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：
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
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