接觸Web時間比較久，雖然知道Servlet的生命周期但是理解卻還是不夠，今天剛好debug代碼涉及這塊就利用余下時間研究了一下。
Servlet的生命周期以及處理瀏覽器請求的過程。Servlet接口中定義的方法有：
而init -> service -> destory剛好就是servlet的生命周期。通常當web容器（即web服務器，例如tomct等）啟動時候回根據配置文件進行加載servlet，加載servlet過程就是初始化servlet的過程。
由web容器進行調用生命周期init方法進行初始化。初始化完成便可以進行處理瀏覽器發送的請求了。
當瀏覽器向Web容器發送請求時候便由Web容器調用Servelt的service方法（此處實現是HttpServlet的service方法）進行處理瀏覽器請求。HttpService的service方法關鍵源碼實現如下:
protected void service(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException {
String method = req.getMethod();
//判斷客戶端的請求是get、post、...
if (method.equals(METHOD_GET)) {
long lastModified = getLastModified(req);
if (lastModified == -1) {
doGet(req, resp);
} else {
long ifModifiedSince;
try {
ifModifiedSince = req.getDateHeader(HEADER_IFMODSINCE);
} catch (IllegalArgumentException iae) {
ifModifiedSince = -1;
}
if (ifModifiedSince < (lastModified / 1000 * 1000)) {
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);//處理post請求
} 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 {
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);
}
}
本例以post請求進行源碼跟蹤，處理post請求調用doPost方法。此處回顧一下我們一般寫一個servlet時候需要的步驟了，我們都是自己定義一個Servlet類然后一般都是實現doPost和doGet方法，回顧到此就可以回到本文了，本例中調用的doPost方法就是調用我們自己實現的doPost方法，這個流程下來就完成了瀏覽器的請求。而對於Servlet的銷毀由Web容器調用destory方法進行銷毀。
Spring MVC中DispatherServlet的繼承結構：
在使用Spring MVC進行開發時候，在web.xml中需要配置DispatherServlet，讓Web容器啟動時候加載Servlet。通過繼承結構可以發現DispatherServlet是HttpServlet的子類。
當瀏覽器進行請求時候Web容器會調用DispatherServlet的service方法，DispatherServlet的service方法在父類FrameWorkServlet中實現；FrameWorkServlet#service方法源碼如下：
/**
* Override the parent class implementation in order to intercept PATCH requests.
*/
@Override
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);
}
}
最后調用super.service調用HttpServlet#service方法，進行判斷請求的方法是get、post還是...然后調用對應的doXXX方法，此處的doXXX方法在FrameWorkServlet中實現（FrameWorkServlet是框架實現的一個Servlet，與正常Servlet開發的servlet類似）。
org.springframework.web.servlet.FrameworkServlet#doServiceorg.springframework.web.servlet.FrameworkServlet#doPost源碼如下：
@Override protected final void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { processRequest(request, response); }
org.springframework.web.servlet.FrameworkServlet#processRequest源碼如下：
protected final void processRequest(HttpServletRequest request, HttpServletResponse response) //DispatcherServlet的processRequst方法在FrameWork中實現
throws ServletException, IOException {
long startTime = System.currentTimeMillis();
Throwable failureCause = null;
LocaleContext previousLocaleContext = LocaleContextHolder.getLocaleContext();
LocaleContext localeContext = buildLocaleContext(request);
RequestAttributes previousAttributes = RequestContextHolder.getRequestAttributes();
ServletRequestAttributes requestAttributes = buildRequestAttributes(request, response, previousAttributes);
WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
asyncManager.registerCallableInterceptor(FrameworkServlet.class.getName(), new RequestBindingInterceptor());
initContextHolders(request, localeContext, requestAttributes);
try {
doService(request, response); //重點
}
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 {
resetContextHolders(request, previousLocaleContext, previousAttributes);
if (requestAttributes != null) {
requestAttributes.requestCompleted();
}
if (logger.isDebugEnabled()) {
if (failureCause != null) {
this.logger.debug("Could not complete request", failureCause);
}
else {
if (asyncManager.isConcurrentHandlingStarted()) {
logger.debug("Leaving response open for concurrent processing");
}
else {
this.logger.debug("Successfully completed request");
}
}
}
publishRequestHandledEvent(request, response, startTime, failureCause);
}
}
org.springframework.web.servlet.DispatcherServlet#doService源碼：
@Override
protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
if (logger.isDebugEnabled()) {
String resumed = WebAsyncUtils.getAsyncManager(request).hasConcurrentResult() ? " resumed" : "";
logger.debug("DispatcherServlet with name '" + getServletName() + "'" + resumed +
" processing " + request.getMethod() + " request for [" + getRequestUri(request) + "]");
}
// Keep a snapshot of the request attributes in case of an include,
// to be able to restore the original attributes after the include.
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(DEFAULT_STRATEGIES_PREFIX)) {
attributesSnapshot.put(attrName, request.getAttribute(attrName));
}
}
}
// 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());
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 {
doDispatch(request, response); //重點
}
finally {
if (!WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
// Restore the original attribute snapshot, in case of an include.
if (attributesSnapshot != null) {
restoreAttributesAfterInclude(request, attributesSnapshot);
}
}
}
}
最后調用doDispatch(request, response)方法完成處理（后續代碼有時間在跟蹤）。
過程總結：