SpringMVC框架中DispatcherServlet分發請求給對應處理器的流程
寫在前面
前面已經講解了 詳細解讀 DispatcherServlet 初始化過程(帶時序圖),瞭解到 DispatcherServlet 在 initStrategies(ApplicationContext context) 方法中初始化 HandlerMapping(處理器對映),HandlerAdapter(處理器介面卡),HandlerExceptionResolver(異常解析器),ViewResolver(檢視解析器)等等。
也掌握了 URL 對映是如何註冊到 HandlerMapping 實現中的。
當然,我們常用的 @RequestMapping 比較複雜,所以再往後放放。先掌握攔截器和異常處理器的使用。
攔截器和異常處理器,都是在客戶端(比如 Chrome 瀏覽器)請求時發揮作用的,所以在學習他們之前,我們先來了解一下 SpringMVC 框架中,DispatcherServlet 分發請求到對應處理器的流程。
從 Servlet 規範說起
當 Servlet 容器允許某個 servlet 物件響應請求時,就會呼叫 Servlet 的 void service(ServletRequest request, ServletResponse response)
對於選擇哪個 servlet 來響應請求的規則,百度 “servlet-mapping url-pattern”,一抓一大把。連結一篇排名靠前的 servlet的url-pattern匹配規則,介紹了四種匹配規則:
- 精準匹配(
/hello
,/admin/home
) - 路徑匹配(
/hi/*
,/hi/kendoizyu/*
,/*
) - 副檔名匹配(
*.action
,*.jsp
,*.htm
) - 預設匹配(
/
)
下面這張圖是 Servlet 容器呼叫 service 方法執行到 doDispatch 的時序圖:
-
HttpServlet # service(HttpServletRequest req, HttpServletResponse resp)
-
FrameworkServet # processRequest 負責“請求上下文”的儲存和清理。這部分涉及到 ThreadLocal 相關的知識,更多分析可參考 SpringMVC之RequestContextHolder分析
-
FrameworkServet # doService 為 HttpServletRequest 暴露 DispatcherServlet 框架相關的屬性。
-
FrameworkServet # doDispatch 為 HttpServletRequest 獲取對應的處理器,介面卡,執行處理方法,並且包含攔截邏輯。
doDispatch 中跟處理器相關的方法分別是 getHandler,getHandlerAdapter 和 HandlerAdapter # handle
doDispatch 在 HandlerAdapter # handle 前後分別有攔截器相關的方法 HandlerExecutionChain # applyPreHandle,applyPostHandle 和 triggerAfterCompletion
本文重點關注和處理器相關的方法。
getHandler
為當前請求決定一個處理器。
DispatcherServlet # getHandler
@Nullable
protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
if (this.handlerMappings != null) {
// 遍歷 DispatcherServlet 中的 HandlerMapping 例項物件列表
for (HandlerMapping mapping : this.handlerMappings) {
// 具體的 HandlerMapping 例項物件呼叫 getHandler 方法
HandlerExecutionChain handler = mapping.getHandler(request);
if (handler != null) {
return handler;
}
}
}
return null;
}
問題1:成員變數 handlerMappings 是怎麼來的呢?
DispatcherServlet # initHandlerMappings -> 點選展開檢視原始碼
private void initHandlerMappings(ApplicationContext context) {
this.handlerMappings = null;
if (this.detectAllHandlerMappings) {
// 找出 DispatcherServlet 和 ContextLoaderListener 上下文中所有實現 HandlerMapping 介面的 Bean
// Find all HandlerMappings in the ApplicationContext, including ancestor contexts.
Map matchingBeans =
BeanFactoryUtils.beansOfTypeIncludingAncestors(context, HandlerMapping.class, true, false);
if (!matchingBeans.isEmpty()) {
this.handlerMappings = new ArrayList<>(matchingBeans.values());
// We keep HandlerMappings in sorted order.
AnnotationAwareOrderComparator.sort(this.handlerMappings);
}
}
else {
// 找出上下文中,名稱為 handlerMapping 且實現 HandlerMapping 的 Bean
try {
HandlerMapping hm = context.getBean(HANDLER_MAPPING_BEAN_NAME, HandlerMapping.class);
this.handlerMappings = Collections.singletonList(hm);
}
catch (NoSuchBeanDefinitionException ex) {
// Ignore, we'll add a default HandlerMapping later.
}
}
// Ensure we have at least one HandlerMapping, by registering
// a default HandlerMapping if no other mappings are found.
