all interceptors whose preHandle invocation
	 * has successfully completed and returned true.
	 *///调用拦截器的afterCompletion方法
	void triggerAfterCompletion(HttpServletRequest request, HttpServletResponse response, Exception ex)
			throws Exception {
 
		HandlerInterceptor[] interceptors = getInterceptors();
		if (!ObjectUtils.isEmpty(interceptors)) {
			for (int i = this.interceptorIndex; i >= 0; i--) {
				HandlerInterceptor interceptor = interceptors[i];
				try {
					interceptor.afterCompletion(request, response, this.handler, ex);
				}
				catch (Throwable ex2) {
					logger.error("HandlerInterceptor.afterCompletion threw exception", ex2);
				}
			}
		}
	}
 
	/**
	 * Apply afterConcurrentHandlerStarted callback on mapped AsyncHandlerInterceptors.
	 */
	void applyAfterConcurrentHandlingStarted(HttpServletRequest request, HttpServletResponse response) {
		HandlerInterceptor[] interceptors = getInterceptors();
		if (!ObjectUtils.isEmpty(interceptors)) {
			for (int i = interceptors.length - 1; i >= 0; i--) {
				if (interceptors[i] instanceof AsyncHandlerInterceptor) {
					try {
						AsyncHandlerInterceptor asyncInterceptor = (AsyncHandlerInterceptor) interceptors[i];
						asyncInterceptor.afterConcurrentHandlingStarted(request, response, this.handler);
					}
					catch (Throwable ex) {
						logger.error("Interceptor [" + interceptors[i] + "] failed in afterConcurrentHandlingStarted", ex);
					}
				}
			}
		}
	}
 
	/**
	 * Delegates to the handler's {@code toString()}.
	 */
	@Override
	public String toString() {
		if (this.handler == null) {
			return "HandlerExecutionChain with no handler";
		}
		StringBuilder sb = new StringBuilder();
		sb.append("HandlerExecutionChain with handler [").append(this.handler).append("]");
		if (!CollectionUtils.isEmpty(this.interceptorList)) {
			sb.append(" and ").append(this.interceptorList.size()).append(" interceptor");
			if (this.interceptorList.size() > 1) {
				sb.append("s");
			}
		}
		return sb.toString();
	}
 
}

　下面是拦截器的执行图

对应的applyPreHandle和applyPostHandle方法代码如下：

/**
	 * Apply preHandle methods of registered interceptors.
	 * @return {@code true} if the execution chain should proceed with the
	 * next interceptor or the handler itself. Else, DispatcherServlet assumes
	 * that this interceptor has already dealt with the response itself.
	 */
	boolean applyPreHandle(HttpServletRequest request, HttpServletResponse response) throws Exception {
		HandlerInterceptor[] interceptors = getInterceptors();
		if (!ObjectUtils.isEmpty(interceptors)) {
			for (int i = 0; i < interceptors.length; i++) {
				HandlerInterceptor interceptor = interceptors[i];
				if (!interceptor.preHandle(request, response, this.handler)) {
					triggerAfterCompletion(request, response, null);
					return false;
				}
				this.interceptorIndex = i;
			}
		}
		return true;
	}
 
	/**
	 * Apply postHandle methods of registered interceptors.
	 */
	void applyPostHandle(HttpServletRequest request, HttpServletResponse response, ModelAndView mv) throws Exception {
		HandlerInterceptor[] interceptors = getInterceptors();
		if (!ObjectUtils.isEmpty(interceptors)) {
			for (int i = interceptors.length - 1; i >= 0; i--) {
				HandlerInterceptor interceptor = interceptors[i];
				interceptor.postHandle(request, response, this.handler, mv);
			}
		}
	}

　　回到SimpleUrlHandlerMapping中，有一个initApplicationContext方法

/**
	 * Calls the {@link #registerHandlers} method in addition to the
	 * superclass's initialization.
	 */
	@Override
	public void initApplicationContext() throws BeansException {
		super.initApplicationContext();
		registerHandlers(this.urlMap);
	}

