Servlet

接口，一个规范，

SpringBoot

Spring Boot 是 Spring 的子项目，正如其名字，提供 Spring 的引导( Boot )的功能。

通过 Spring Boot ，开发者可以快速配置 Spring 项目，引入各种 Spring MVC、Spring Transaction、Spring AOP、等等框架，而无需不断重复编写繁重的 Spring 配置，降低了 Spring 的使用成本。

Servlet和SpringBoot

搭建一个SpringBoot项目：

引入依赖：

<dependency>
    <groupid>org.springframework.boot</groupid>
    <artifactid>spring-boot-starter-web</artifactid>
</dependency>

添加Controller

@Controller
public class IndexController {
 
    @RequestMapping("/hello")
    public String hello(){
        return "hello";
    }
}

添加启动类

@SpringBootApplication
public class DemoApplication {
    public static void main(String[] args) {
        SpringApplication.run(DemoApplication.class, args);
    }
}

运行DemoApplication.main即可启动一个简单的Springboot web 项目。

项目是通过一个内嵌的Tomcat容器来提供web服务。Tomcat是Servlet容器，SpringBoot想要使用Servlet容器就需要有servlet 那么SpringBoot中有哪些Servlet?是怎么通过这些Servlet来提供web服务的呢？

SpringBoot中的Servlet

SpringBoot中涉及到的Servlet及它们的关系

HttpServletBean

主要方法：

设置一些配置信息，并提供子类初始化入口。无具体业务逻辑

@Override
	public final void init() throws ServletException {

		// Set bean properties from init parameters.
		PropertyValues pvs = new ServletConfigPropertyValues(getServletConfig(), this.requiredProperties);
		if (!pvs.isEmpty()) {
			try {
				BeanWrapper bw = PropertyAccessorFactory.forBeanPropertyAccess(this);
				ResourceLoader resourceLoader = new ServletContextResourceLoader(getServletContext());
				bw.registerCustomEditor(Resource.class, new ResourceEditor(resourceLoader, getEnvironment()));
				initBeanWrapper(bw);
				bw.setPropertyValues(pvs, true);
			}
			catch (BeansException ex) {
				if (logger.isErrorEnabled()) {
					logger.error("Failed to set bean properties on servlet ‘" + getServletName() + "‘", ex);
				}
				throw ex;
			}
		}

		// Let subclasses do whatever initialization they like.
    //增加子类实例化入口
		initServletBean();
	}

FrameworkServlet

FrameworkServlet 会实现

• #doGet(HttpServletRequest request, HttpServletResponse response)
• #doPost(HttpServletRequest request, HttpServletResponse response)
• #doPut(HttpServletRequest request, HttpServletResponse response)
• #doDelete(HttpServletRequest request, HttpServletResponse response)
• #doOptions(HttpServletRequest request, HttpServletResponse response)
• #doTrace(HttpServletRequest request, HttpServletResponse response)
• #service(HttpServletRequest request, HttpServletResponse response)

等方法。而这些实现，最终会调用 #processRequest(HttpServletRequest request, HttpServletResponse response) 方法，处理请求。

	@Override
	protected void service(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {

		HttpMethod httpMethod = HttpMethod.resolve(request.getMethod());
		if (httpMethod == HttpMethod.PATCH || httpMethod == null) {
			processRequest(request, response);
		}
		else {
			super.service(request, response);
		}
	}

	@Override
	protected final void doGet(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {

		processRequest(request, response);
	}

	@Override
	protected final void doPost(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {

		processRequest(request, response);
	}


	@Override
	protected final void doPut(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {

		processRequest(request, response);
	}


	@Override
	protected final void doDelete(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {

		processRequest(request, response);
	}
...

处理请求：

protected final void processRequest(HttpServletRequest request, HttpServletResponse response)
		throws ServletException, IOException {
	// <1> 记录当前时间，用于计算 web 请求的处理时间
	long startTime = System.currentTimeMillis();
	// <2> 记录异常
	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 {
		//  执行真正的逻辑
		doService(request, response);
	} catch (ServletException | 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();
		}
		// 打印请求日志，并且日志级别为 DEBUG 
		logResult(request, response, failureCause, asyncManager);
		// 发布 ServletRequestHandledEvent 事件
		publishRequestHandledEvent(request, response, startTime, failureCause);
	}
}

//抽象类给子类实现
protected abstract void doService(HttpServletRequest request, HttpServletResponse response)
      throws Exception;

DispatcherServlet

doService

#doService(HttpServletRequest request, HttpServletResponse response) 方法，DispatcherServlet 的处理请求的入口方法，代码如下：

// DispatcherServlet.java

@Override
protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
  logRequest(request);

	// 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)) {
		attributesSnapshot = new HashMap<>();
		Enumeration<!--?--> attrNames = request.getAttributeNames();
		while (attrNames.hasMoreElements()) {
			String attrName = (String) attrNames.nextElement();
			if (this.cleanupAfterInclude || attrName.startsWith(DEFAULT_STRATEGIES_PREFIX)) {
				attributesSnapshot.put(attrName, request.getAttribute(attrName));
			}
		}
	}

	// Make framework objects available to handlers and view objects.
	// 设置 Spring 框架中的常用对象到 request 属性中
	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());

	...

