SpringMVC之請求處理原始碼分析從service到doDispatch(一)
阿新 • • 發佈：2019-02-17
說明：本文所用的SpringMVC版本為4.3.4.RELEASE，應用伺服器為TomCat8.0.33。
前面幾篇文章我們簡單的分析了一下SpringMVC初始化的過程，接下來的這幾篇文章我們從原始碼的角度分析一下SpringMVC對請求的處理過程。這一篇文章我們來分析請求從service到DispatcherServlet的doDispatch的過程。我們做web開發的同學大都對Servlet的生命週期比較瞭解，對Servlet規範也有一些瞭解。我們知道請求在被Servlet處理之前會先被過濾器處理，過濾器處理完之後會呼叫Servlet的service方法來對相應的請求進行處理響應(關於過濾器可以看 這裡)。所以我們這裡分析的入口是Servlet的service方法。我們在用SpringMVC的時候，通常都會在web.xml中進行這樣的配置：
我們瞭解Servlet的同學都知道這樣配置的話，所有的請求(靜態資源除外)將由DispatcherServlet進行處理。即我們這裡處理請求的Servlet類是DispatcherServlet。下面首先我們來看一下DispatcherServlet的UML類圖：<servlet> <servlet-name>spring-mvc</servlet-name> <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class> <init-param> <param-name>contextConfigLocation</param-name> <param-value>classpath:learn-spring-mvc.xml</param-value> </init-param> <load-on-startup>1</load-on-startup> </servlet> <servlet-mapping> <servlet-name>spring-mvc</servlet-name> <url-pattern>/</url-pattern> </servlet-mapping>
在上面的類圖中我們先關注藍色線的這一部分，即左面的這一部分內容。從上面的類圖中我們可以看到DispatcherServlet繼承了FrameworkServlet，FrameworkServlet繼承了HttpServletBean，HttpServletBean繼承了HttpServlet，HttpServlet繼承了GenericServlet，GenericServlet則實現了我們最頂級的介面Servlet和ServletConfig。請求的主要時序圖如下：
從DispatcherServlet的原始碼中我們沒有找到service(ServletRequest req, ServletResponse res)這個方法，但是我們在DispatcherServlet的父類HttpServlet中找到了這個方法，我們去HttpServlet中看看這個方法的內容：
HttpServlet#service(ServletRequest req, ServletResponse res)
@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("non-HTTP request or response");
}
service(request, response);
}
}FrameworkServlet#service(HttpServletRequest request, HttpServletResponse response)
protected void service(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
HttpMethod httpMethod = HttpMethod.resolve(request.getMethod());
if (HttpMethod.PATCH == httpMethod || httpMethod == null) {
processRequest(request, response);
}
else {
super.service(request, response);
}
}public enum HttpMethod {
GET, HEAD, POST, PUT, PATCH, DELETE, OPTIONS, TRACE;
private static final Map<String, HttpMethod> mappings = new HashMap<String, HttpMethod>(8);
static {
for (HttpMethod httpMethod : values()) {
mappings.put(httpMethod.name(), httpMethod);
}
}
public static HttpMethod resolve(String method) {
return (method != null ? mappings.get(method) : null);
}
public boolean matches(String method) {
return (this == resolve(method));
}
}HttpServlet#service(HttpServletRequest req, HttpServletResponse resp)
protected void service(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException {
//獲取請求型別
String method = req.getMethod();
//如果是get請求
if (method.equals(METHOD_GET)) {
//檢查是不是開啟了頁面快取 通過header頭的 Last-Modified/If-Modified-Since
//獲取Last-Modified的值
long lastModified = getLastModified(req);
if (lastModified == -1) {
// servlet doesn't support if-modified-since, no reason
// to go through further expensive logic
//沒有開啟頁面快取呼叫doGet方法
doGet(req, resp);
} else {
long ifModifiedSince;
try {
//獲取If-Modified-Since的值
ifModifiedSince = req.getDateHeader(HEADER_IFMODSINCE);
} catch (IllegalArgumentException iae) {
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
//更新Last-Modified
maybeSetLastModified(resp, lastModified);
//呼叫doGet方法
doGet(req, resp);
} else {
//設定304狀態碼 在HttpServletResponse中定義了很多常用的狀態碼
resp.setStatus(HttpServletResponse.SC_NOT_MODIFIED);
}
}
} else if (method.equals(METHOD_HEAD)) {
//呼叫doHead方法
long lastModified = getLastModified(req);
maybeSetLastModified(resp, lastModified);
doHead(req, resp);
} else if (method.equals(METHOD_POST)) {
//呼叫doPost方法
doPost(req, resp);
} else if (method.equals(METHOD_PUT)) {
//呼叫doPost方法
doPut(req, resp);