　　继续看看registerHandlers方法，注册Handler

protected void registerHandlers(Map<String, Object> urlMap) throws BeansException {
		if (urlMap.isEmpty()) {
			logger.warn("Neither 'urlMap' nor 'mappings' set on SimpleUrlHandlerMapping");
		}
		else {
			for (Map.Entry<String, Object> entry : urlMap.entrySet()) {
				String url = entry.getKey();
				Object handler = entry.getValue();
				// Prepend with slash if not already present.
				if (!url.startsWith("/")) {//不是以"/"开头的，加上"/"
					url = "/" + url;
				}
				// Remove whitespace from handler bean name.
				if (handler instanceof String) {
					handler = ((String) handler).trim();
				}
				registerHandler(url, handler);
			}
		}
	}

　　AbstractUrlHandlerMapping对handler的注册

/**
	 * Register the specified handler for the given URL path.
	 * @param urlPath the URL the bean should be mapped to
	 * @param handler the handler instance or handler bean name String
	 * (a bean name will automatically be resolved into the corresponding handler bean)
	 * @throws BeansException if the handler couldn't be registered
	 * @throws IllegalStateException if there is a conflicting handler registered
	 */
	protected void registerHandler(String urlPath, Object handler) throws BeansException, IllegalStateException {
		Assert.notNull(urlPath, "URL path must not be null");
		Assert.notNull(handler, "Handler object must not be null");
		Object resolvedHandler = handler;
 
		// Eagerly resolve handler if referencing singleton via name.
		if (!this.lazyInitHandlers && handler instanceof String) {
			String handlerName = (String) handler;
			if (getApplicationContext().isSingleton(handlerName)) {
				resolvedHandler = getApplicationContext().getBean(handlerName);//直接从spring容器中获取handler
			}
		}
 
		Object mappedHandler = this.handlerMap.get(urlPath);//从map中获取handler
		if (mappedHandler != null) {
			if (mappedHandler != resolvedHandler) {
				throw new IllegalStateException(
						"Cannot map " + getHandlerDescription(handler) + " to URL path [" + urlPath +
						"]: There is already " + getHandlerDescription(mappedHandler) + " mapped.");
			}
		}
		else {
			if (urlPath.equals("/")) {
				if (logger.isInfoEnabled()) {
					logger.info("Root mapping to " + getHandlerDescription(handler));
				}
				setRootHandler(resolvedHandler);//url是"/"，将其设置为根handler
			}
			else if (urlPath.equals("/*")) {
				if (logger.isInfoEnabled()) {
					logger.info("Default mapping to " + getHandlerDescription(handler));
				}
				setDefaultHandler(resolvedHandler);//默认handler
			}
			else {
				this.handlerMap.put(urlPath, resolvedHandler);//注册handler，url为key，controller为value的map
				if (logger.isInfoEnabled()) {
					logger.info("Mapped URL path [" + urlPath + "] onto " + getHandlerDescription(handler));
				}
			}
		}
	}

　　有了上面的配置好了Url和Controller的映射关系，为SpringMVC响应HTTP请求准备好了基础的映射数据，至此SpringMVC就可以等待HTTP的请求的到来。

2.使用HandlerMapping完成请求的映射处理

当请求来时，首先AbstractHandlerMapping的getHandler被调用,方法参数是request

@Override
	public final HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
		Object handler = getHandlerInternal(request);
		if (handler == null) {//为空走默认的handler
			handler = getDefaultHandler();
		}
		if (handler == null) {
			return null;
		}
		// Bean name or resolved handler?
		if (handler instanceof String) {
			String handlerName = (String) handler;//从spring容器中获取
			handler = getApplicationContext().getBean(handlerName);
		}
		return getHandlerExecutionChain(handler, request);//把handler封装到HandlerExecutionChain中并加上拦截器
	}

　　封装HandlerExecutionChain

protected HandlerExecutionChain getHandlerExecutionChain(Object handler, HttpServletRequest request) {
		HandlerExecutionChain chain = (handler instanceof HandlerExecutionChain ?
				(HandlerExecutionChain) handler : new HandlerExecutionChain(handler));
		chain.addInterceptors(getAdaptedInterceptors());
 