if (this.handlerMappings == null) {
// Spring 框架預設的 HandlerMapping
this.handlerMappings = getDefaultStrategies(context, HandlerMapping.class);
if (logger.isTraceEnabled()) {
logger.trace("No HandlerMappings declared for servlet '" + getServletName() +
"': using default strategies from DispatcherServlet.properties");
}
}
}
閱讀原始碼之後,我們發現初始化 handlerMappings 的方法中,分為自定義初始化多個或者單個 HandlerMapping Bean,如果沒有自定義就按照框架預設的來:
-
預設檢測所有 HandlerMapping Bean:所有實現 HandlerMapping 介面的 Bean
-
設定檢測唯一 HandlerMapping Bean:名稱為 handlerMapping 且實現 HandlerMapping 介面的 Bean。需要對 web.xml 的 <servlet> 做一點修改:
<init-param>
<param-name>detectAllHandlerMappings</param-name>
<param-value>false</param-value>
</init-param>
- 讀取框架預設檔案 DipatcherServlet.properties,獲取 org.springframework.web.servlet.HandlerMapping 對應的 value。(開發者無法修改)
org.springframework.web.servlet.HandlerMapping=org.springframework.web.servlet.handler.BeanNameUrlHandlerMapping,\
org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerMapping
AbstractHandlerMapping # getHandler
這個基類方法是通用的,學習掌握這個方法,基本上就知道 SpringMVC 獲取處理器的方法。
@Override
@Nullable
public final HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
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);
}
HandlerExecutionChain executionChain = getHandlerExecutionChain(handler, request);
if (logger.isTraceEnabled()) {
logger.trace("Mapped to " + handler);
}
else if (logger.isDebugEnabled() && !request.getDispatcherType().equals(DispatcherType.ASYNC)) {
logger.debug("Mapped to " + executionChain.getHandler());
}
if (CorsUtils.isCorsRequest(request)) {
CorsConfiguration globalConfig = this.corsConfigurationSource.getCorsConfiguration(request);
CorsConfiguration handlerConfig = getCorsConfiguration(handler, request);
CorsConfiguration config = (globalConfig != null ? globalConfig.combine(handlerConfig) : handlerConfig);
executionChain = getCorsHandlerExecutionChain(request, executionChain, config);
}
return executionChain;
}
getHandlerInternal 方法實現目前有 2 種,一種在子類 AbstractUrlHandlerMapping 中,另一種在子類 AbstractHandlerMethodMapping 中。
getHandlerInternal
URL 與 類級別的處理器 對映
AbstractUrlHandlerMapping # getHandlerInternal -> 點選展開檢視原始碼
@Override
@Nullable
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 = obtainApplicationContext().getBean(handlerName);
}
validateHandler(rawHandler, request);
handler = buildPathExposingHandler(rawHandler, lookupPath, lookupPath, null);
}
}
return handler;
}
執行時序圖如下:
URL 與 方法級別的處理器 對映
AbstractHandlerMethodMapping # getHandlerInternal -> 點選展開檢視原始碼
@Nullable
protected HandlerMethod lookupHandlerMethod(String lookupPath, HttpServletRequest request) throws Exception {
List matches = new ArrayList<>();
List directPathMatches = this.mappingRegistry.getMappingsByUrl(lookupPath);
if (directPathMatches != null) {
addMatchingMappings(directPathMatches, matches, request);
}
if (matches.isEmpty()) {
// No choice but to go through all mappings...
addMatchingMappings(this.mappingRegistry.getMappings().keySet(), matches, request);
}
if (!matches.isEmpty()) {
Comparator comparator = new MatchComparator(getMappingComparator(request));
matches.sort(comparator);
Match bestMatch = matches.get(0);
if (matches.size() > 1) {
if (logger.isTraceEnabled()) {
logger.trace(matches.size() + " matching mappings: " + matches);
}
if (CorsUtils.isPreFlightRequest(request)) {
return PREFLIGHT_AMBIGUOUS_MATCH;
}
Match secondBestMatch = matches.get(1);
if (comparator.compare(bestMatch, secondBestMatch) == 0) {
Method m1 = bestMatch.handlerMethod.getMethod();
Method m2 = secondBestMatch.handlerMethod.getMethod();
String uri = request.getRequestURI();
throw new IllegalStateException(
"Ambiguous handler methods mapped for '" + uri + "': {" + m1 + ", " + m2 + "}");
}
}
request.setAttribute(BEST_MATCHING_HANDLER_ATTRIBUTE, bestMatch.handlerMethod);
handleMatch(bestMatch.mapping, lookupPath, request);
return bestMatch.handlerMethod;
}
else {
return handleNoMatch(this.mappingRegistry.getMappings().keySet(), lookupPath, request);
}
}
private void addMatchingMappings(Collection mappings, List matches, HttpServletRequest request) {
for (T mapping : mappings) {
T match = getMatchingMapping(mapping, request);
if (match != null) {
matches.add(new Match(match, this.mappingRegistry.getMappings().get(mapping)));
}
}
}
執行時序圖如下:
這兩個處理器要詳細講解還得結合例項,這裡就不多說了
getHandlerAdapter
通過 getHandler 方法,我們已經得到了“處理器”物件,但是“處理器”物件沒有統一的介面,所以使用介面卡模式進行統一適配。
protected HandlerAdapter getHandlerAdapter(Object handler) throws ServletException {
if (this.handlerAdapters != null) {
for (HandlerAdapter adapter : this.handlerAdapters) {
if (adapter.supports(handler)) {
return adapter;
}
}
}
throw new ServletException("No adapter for handler [" + handler +
"]: The DispatcherServlet configuration needs to include a HandlerAdapter that supports this handler");
}
這裡用到了 介面卡模式,示意圖如下:
除了 HandlerMethod 以外,其他的幾個都是方法的直接轉發。右圖 HandlerAdapter 虛線框內的類,這是 supports(Object handler) 的目標物件,拿其中一個舉例:
@Override
public boolean supports(Object handler) {
return (handler instanceof HttpRequestHandler);
}
所謂的直接轉發,看原始碼:
@Override
@Nullable
public ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler)
throws Exception {
// 做了一步強轉,就把剩下的交給介面方法處理了!
((HttpRequestHandler) handler).handleRequest(request, response);
return null;
}
doDispatch 中執行的“處理器”方法的正是這個 HandlerAdapter # handle(HttpServletRequest request, HttpServletResponse response, Object handler)
總結
doDispatch 的主要流程就是 獲取處理器 getHandler, 獲取處理器介面卡 getHandlerAdapter,執行處理器介面卡的 handle 方法
其中,getHandlerInternal 因子類的不同,而有二類不同的行為,一類是 AbstractUrlHandlerMapping,另一類是AbstractHandlerMethodMapping