} else if (method.equals(METHOD_DELETE)) {
//呼叫doPost方法
doDelete(req, resp);
} else if (method.equals(METHOD_OPTIONS)) {
//呼叫doPost方法
doOptions(req,resp);
} else if (method.equals(METHOD_TRACE)) {
//呼叫doPost方法
doTrace(req,resp);
} else {
//伺服器不支援的方法 直接返回錯誤資訊
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#processRequest(HttpServletRequest request, HttpServletResponse response)
protected final void processRequest(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
long startTime = System.currentTimeMillis();
Throwable failureCause = null;
LocaleContext previousLocaleContext = LocaleContextHolder.getLocaleContext();
//國際化
LocaleContext localeContext = buildLocaleContext(request);
RequestAttributes previousAttributes = RequestContextHolder.getRequestAttributes();
//構建ServletRequestAttributes物件
ServletRequestAttributes requestAttributes = buildRequestAttributes(request, response, previousAttributes);
//非同步管理
WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
asyncManager.registerCallableInterceptor(FrameworkServlet.class.getName(), new RequestBindingInterceptor());
//初始化ContextHolders
initContextHolders(request, localeContext, requestAttributes);
//執行doService
try {
doService(request, response);
}
finally {
//重新設定ContextHolders
resetContextHolders(request, previousLocaleContext, previousAttributes);
if (requestAttributes != null) {
requestAttributes.requestCompleted();
}
//釋出請求處理事件
publishRequestHandledEvent(request, response, startTime, failureCause);
}
} protected LocaleContext buildLocaleContext(final HttpServletRequest request) {
if (this.localeResolver instanceof LocaleContextResolver) {
return ((LocaleContextResolver) this.localeResolver).resolveLocaleContext(request);
}
else {
return new LocaleContext() {
@Override
public Locale getLocale() {
return localeResolver.resolveLocale(request);
}
};
}
}initContextHolders這個方法主要是將Request請求、ServletRequestAttribute物件和國際化物件放入到上下文中。其原始碼如下：
private void initContextHolders(
HttpServletRequest request, LocaleContext localeContext, RequestAttributes requestAttributes) {
if (localeContext != null) {
LocaleContextHolder.setLocaleContext(localeContext, this.threadContextInheritable);//threadContextInheritable預設為false
}
if (requestAttributes != null) {//threadContextInheritable預設為false
RequestContextHolder.setRequestAttributes(requestAttributes, this.threadContextInheritable);
}
}HttpServletRequest request = (HttpServletRequest) RequestContextHolder.getRequestAttributes().resolveReference(RequestAttributes.REFERENCE_REQUEST);DispatcherServlet#doService
我們接下來分析一下DispatcherServlet中的doService這個方法 @Override
protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
//include這種的
Map<String, Object> attributesSnapshot = null;
if (WebUtils.isIncludeRequest(request)) {
attributesSnapshot = new HashMap<String, Object>();
Enumeration<?> attrNames = request.getAttributeNames();
while (attrNames.hasMoreElements()) {
String attrName = (String) attrNames.nextElement();
if (this.cleanupAfterInclude || attrName.startsWith("org.springframework.web.servlet")) {
attributesSnapshot.put(attrName, request.getAttribute(attrName));
}
}
}
//Spring上下文
request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE, getWebApplicationContext());
//國際化解析器
request.setAttribute(LOCALE_RESOLVER_ATTRIBUTE, this.localeResolver);
//主題解析器
request.setAttribute(THEME_RESOLVER_ATTRIBUTE, this.themeResolver);
//主題
request.setAttribute(THEME_SOURCE_ATTRIBUTE, getThemeSource());
//重定向的資料
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);
try {
//呼叫doDispatch方法-核心方法
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);
}
}
}
}