8.SpringMVC源码剖析其运行过程

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探索SpringMVC的秘密

我们都知道在Springmvc中前端控制器，是执行的核心控制器，从继承关系上看其实DispatcherServlet就是一个servlet。这时候我们回顾一下DispatcherServlet是怎么配置的？是不是在<servlet></servlet>里面配置的，我们刚学习servlet的时候是不是自定义servlet也是在web.xml中通过<servlet></servlet>配置的呢？如果你很熟悉javaweb基础，你会回答yes

我们学过javaweb部分都知道servlet的生命周期中有一个service方法是提供服务的方法，每次访问servlet的时候，service方法就会被调用一次。

在FrameworkServlet中有service方法，源码如下：

重点关注：

• processRequest(request, response);

• super.service(request, response);

/**
 * Override the parent class implementation in order to intercept PATCH
 * requests.
 * 重写父类实现以拦截PATCH请求。
 */
@Override
protected void service(HttpServletRequest request, HttpServletResponse response)
		throws ServletException, IOException {

	String method = request.getMethod();
	if (method.equalsIgnoreCase(RequestMethod.PATCH.name())) {
		processRequest(request, response);
	}
	else {
           // 
		super.service(request, response);
	}
}

这里做了 一下请求方式的判断如果请求的方式是下面这几种的话调用processRequest()方法，否则调用父类的service方法，即HttpServlet的service方法。

public enum RequestMethod {
	GET, HEAD, POST, PUT, PATCH, DELETE, OPTIONS, TRACE
}

这里我们进入processRequest()方法，源码如下：

处理此请求，发布事件，无论结果如何。

实际的事件处理由抽象的doService模板方法执行，所以我们只需要关注其中调用的doService()方法，其他代码一笑而过...

/**
	 * Process this request, publishing an event regardless of the outcome.
	 * <p>The actual event handling is performed by the abstract
	 * {@link #doService} template method.
	 */
	protected final void processRequest(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {

		long startTime = System.currentTimeMillis();
		Throwable failureCause = null;

		LocaleContext previousLocaleContext = LocaleContextHolder.getLocaleContext();
		LocaleContext localeContext = buildLocaleContext(request);

		RequestAttributes previousAttributes = RequestContextHolder.getRequestAttributes();
		ServletRequestAttributes requestAttributes = buildRequestAttributes(request, response, previousAttributes);

		WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
		asyncManager.registerCallableInterceptor(FrameworkServlet.class.getName(), new RequestBindingInterceptor());

		initContextHolders(request, localeContext, requestAttributes);

		try {
			doService(request, response);
		}
		catch (ServletException ex) {
			failureCause = ex;
			throw ex;
		}
		catch (IOException ex) {
			failureCause = ex;
			throw ex;
		}
		catch (Throwable ex) {
			failureCause = ex;
			throw new NestedServletException("Request processing failed", ex);
		}

		finally {
			resetContextHolders(request, previousLocaleContext, previousAttributes);
			if (requestAttributes != null) {
				requestAttributes.requestCompleted();
			}

			if (logger.isDebugEnabled()) {
				if (failureCause != null) {
					this.logger.debug("Could not complete request", failureCause);
				}
				else {
					if (asyncManager.isConcurrentHandlingStarted()) {
						logger.debug("Leaving response open for concurrent processing");
					}
					else {
						this.logger.debug("Successfully completed request");
					}
				}
			}

			publishRequestHandledEvent(request, startTime, failureCause);
		}
	}

点击doService()方法之后，我们发现它在 FrameworkServlet中是一个抽象的方法，所以它的子类一定会实现它，我们回到DispatcherServlet中寻找doService()方法，其源代码如下：

公开特定于DispatcherServlet的请求属性，并将其委托给doDispatch()进行实际的调度,同上我们只需要找到doDispatch()方法，其他代码一笑而过....

/**
 * Exposes the DispatcherServlet-specific request attributes and delegates to {@link 	  *#doDispatch} for the actual dispatching.
 */
@Override
protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
	if (logger.isDebugEnabled()) {
		String requestUri = urlPathHelper.getRequestUri(request);
		String resumed = WebAsyncUtils.getAsyncManager(request).hasConcurrentResult() ? " resumed" : "";
		logger.debug("DispatcherServlet with name '" + getServletName() + "'" + resumed +
				" processing " + request.getMethod() + " request for [" + requestUri + "]");
	}

	// Keep a snapshot of the request attributes in case of an include,
	// to be able to restore the original attributes after the include.
	Map<String, Object> attributesSnapshot = null;
	if (WebUtils.isIncludeRequest(request)) {
		logger.debug("Taking snapshot of request attributes before include");
		attributesSnapshot = new HashMap<String, Object>();
		Enumeration<?> attrNames = request.getAttributeNames();
		while (attrNames.hasMoreElements()) {
			String attrName = (String) attrNames.nextElement();
			if (this.cleanupAfterInclude || attrName.startsWith("org.springframework.web.servlet")) {
				attributesSnapshot.put(attrName, request.getAttribute(attrName));
			}
		}
	}

	// Make framework objects available to handlers and view objects.
	request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE, getWebApplicationContext());
	request.setAttribute(LOCALE_RESOLVER_ATTRIBUTE, this.localeResolver);
	request.setAttribute(THEME_RESOLVER_ATTRIBUTE, this.themeResolver);
	request.setAttribute(THEME_SOURCE_ATTRIBUTE, getThemeSource());

	FlashMap inputFlashMap = this.flashMapManager.retrieveAndUpdate(request, response);
	if (inputFlashMap != null) {
		request.setAttribute(INPUT_FLASH_MAP_ATTRIBUTE, Collections.unmodifiableMap(inputFlashMap));
	}
	request.setAttribute(OUTPUT_FLASH_MAP_ATTRIBUTE, new FlashMap());
	request.setAttribute(FLASH_MAP_MANAGER_ATTRIBUTE, this.flashMapManager);

	try {
		doDispatch(request, response);
	}
	finally {
		if (WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
			return;
		}
		// Restore the original attribute snapshot, in case of an include.
		if (attributesSnapshot != null) {
			restoreAttributesAfterInclude(request, attributesSnapshot);
		}
	}
}

doDispatch()方法

当我们进入doDispatch()方法，下面好戏就开始了….