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

doDispatch

#doDispatch(HttpServletRequest request, HttpServletResponse response) 方法，执行请求的分发。

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);

			//  <2> 映射处理器 
      		//获得请求对应的 HandlerExecutionChain 对象
			mappedHandler = getHandler(processedRequest);
			if (mappedHandler == null) { // 如果获取不到，则根据配置抛出异常或返回 404 错误
				noHandlerFound(processedRequest, response);
				return;
			}

 			//  <3> 处理器适配器 
			// 获得当前 handler 对应的 HandlerAdapter 对象
			HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

			// last-modified
			String method = request.getMethod();
			boolean isGet = "GET".equals(method);
			if (isGet || "HEAD".equals(method)) {
				long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
				if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
					return;
				}
			}

			// 前置处理 拦截器
			if (!mappedHandler.applyPreHandle(processedRequest, response)) {
				return;
			}
			//  <4> 调用处理器方法 
			//  真正的调用 handler 方法，并返回视图
			mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

			if (asyncManager.isConcurrentHandlingStarted()) {
				return;
			}

			// 视图
			applyDefaultViewName(processedRequest, mv);
			//  后置处理 拦截器
			mappedHandler.applyPostHandle(processedRequest, response, mv);
		} catch (Exception ex) {
			dispatchException = ex; // 记录异常
		} catch (Throwable err) {
			dispatchException = new NestedServletException("Handler dispatch failed", err); // <10> 记录异常
		}

		// 处理正常和异常的请求调用结果。
		processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
	} catch (Exception ex) {
	...
	} 
  ...
}

processDispatchResult

#processDispatchResult(HttpServletRequest request, HttpServletResponse response, HandlerExecutionChain mappedHandler, ModelAndView mv, Exception exception) 方法，处理正常和异常的请求调用结果。代码如下：

private void processDispatchResult(HttpServletRequest request, HttpServletResponse response,
		@Nullable HandlerExecutionChain mappedHandler, @Nullable ModelAndView mv,
		@Nullable Exception exception) throws Exception {
	// 标记，是否是生成的 ModelAndView 对象
	boolean errorView = false;

	//如果是否异常的结果
	if (exception != null) {
		if (exception instanceof ModelAndViewDefiningException) {
			logger.debug("ModelAndViewDefiningException encountered", exception);
			mv = ((ModelAndViewDefiningException) exception).getModelAndView();
		} else {
			Object handler = (mappedHandler != null ? mappedHandler.getHandler() : null);
			mv = processHandlerException(request, response, handler, exception);
			errorView = (mv != null);
		}
	}

	if (mv != null && !mv.wasCleared()) {
		//  渲染页面
		render(mv, request, response);
		if (errorView) {
			WebUtils.clearErrorRequestAttributes(request);
		}
	} else {
		if (logger.isTraceEnabled()) {
			logger.trace("No view rendering, null ModelAndView returned.");
		}
	}

	...
}

render

#render(ModelAndView mv, HttpServletRequest request, HttpServletResponse response)

protected void render(ModelAndView mv, HttpServletRequest request, HttpServletResponse response) throws Exception {
		// Determine locale for request and apply it to the response.
		Locale locale =
				(this.localeResolver != null ? this.localeResolver.resolveLocale(request) : request.getLocale());
		response.setLocale(locale);

		View view;
		String viewName = mv.getViewName();
		if (viewName != null) {
      //<5> 解析视图
      // 视图解析器ViewResolver
			view = resolveViewName(viewName, mv.getModelInternal(), locale, request);
			if (view == null) {
				throw new ServletException("Could not resolve view with name ‘" + mv.getViewName() +
						"‘ in servlet with name ‘" + getServletName() + "‘");
			}
		}
		else {
			// No need to lookup: the ModelAndView object contains the actual View object.
			view = mv.getView();
			if (view == null) {
				throw new ServletException("ModelAndView [" + mv + "] neither contains a view name nor a " +
						"View object in servlet with name ‘" + getServletName() + "‘");
			}
		}

		// Delegate to the View object for rendering.
		if (logger.isTraceEnabled()) {
			logger.trace("Rendering view [" + view + "] ");
		}
		try {
			if (mv.getStatus() != null) {
				response.setStatus(mv.getStatus().value());
			}
      //<6> 渲染视图
      //视图渲染
			view.render(mv.getModelInternal(), request, response);
		}
		catch (Exception ex) {
			if (logger.isDebugEnabled()) {
				logger.debug("Error rendering view [" + view + "]", ex);
			}
			throw ex;
		}
	}

至此，对照图中的一个请求的处理流程在 DispatcherServlet 中的流转结束。

End

SpringBoot里的Servlet和实现