		String lookupPath = this.urlPathHelper.getLookupPathForRequest(request);
		for (MappedInterceptor mappedInterceptor : this.mappedInterceptors) {
			if (mappedInterceptor.matches(lookupPath, this.pathMatcher)) {
				chain.addInterceptor(mappedInterceptor.getInterceptor());
			}
		}
 
		return chain;
	}

　　上面的getHnadler会调用getHandlerInternal

protected Object getHandlerInternal(HttpServletRequest request) throws Exception {
		String lookupPath = getUrlPathHelper().getLookupPathForRequest(request);
		Object handler = lookupHandler(lookupPath, request);
		if (handler == null) {
			// We need to care for the default handler directly, since we need to
			// expose the PATH_WITHIN_HANDLER_MAPPING_ATTRIBUTE for it as well.
			Object rawHandler = null;
			if ("/".equals(lookupPath)) {
				rawHandler = getRootHandler();
			}
			if (rawHandler == null) {
				rawHandler = getDefaultHandler();
			}
			if (rawHandler != null) {
				// Bean name or resolved handler?
				if (rawHandler instanceof String) {
					String handlerName = (String) rawHandler;
					rawHandler = getApplicationContext().getBean(handlerName);
				}
				validateHandler(rawHandler, request);
				handler = buildPathExposingHandler(rawHandler, lookupPath, lookupPath, null);
			}
		}
		if (handler != null && logger.isDebugEnabled()) {
			logger.debug("Mapping [" + lookupPath + "] to " + handler);
		}
		else if (handler == null && logger.isTraceEnabled()) {
			logger.trace("No handler mapping found for [" + lookupPath + "]");
		}
		return handler;
	}

　　这里调用的是lookupHandler方法，在来看一下这个方法。

protected Object lookupHandler(String urlPath, HttpServletRequest request) throws Exception {
		// Direct match?
		Object handler = this.handlerMap.get(urlPath);
		if (handler != null) {
			// Bean name or resolved handler?
			if (handler instanceof String) {
				String handlerName = (String) handler;
				handler = getApplicationContext().getBean(handlerName);
			}
			validateHandler(handler, request);
			return buildPathExposingHandler(handler, urlPath, urlPath, null);
		}
		// Pattern match?
		List<String> matchingPatterns = new ArrayList<String>();
		for (String registeredPattern : this.handlerMap.keySet()) {
			if (getPathMatcher().match(registeredPattern, urlPath)) {
				matchingPatterns.add(registeredPattern);
			}
		}
		String bestPatternMatch = null;
		Comparator<String> patternComparator = getPathMatcher().getPatternComparator(urlPath);
		if (!matchingPatterns.isEmpty()) {
			Collections.sort(matchingPatterns, patternComparator);
			if (logger.isDebugEnabled()) {
				logger.debug("Matching patterns for request [" + urlPath + "] are " + matchingPatterns);
			}
			bestPatternMatch = matchingPatterns.get(0);
		}
		if (bestPatternMatch != null) {
			handler = this.handlerMap.get(bestPatternMatch);
			// Bean name or resolved handler?
			if (handler instanceof String) {
				String handlerName = (String) handler;
				handler = getApplicationContext().getBean(handlerName);
			}
			validateHandler(handler, request);
			String pathWithinMapping = getPathMatcher().extractPathWithinPattern(bestPatternMatch, urlPath);
 
			// There might be multiple 'best patterns', let's make sure we have the correct URI template variables
			// for all of them
			Map<String, String> uriTemplateVariables = new LinkedHashMap<String, String>();
			for (String matchingPattern : matchingPatterns) {
				if (patternComparator.compare(bestPatternMatch, matchingPattern) == 0) {
					Map<String, String> vars = getPathMatcher().extractUriTemplateVariables(matchingPattern, urlPath);
					Map<String, String> decodedVars = getUrlPathHelper().decodePathVariables(request, vars);
					uriTemplateVariables.putAll(decodedVars);
				}
			}
			if (logger.isDebugEnabled()) {
				logger.debug("URI Template variables for request [" + urlPath + "] are " + uriTemplateVariables);
			}
			return buildPathExposingHandler(handler, bestPatternMatch, pathWithinMapping, uriTemplateVariables);
		}
		// No handler found...
		return null;
	}

　　经过一系列的对HTTP请求进行解析和匹配handler的过程，得到了与请求对应的处理器。在返回handler中，已经完成了对HandlerExecutionChain进行封装工作，为HTTP请求响应做准备。

