HttpServlet
Service層處理HTTP請求的入口是service()方法（更深層次的需要看Tomcat的原理），分別處理HTTP不同的方法，這裏我們看GET方法的邏輯。
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("non-HTTP request or response");
}
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 = req.getDateHeader(HEADER_IFMODSINCE);
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);
}
}
FrameworkServlet
進入到Spring的框架邏輯中，調用doService，然後發佈一個事件，這裏重點看處理請求的doService方法，FrameworkServlet中該方法爲abstract，具體在DispatcherServlet中實現。
@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 {
long startTime = System.currentTimeMillis();
Throwable failureCause = null;
// Expose current LocaleResolver and request as LocaleContext.
LocaleContext previousLocaleContext = LocaleContextHolder.getLocaleContext();
LocaleContextHolder.setLocaleContext(buildLocaleContext(request), this.threadContextInheritable);
// Expose current RequestAttributes to current thread.
RequestAttributes previousRequestAttributes = RequestContextHolder.getRequestAttributes();
ServletRequestAttributes requestAttributes = null;
if (previousRequestAttributes == null || previousRequestAttributes.getClass().equals(ServletRequestAttributes.class)) {
requestAttributes = new ServletRequestAttributes(request);
RequestContextHolder.setRequestAttributes(requestAttributes, this.threadContextInheritable);
}
if (logger.isTraceEnabled()) {
logger.trace("Bound request context to thread: " + request);
}
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 {
// Clear request attributes and reset thread-bound context.
LocaleContextHolder.setLocaleContext(previousLocaleContext, this.threadContextInheritable);
if (requestAttributes != null) {
RequestContextHolder.setRequestAttributes(previousRequestAttributes, this.threadContextInheritable);
requestAttributes.requestCompleted();
}
if (logger.isTraceEnabled()) {
logger.trace("Cleared thread-bound request context: " + request);
}
if (failureCause != null) {
this.logger.debug("Could not complete request", failureCause);
} else {
this.logger.debug("Successfully completed request");
}
if (this.publishEvents) {
// Whether or not we succeeded, publish an event.
long processingTime = System.currentTimeMillis() - startTime;
this.webApplicationContext.publishEvent(
new ServletRequestHandledEvent(this,
request.getRequestURI(), request.getRemoteAddr(),
request.getMethod(), getServletConfig().getServletName(),
WebUtils.getSessionId(request), getUsernameForRequest(request),
processingTime, failureCause));
}
}
}
DispatcherServlet
doDispatch中具體處理每個請求，包括Interceptor的攔截邏輯和Controller handler的業務邏輯。
@Override
protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
if (logger.isDebugEnabled()) {
String requestUri = urlPathHelper.getRequestUri(request);
logger.debug("DispatcherServlet with name '" + getServletName() + "' processing " + request.getMethod() +
" request for [" + requestUri + "]");
}
/**
* Make framework objects available to handlers and view objects.
* 爲了處理器對象，視圖對象可以訪問某些某些全局的對象，設置到HTTP request的屬性中
* */
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 {
doDispatch(request, response);
} finally {
}
}
/**
* 具體分發請求到handler
* <p>The handler will be obtained by applying the servlet's HandlerMappings in order.
* The HandlerAdapter will be obtained by querying the servlet's installed HandlerAdapters
* to find the first that supports the handler class.
* <p>All HTTP methods are handled by this method. It's up to HandlerAdapters or handlers
* themselves to decide which methods are acceptable.
*
* @param request current HTTP request
* @param response current HTTP response
* @throws Exception in case of any kind of processing failure
*/
protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
HttpServletRequest processedRequest = request;
HandlerExecutionChain mappedHandler = null;
int interceptorIndex = -1;
try {
ModelAndView mv;
boolean errorView = false;
try {
processedRequest = checkMultipart(request);
/**
* 獲取該請求對應的 handler和interceptor（定位到了某個類）
* Determine handler for the current request.
