上篇文章我們走讀了servlet生命周期中的創建和初始化以及servlet中Filter在springboot中的使用。本文繼續進行servlet生命周期中下面的部分，提供服務即service方法。
Servlet的service方法在HttpServlet中實現，代碼為：
@Override
public void service(ServletRequest req, ServletResponse res)
throws ServletException, IOException {
HttpServletRequest request;
HttpServletResponse response;
try {
request = (HttpServletRequest) req;
response = (HttpServletResponse) res;
} catch (ClassCastException e) {
throw new ServletException(lStrings.getString("http.non_http"));
}
service(request, response);
}
@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);
}
}
protected void service(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException {
String method = req.getMethod();
if (method.equals(METHOD_GET)) {
long lastModified = getLastModified(req);
if (lastModified == -1) {
// servlet doesn't support if-modified-since, no reason
// to go through further expensive logic
doGet(req, resp);
} else {
long ifModifiedSince;
try {
ifModifiedSince = req.getDateHeader(HEADER_IFMODSINCE);
} catch (IllegalArgumentException iae) {
// Invalid date header - proceed as if none was set
ifModifiedSince = -1;
}
if (ifModifiedSince < (lastModified / 1000 * 1000)) {
// If the servlet mod time is later, call doGet()
// Round down to the nearest second for a proper compare
// A ifModifiedSince of -1 will always be less
maybeSetLastModified(resp, lastModified);
doGet(req, resp);
} else {
resp.setStatus(HttpServletResponse.SC_NOT_MODIFIED);
}
}
} else if (method.equals(METHOD_HEAD)) {
long lastModified = getLastModified(req);
maybeSetLastModified(resp, lastModified);
doHead(req, resp);
} else if (method.equals(METHOD_POST)) {
doPost(req, resp);
} else if (method.equals(METHOD_PUT)) {
doPut(req, resp);
} else if (method.equals(METHOD_DELETE)) {
doDelete(req, resp);
} else if (method.equals(METHOD_OPTIONS)) {
doOptions(req,resp);
} else if (method.equals(METHOD_TRACE)) {
doTrace(req,resp);
} else {
//
// Note that this means NO servlet supports whatever
// method was requested, anywhere on this server.
//
String errMsg = lStrings.getString("http.method_not_implemented");
Object[] errArgs = new Object[1];
errArgs[0] = method;
errMsg = MessageFormat.format(errMsg, errArgs);
resp.sendError(HttpServletResponse.SC_NOT_IMPLEMENTED, errMsg);
}
}
羅列了上面三段代碼后，發現，我們就進入到了doXXX方法了，這里，我們選doGet方法來走讀代碼。
doGet方法的實現在類FrameworkServlet中。
@Override
protected final void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
processRequest(request, response);
}
protected final void processRequest(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
......
try {
//真正執行操作。
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();
}
logResult(request, response, failureCause, asyncManager);
//廣播一個請求處理完成的事件，這個是一個可擴展的口子
publishRequestHandledEvent(request, response, startTime, failureCause);
}
}
DispatcherServlet中實現了doService.
@Override
protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
......
// 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());
if (this.flashMapManager != null) {
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);
}
RequestPath requestPath = null;
if (this.parseRequestPath && !ServletRequestPathUtils.hasParsedRequestPath(request)) {
requestPath = ServletRequestPathUtils.parseAndCache(request);
}
try {
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);
}
}
if (requestPath != null) {
ServletRequestPathUtils.clearParsedRequestPath(request);
}
}
}
doDispatch方法實現：
protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
HttpServletRequest processedRequest = request;
HandlerExecutionChain mappedHandler = null;
boolean multipartRequestParsed = false;
WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
try {
//定義視圖與模型對象
ModelAndView mv = null;
Exception dispatchException = null;
try {
//判斷是否為文件類操作
processedRequest = checkMultipart(request);
multipartRequestParsed = (processedRequest != request);
// Determine handler for the current request.
