Servlet是什么
1. Servlet是服务器端运行的一个程序，是一个被编译好的Java类。它不是框架等。
2. Web容器的启动，需要依赖Servlet。当web服务器开始执行时，servlet类就被初始化
3. 当用户通过浏览器访问的时候，会输入URI，这个时候，web服务器就通过Servlet来分发请求执行不同的内容。
一般我们会使用Tomcat来运行Java的web项目。通常我们可以在web项目的目录中可以看到WEB-INF这样的文件夹。这个文件夹是受保护的，外部无法通过url来访问的。文件夹里面包含了web.xml以及class和libs目录。我们要将web项目运行起来，就得在web.xml中定义一个Servlet，因为定义了Servlet，web容器才能知道需要如何去分发请求进行业务处理的。
Servlet是有一个Jar包，里面包含了一些Servlet的相关设计和细节。
一个使用Spring的Servlet的web.xml配置例子：
<servlet>
<servlet-name>apiServlet</servlet-name>
<servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
<init-param>
<param-name>contextConfigLocation</param-name>
<param-value>classpath:spring-common.xml</param-value>
</init-param>
<load-on-startup>1</load-on-startup>
</servlet>
<servlet-mapping>
<servlet-name>apiServlet</servlet-name>
<url-pattern>/api/*</url-pattern>
</servlet-mapping>
HttpServlet
1. HttpServlet是一个抽象类，具体的实现自己需要的Servlet需要通过继承该类来实现 。
HttpServlet中主要的几个方法：
| 方法 | 用途 |
|---|---|
| doGet | 处理HTTP GET请求 |
| doPost | 处理HTTP POST请求 |
| doPut | 处理HTTP PUT请求 |
| doDelete | 处理HTTP DELETE请求 |
Servlet最终调用的是service方法，这个方法中会根据request的Method来判断具体是执行doGet还是doPost。具体可以看HttpServlet类源码。
2. HttpServlet最上层是继承Servlet这个接口类。Servlet这个接口类非常简单，只定义了5个方法。具体可以看Servlet接口源码。
package javax.servlet;
import java.io.IOException;
public interface Servlet {
//容器初始化
public void init(ServletConfig config) throws ServletException;
//获取配置信息
public ServletConfig getServletConfig();
//具体的service 请求分发
public void service(ServletRequest req, ServletResponse res) throws ServletException, IOException;
//获取servlet信息
public String getServletInfo();
//容器销毁的时候
public void destroy();
}
Spring的DispatcherServlet
Spring主要通过DispatcherServlet实现了Servlet。DispatcherServlet最上层也是继承的是HttpServlet这个类。
我们主要看下DispatcherServlet两个比较重要的方法：
@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 + "]");
}
// 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)) {
logger.debug("Taking snapshot of request attributes before include");
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));
}
}
}
// 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); //这边最终也是调用了doDispatch方法，该方法主要用来处理SPring框架的具体业务分发逻辑。
}
finally {
// Restore the original attribute snapshot, in case of an include.
if (attributesSnapshot != null) {
restoreAttributesAfterInclude(request, attributesSnapshot);
}
}
}
//Spring框架最终的分发都是通过该方法的
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);
// Determine handler for the current request.
mappedHandler = getHandler(processedRequest, false);
if (mappedHandler == null || mappedHandler.getHandler() == null) {
noHandlerFound(processedRequest, response);
return;
}
// Determine handler adapter for the current request.
HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());
// Process last-modified header, if supported by the handler.
String method = request.getMethod();
boolean isGet = "GET".equals(method);
if (isGet || "HEAD".equals(method)) {
long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
if (logger.isDebugEnabled()) {
String requestUri = urlPathHelper.getRequestUri(request);
logger.debug("Last-Modified value for [" + requestUri + "] is: " + lastModified);
}
if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
return;
}
}
// 这里是处理前置拦截器
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())) {
triggerAfterCompletion(mappedHandler, interceptorIndex, processedRequest, response, null);
return;
}
interceptorIndex = i;
}
}
//处理最终的Action逻辑
mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
// Do we need view name translation?
if (mv != null && !mv.hasView()) {
mv.setViewName(getDefaultViewName(request));
}
//处理后置拦截器
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");
}
}
// 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);
}
}
}