* */
mappedHandler = getHandler(processedRequest, false);
if (mappedHandler == null || mappedHandler.getHandler() == null) {
noHandlerFound(processedRequest, response);
return;
}
/**
* 獲得該請求對應的更具體的handler（適配器模式，包含更多的策略和配置），如AnnotationMethodHandlerAdapter
* Determine handler adapter for the current request.
* */
HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());
/**
* 依次執行我們的Interceptor
* Apply preHandle methods of registered interceptors.
* */
HandlerInterceptor[] interceptors = mappedHandler.getInterceptors();
if (interceptors != null) {
for (int i = 0; i < interceptors.length; i++) {
HandlerInterceptor interceptor = interceptors[i];
if (!interceptor.preHandle(processedRequest, response, mappedHandler.getHandler())) {
/**
* 如果該preHandle方法返回了false，那麼就倒序依次執行前面的Interceptor的 afterCompletion 方法，然後返回
* 所以不會執行後面的 Controller的handler方法，以及Interceptor的postHandle方法
* */
triggerAfterCompletion(mappedHandler, interceptorIndex, processedRequest, response, null);
return;
}
interceptorIndex = i;
}
}
/**
* 真正進入 Controller handler 進行請求處理
* Actually invoke the handler.
* */
mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
// Do we need view name translation?
if (mv != null && !mv.hasView()) {
mv.setViewName(getDefaultViewName(request));
}
//
/**
* 然後逆序依次執行我們的Interceptor的 postHandle 方法
* Apply postHandle methods of registered interceptors.
* */
if (interceptors != null) {
for (int i = interceptors.length - 1; i >= 0; i--) {
HandlerInterceptor interceptor = interceptors[i];
interceptor.postHandle(processedRequest, response, mappedHandler.getHandler(), mv);
}
}
} catch (ModelAndViewDefiningException ex) {
logger.debug("ModelAndViewDefiningException encountered", ex);
mv = ex.getModelAndView();
} catch (Exception ex) {
Object handler = (mappedHandler != null ? mappedHandler.getHandler() : null);
mv = processHandlerException(processedRequest, response, handler, ex);
errorView = (mv != null);
}
// Did the handler return a view to render?
if (mv != null && !mv.wasCleared()) {
render(mv, processedRequest, response);
if (errorView) {
WebUtils.clearErrorRequestAttributes(request);
}
} else {
if (logger.isDebugEnabled()) {
logger.debug("Null ModelAndView returned to DispatcherServlet with name '" + getServletName() +
"': assuming HandlerAdapter completed request handling");
}
}
/** 此時所有的Interceptor都順利完成了（preHandle -> handler -> postHandle），
* 此時倒序依次執行所有的Interceptor的 afterCompletion方法
* Trigger after-completion for successful outcome.
* */
triggerAfterCompletion(mappedHandler, interceptorIndex, processedRequest, response, null);
} catch (Exception ex) {
// Trigger after-completion for thrown exception.
triggerAfterCompletion(mappedHandler, interceptorIndex, processedRequest, response, ex);
throw ex;
} catch (Error err) {
ServletException ex = new NestedServletException("Handler processing failed", err);
// Trigger after-completion for thrown exception.
triggerAfterCompletion(mappedHandler, interceptorIndex, processedRequest, response, ex);
throw ex;
} finally {
// Clean up any resources used by a multipart request.
if (processedRequest != request) {
cleanupMultipart(processedRequest);
}
}
}
private void triggerAfterCompletion(HandlerExecutionChain mappedHandler,
int interceptorIndex,
HttpServletRequest request,
HttpServletResponse response,
Exception ex) throws Exception {
// Apply afterCompletion methods of registered interceptors.
if (mappedHandler != null) {
HandlerInterceptor[] interceptors = mappedHandler.getInterceptors();
if (interceptors != null) {
for (int i = interceptorIndex; i >= 0; i--) {
HandlerInterceptor interceptor = interceptors[i];
try {
interceptor.afterCompletion(request, response, mappedHandler.getHandler(), ex);
} catch (Throwable ex2) {
logger.error("HandlerInterceptor.afterCompletion threw exception", ex2);
}
}
}
}
}
總結
轉載:https://zhuanlan.zhihu.com/p/23626598
