SpringMVC之請求處理原始碼分析從service到doDispatch(一)
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
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);
}
}
service這個方法的內容很簡單,就是將ServletRequest和ServletResponse轉換為HttpServletRequest和HttpServletResponse。因為我們是做web開發,通常用的是HTTP協議,所以這裡我們需要的時候HttpServletRequest和HttpServletResponse。接下來就是呼叫service(HttpServletRequest request, HttpServletResponse response),我們在HttpServlet和FrameworkServlet中都找到了這個方法,但是HttpServlet是FrameworkServlet的父類,即FrameworkServlet中重寫了service這個方法,所以我們這裡取FrameworkServlet中去看看這個方法的內容:
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);
}
}
這個方法的內容也很簡單,第一步根據請求的方法型別轉換對應的列舉類。我們可以看一下HttpMethod這個列舉類:
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));
}
}
HttpMethod這個定義了這樣的幾種列舉型別:GET, HEAD, POST, PUT, PATCH, DELETE, OPTIONS, TRACE;而這些也是RFC標準中幾種請求型別。如果請求型別為PATCH或者沒有找到相應的請求型別的話,則直接呼叫processRequest這個方法。但是這種情況我們很少很少會遇到。所以這裡會執行super.service這個方法。即呼叫HttpServlet中的service方法。我們看一下HttpServlet中這個service方法的內容:
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);
}
}
這個方法的主要作用是根據請求型別呼叫響應的請求方法如果GET型別,呼叫doGet方法;POST型別,呼叫doPost方法。這些方法都是在HttpServlet中定義的,平時我們做web開發的時候主要是繼承HttpServlet這個類,然後重寫它的doPost或者doGet方法。我們的FrameworkServlet這個子類就重寫了這些方法中的一部分:doGet、doPost、doPut、doDelete、doOption、doTrace。這裡我們只說我們最常用的doGet和doPost這兩個方法。通過翻開原始碼我們發現,這兩個方法體的內容是一樣的,都是呼叫了processRequest這個方法。processRequest這個方法是我們接下來要分析的:
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);
}
}
在這個方法裡大概做了這樣幾件事:國際化的設定,建立ServletRequestAttributes物件,初始化上下文holders(即將Request物件放入到執行緒上下文中),呼叫doService方法。我們分析主要的幾個:
國際化的設定主要是在DispatcherServlet#buildLocaleContext這個方法中完成的,其原始碼如下:
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);
}
};
}
}
根據我們之前文章的分析,如果我們沒有配置國際化解析器的話,那麼它會使用預設的解析器:AcceptHeaderLocaleResolver,即從Header中獲取國際化的資訊。除了AcceptHeaderLocaleResolver之外,SpringMVC中還提供了這樣的幾種解析器:CookieLocaleResolver、SessionLocaleResolver、FixedLocaleResolver。分別從cookie、session中去國際化資訊和JVM預設的國際化資訊(Local.getDefault())。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);
}
}
RequestContextHolder這個類有什麼用呢?有時候我們想在某些類中獲取HttpServletRequest物件,比如在AOP攔截的類中,那麼我們就可以這樣來獲取Request的物件了,
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);
}
}
}
}
在這個方法中主要做了這幾件事:處理include標籤的請求,將上下文放到request的屬性中,將國際化解析器放到request的屬性中,將主題解析器放到request屬性中,將主題放到request的屬性中,處理重定向的請求資料最後呼叫doDispatch這個核心的方法對請求進行處理,我們在下一章中詳細分析一下doDispatch這個方法。