3.SpringMVC对HTTP请求的分发处理

回到DispatcherServlet中，对HTTP的请求是在doService中，DispatcherServlet是HttpServlet的子类，和其他Servlet一样，通过doService来响应HTTP请求。doService直接调用的是doDispatch方法

主要看一下这个方法。

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
		HttpServletRequest processedRequest = request;
		HandlerExecutionChain mappedHandler = null;
		boolean multipartRequestParsed = false;
 
		WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
 
		try {
			ModelAndView mv = null;
			Exception dispatchException = null;
 
			try {
				processedRequest = checkMultipart(request);
				multipartRequestParsed = (processedRequest != request);
 
				// Determine handler for the current request.
				mappedHandler = getHandler(processedRequest);//获取handler
				if (mappedHandler == null || mappedHandler.getHandler() == null) {
					noHandlerFound(processedRequest, response);
					return;
				}
 
				// Determine handler adapter for the current request.//获取adapter
				HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());
 
				// Process last-modified header, if supported by the handler.
				String method = request.getMethod();
				boolean isGet = "GET".equals(method);
				if (isGet || "HEAD".equals(method)) {
					long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
					if (logger.isDebugEnabled()) {
						logger.debug("Last-Modified value for [" + getRequestUri(request) + "] is: " + lastModified);
					}
					if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
						return;
					}
				}
 
				if (!mappedHandler.applyPreHandle(processedRequest, response)) {
					return;
				}
 
				// Actually invoke the handler.handler处理的结果封装到mv中
				mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
 
				if (asyncManager.isConcurrentHandlingStarted()) {
					return;
				}
 
				applyDefaultViewName(request, mv);
				mappedHandler.applyPostHandle(processedRequest, response, mv);
			}
			catch (Exception ex) {
				dispatchException = ex;
			}
			processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
		}
		catch (Exception ex) {
			triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
		}
		catch (Error err) {
			triggerAfterCompletionWithError(processedRequest, response, mappedHandler, err);
		}
		finally {
			if (asyncManager.isConcurrentHandlingStarted()) {
				// Instead of postHandle and afterCompletion
				if (mappedHandler != null) {
					mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
				}
			}
			else {
				// Clean up any resources used by a multipart request.
				if (multipartRequestParsed) {
					cleanupMultipart(processedRequest);
				}
			}
		}
	}

　　看一下getHandlerAdapter方法

protected HandlerAdapter getHandlerAdapter(Object handler) throws ServletException {
		for (HandlerAdapter ha : this.handlerAdapters) {
			if (logger.isTraceEnabled()) {
				logger.trace("Testing handler adapter [" + ha + "]");
			}
			if (ha.supports(handler)) {//判断是否支持
				return ha;
			}
		}
		throw new ServletException("No adapter for handler [" + handler +
				"]: The DispatcherServlet configuration needs to include a HandlerAdapter that supports this handler");
	}

　继续看一下supports方法的实现，以SimpleControlerHandlerAdapter类为例

public class SimpleControllerHandlerAdapter implements HandlerAdapter {
 
	@Override
	public boolean supports(Object handler) {
		return (handler instanceof Controller);
	}
 
	@Override
	public ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler)
			throws Exception {
 
		return ((Controller) handler).handleRequest(request, response);
	}
 
	@Override
	public long getLastModified(HttpServletRequest request, Object handler) {
		if (handler instanceof LastModified) {
			return ((LastModified) handler).getLastModified(request);
		}
		return -1L;
	}
 
}

　　经过上面一系列的操作，得到handler对象，handler调用handleRequest方法，返回ModelAndView对象，最后通过render方法进行渲染。

问题：handlerMapping是在什么时候初始化的？

在DispatcherServlet调用init->initStrategies方法中，进行初始化。

protected void initStrategies(ApplicationContext context) {
		initMultipartResolver(context);
		initLocaleResolver(context);
		initThemeResolver(context);
               //初始化handlerMapping
		initHandlerMappings(context);
		initHandlerAdapters(context);
		initHandlerExceptionResolvers(context);
		initRequestToViewNameTranslator(context);
		initViewResolvers(context);
		initFlashMapManager(context);
	}

　　最后一个请求的流程图。