//獲取處理器。
mappedHandler = getHandler(processedRequest);
if (mappedHandler == null) {
noHandlerFound(processedRequest, response);
return;
}
// Determine handler adapter for the current request.
//獲取處理器適配器
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 (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
return;
}
}
//我們定義的interceptor的preHandler方法調用
if (!mappedHandler.applyPreHandle(processedRequest, response)) {
return;
}
// Actually invoke the handler.
//真正調用處理器，返回modelAndView
mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
if (asyncManager.isConcurrentHandlingStarted()) {
return;
}
applyDefaultViewName(processedRequest, mv);
//調用interceptor的postHandler方法。
mappedHandler.applyPostHandle(processedRequest, response, mv);
}
catch (Exception ex) {
dispatchException = ex;
}
catch (Throwable err) {
// As of 4.3, we're processing Errors thrown from handler methods as well,
// making them available for @ExceptionHandler methods and other scenarios.
dispatchException = new NestedServletException("Handler dispatch failed", err);
}
//處理結果，將ModelAndView解析成view，或者異常的處理
processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
}
catch (Exception ex) {
//inteceptor中preHandler和postHandler一一對應
triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
}
catch (Throwable err) {
triggerAfterCompletion(processedRequest, response, mappedHandler,
new NestedServletException("Handler processing failed", 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);
}
}
}
}
接下來，我們看每步操作的具體實現。
Handler的獲取
先看getHandler方法的代碼：
@Nullable
protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
if (this.handlerMappings != null) {
for (HandlerMapping mapping : this.handlerMappings) {
HandlerExecutionChain handler = mapping.getHandler(request);
if (handler != null) {
return handler;
}
}
}
return null;
}
這里從handlerMappings中獲取mapping，handlerMappings值是有順序的，排第一的一般是RequestMappingHandlerMapping,這里handlerMappings在前文中說的initHandlerMappings里做的初始化，主要取HandlerMapping類型的bean。默認情況下，springboot里有5個，分別為RequestMappingHandlerMapping,WelcomePageHandlerMapping,BeanNameUrlHandlerMapping,RouterFunctionMapping和SimpleUrlHandlerMapping,這些bean定義在WebMvcConfiguration及其父類中。這里就不做過詳細的贅述了。
RequestMappingHandlerMapping比較特殊一些，這里拿出來詳細看一下其bean的初始化過程，這個跟getHandler有關系。
先看類圖：
RequestMappingHandlerMapping.png
RequestMappingHandlerMapping實現了InitializingBean接口，就是說在該bean初始化完成后，會調用
afterPropertiesSet進行后置處理。我們來看該方法的實現：
public void afterPropertiesSet() {
this.config = new RequestMappingInfo.BuilderConfiguration();
this.config.setTrailingSlashMatch(useTrailingSlashMatch());
this.config.setContentNegotiationManager(getContentNegotiationManager());
if (getPatternParser() != null) {
this.config.setPatternParser(getPatternParser());
Assert.isTrue(!this.useSuffixPatternMatch && !this.useRegisteredSuffixPatternMatch,
"Suffix pattern matching not supported with PathPatternParser.");
}
else {
this.config.setSuffixPatternMatch(useSuffixPatternMatch());
this.config.setRegisteredSuffixPatternMatch(useRegisteredSuffixPatternMatch());
this.config.setPathMatcher(getPathMatcher());
}
super.afterPropertiesSet();
}
這里會調用父類的afterPropertiesSet方法。
@Override
public void afterPropertiesSet() {
initHandlerMethods();
}
protected void initHandlerMethods() {
//遍歷工廠中所有的bean
for (String beanName : getCandidateBeanNames()) {
if (!beanName.startsWith(SCOPED_TARGET_NAME_PREFIX)) {
//處理選中的bean
processCandidateBean(beanName);
}
}
handlerMethodsInitialized(getHandlerMethods());
}
protected void processCandidateBean(String beanName) {
Class<?> beanType = null;
try {
beanType = obtainApplicationContext().getType(beanName);
}
catch (Throwable ex) {
// An unresolvable bean type, probably from a lazy bean - let's ignore it.