下面的内容是翻译自doDispatch()方法的注释：

1. 将实际的分派处理程序处理。
2. 该处理程序将通过依次应用servlet的处理程序映射来获得。
3. HandlerAdapter将通过查询servlet已安装的HandlerAdapter来找到第一个支持handler类的HandlerAdapter。
4. 所有HTTP方法都由此方法处理。
5. 由处理器适配器或处理器本身来决定哪些方法是可接受的。

特别是第3点，是促使我探究源码的原因，因为这里使用了设计模式中适配器模式的思想实现的。

下面将对这个方法进行详细的解释：

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.
                // 确定当前请求的处理程序。
				mappedHandler = getHandler(processedRequest);
				if (mappedHandler == null || mappedHandler.getHandler() == null) {
					noHandlerFound(processedRequest, response);
					return;
				}

				// Determine handler adapter for the current request.
				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()) {
						String requestUri = urlPathHelper.getRequestUri(request);
						logger.debug("Last-Modified value for [" + requestUri + "] is: " + lastModified);
					}
					if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
						return;
					}
				}

				if (!mappedHandler.applyPreHandle(processedRequest, response)) {
					return;
				}

				try {
					// Actually invoke the handler.
                    // 实际调用处理程序。
					mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
				}
				finally {
					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
				mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
				return;
			}
			// Clean up any resources used by a multipart request.
			if (multipartRequestParsed) {
				cleanupMultipart(processedRequest);
			}
		}
	}

确定当前请求的处理程序getHandler

mappedHandler = getHandler(processedRequest);

我们进入到DispatcherServlet的getHandler()方法，源码如下：

1. 返回此请求的HandlerExecutionChain即处理程序执行链，由处理程序对象和任何处理程序拦截器组成。
2. 依次尝试所有处理程序映射。
3. 进入到此方法中就拿到this.handlerMappings,而this.handlerMappings是此Servlet使用的HandlerMapping列表。
4. 
5. 遍历this.handlerMappings拿到处理程序执行链，我测试使用的控制器是使用注解@Controller进行配置的，所以通过RequestMappingHandlerMapping就拿到了处理程序执行链。当然下面那个也是有用的，具体看我们的控制器是有什么方式配置的。

/**
 * Return the HandlerExecutionChain for this request.
 * <p>Tries all handler mappings in order.
 * @param request current HTTP request
 * @return the HandlerExecutionChain, or {@code null} if no handler could be found
 */
protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
	for (HandlerMapping hm : this.handlerMappings) {
		if (logger.isTraceEnabled()) {
			logger.trace(
					"Testing handler map [" + hm + "] in DispatcherServlet with name '" + getServletName() + "'");
		}
		HandlerExecutionChain handler = hm.getHandler(request);
		if (handler != null) {
			return handler;
		}
	}
	return null;
}

返回结果为：处理程序执行链HandlerExecutionChain。

确定当前请求的处理程序适配器getHandlerAdapter

HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

返回此处理程序对象的HandlerAdapter。根据不同的handler获取不同的handlerAdapter。

结果返回的是：RequestMappingHandlerAdapter

/**
	 * Return the HandlerAdapter for this handler object.
	 * @param handler the handler object to find an adapter for
	 * @throws ServletException if no HandlerAdapter can be found for the handler. This is a fatal error.
	 */
	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)) {
				return ha;
			}
		}
		throw new ServletException("No adapter for handler [" + handler +
				"]: The DispatcherServlet configuration needs to include a HandlerAdapter that supports this handler");
	}

通过拿到的RequestMappingHandlerAdapter去执行handler，即去执行控制器也就是我配置的HelloController

实际调用处理程序。
mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

到了这一步我们就得到了ModelAndView对象，然后根据ModelAndView对象去找对应的视图解析器去解析视图，返回逻辑视图，然后渲染视图返回就可以了。

小结

如果你跟着上面的思路，并熟悉配置Controller，ModelAndView，视图解析器，设计模式-适配器模式的话再结合着下面这张图，我相信你会非常通透。

附录：

DispatcherServlet.properties

两种HandlerMapping

1. BeanNameUrlHandlerMapping
2. DefaultAnnotationHandlerMapping

三种HandlerAdapter

1. HttpRequestHandlerAdapter
2. SimpleControllerHandlerAdapter
3. AnnotationMethodHandlerAdapter

org.springframework.web.servlet.HandlerMapping=org.springframework.web.servlet.handler.BeanNameUrlHandlerMapping,\
	org.springframework.web.servlet.mvc.annotation.DefaultAnnotationHandlerMapping

org.springframework.web.servlet.HandlerAdapter=org.springframework.web.servlet.mvc.HttpRequestHandlerAdapter,\
	org.springframework.web.servlet.mvc.SimpleControllerHandlerAdapter,\
	org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapter

定义的Controller

调试的过程中断点大多在都doDispatch()方法中。

/**
 * @Date 2020/6/16 17:22
 * @Version 10.21
 * @Author DuanChaojie
 */
@Controller
@RequestMapping("/user")
public class HelloWorldController {

    @RequestMapping(value = "/justweb" )
    public String hello(){
        System.out.println("使用注解配置的Controller，访问成功了...\n\r开始源码分析....");
        return "success";
    }

}