if (logger.isTraceEnabled()) {
logger.trace("Could not resolve type for bean '" + beanName + "'", ex);
}
}
if (beanType != null && isHandler(beanType)) {
detectHandlerMethods(beanName);
}
}
@Override
protected boolean isHandler(Class<?> beanType) {
return (AnnotatedElementUtils.hasAnnotation(beanType, Controller.class) ||
AnnotatedElementUtils.hasAnnotation(beanType, RequestMapping.class));
}
只有被Controller或RequestMapping注解的bean，才被當成處理器。即需要業務實現的bean，也就是我們平時寫的controller。
protected void detectHandlerMethods(Object handler) {
Class<?> handlerType = (handler instanceof String ?
obtainApplicationContext().getType((String) handler) : handler.getClass());
if (handlerType != null) {
Class<?> userType = ClassUtils.getUserClass(handlerType);
Map<Method, T> methods = MethodIntrospector.selectMethods(userType,
(MethodIntrospector.MetadataLookup<T>) method -> {
try {
return getMappingForMethod(method, userType);
}
catch (Throwable ex) {
throw new IllegalStateException("Invalid mapping on handler class [" +
userType.getName() + "]: " + method, ex);
}
});
if (logger.isTraceEnabled()) {
logger.trace(formatMappings(userType, methods));
}
methods.forEach((method, mapping) -> {
Method invocableMethod = AopUtils.selectInvocableMethod(method, userType);
registerHandlerMethod(handler, invocableMethod, mapping);
});
}
}
registerHandlerMethod會調用AbstractHandlerMethodMapping的register方法，將handler注冊到mapping中。
此后，使用getHandler時，就能得到對應的handler了。返回頭來，再繼續看getHandler方法。
先看getHandler方法的代碼：
@Nullable
protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
if (this.handlerMappings != null) {
for (HandlerMapping mapping : this.handlerMappings) {
HandlerExecutionChain handler = mapping.getHandler(request);
if (handler != null) {
return handler;
}
}
}
return null;
}
這里，for循環里第一個一般取的是RequestMappingHandlerMapping，看RequestMappingHandlerMapping的getHandler方法。
public final HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
//在mappingRegistry中獲取到處理器。
Object handler = getHandlerInternal(request);
if (handler == null) {
handler = getDefaultHandler();
}
if (handler == null) {
return null;
}
// Bean name or resolved handler?
if (handler instanceof String) {
String handlerName = (String) handler;
handler = obtainApplicationContext().getBean(handlerName);
}
//封裝成HandlerExecutionChain返回。
HandlerExecutionChain executionChain = getHandlerExecutionChain(handler, request);
......
if (hasCorsConfigurationSource(handler) || CorsUtils.isPreFlightRequest(request)) {
CorsConfiguration config = getCorsConfiguration(handler, request);
if (getCorsConfigurationSource() != null) {
CorsConfiguration globalConfig = getCorsConfigurationSource().getCorsConfiguration(request);
config = (globalConfig != null ? globalConfig.combine(config) : config);
}
executionChain = getCorsHandlerExecutionChain(request, executionChain, config);
}
return executionChain;
}
至此，就獲取到一個mappedHandler。下面，我們看如何獲取處理器適配器。
HandlerAdapter的獲取
HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());在工廠中獲取HandlerAdapter類型的bean。bean的定義也在WebMvcAutoConfiguration及其父類中。
具體的業務處理
mappedHandler和HandlerAdapter都獲取完了，接下來就是具體的業務處理了。首先處理interceptor的preHandler。接下來通過HandlerAdapter的handle方法通過反射的方式調用具體的處理器里的接口，返回ModelAndView，然后處理interceptor的postHandler。最后解析ModelAndView進行視圖渲染。
自此，客戶端請求在springboot中的流轉全部完成。后面代碼邏輯比較清晰，就沒有專門貼代碼來做詳細的說明了。以后用到的時候再進行說明吧。
