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
可以看下HttpServlet类:
package javax.servlet.http; import java.io.IOException; import java.io.PrintWriter; import java.io.OutputStreamWriter; import java.io.UnsupportedEncodingException; import java.lang.reflect.Method; import java.text.MessageFormat; import java.util.Enumeration; import java.util.Locale; import java.util.ResourceBundle; import javax.servlet.GenericServlet; import javax.servlet.ServletException; import javax.servlet.ServletOutputStream; import javax.servlet.ServletRequest; import javax.servlet.ServletResponse; public abstract class HttpServlet extends GenericServlet implements java.io.Serializable { private static final String METHOD_DELETE = "DELETE"; private static final String METHOD_HEAD = "HEAD"; private static final String METHOD_GET = "GET"; private static final String METHOD_OPTIONS = "OPTIONS"; private static final String METHOD_POST = "POST"; private static final String METHOD_PUT = "PUT"; private static final String METHOD_TRACE = "TRACE"; private static final String HEADER_IFMODSINCE = "If-Modified-Since"; private static final String HEADER_LASTMOD = "Last-Modified"; private static final String LSTRING_FILE = "javax.servlet.http.LocalStrings"; private static ResourceBundle lStrings = ResourceBundle.getBundle(LSTRING_FILE); public HttpServlet() { } //处理HTTP的GET请求,你需要实现一个Servlet,然后实现该方法 protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { String protocol = req.getProtocol(); String msg = lStrings.getString("http.method_get_not_supported"); if (protocol.endsWith("1.1")) { resp.sendError(HttpServletResponse.SC_METHOD_NOT_ALLOWED, msg); } else { resp.sendError(HttpServletResponse.SC_BAD_REQUEST, msg); } } protected long getLastModified(HttpServletRequest req) { return -1; } protected void doHead(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { NoBodyResponse response = new NoBodyResponse(resp); doGet(req, response); response.setContentLength(); } //实现HTTP POST请求 protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { String protocol = req.getProtocol(); String msg = lStrings.getString("http.method_post_not_supported"); if (protocol.endsWith("1.1")) { resp.sendError(HttpServletResponse.SC_METHOD_NOT_ALLOWED, msg); } else { resp.sendError(HttpServletResponse.SC_BAD_REQUEST, msg); } } protected void doPut(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { String protocol = req.getProtocol(); String msg = lStrings.getString("http.method_put_not_supported"); if (protocol.endsWith("1.1")) { resp.sendError(HttpServletResponse.SC_METHOD_NOT_ALLOWED, msg); } else { resp.sendError(HttpServletResponse.SC_BAD_REQUEST, msg); } } protected void doDelete(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { String protocol = req.getProtocol(); String msg = lStrings.getString("http.method_delete_not_supported"); if (protocol.endsWith("1.1")) { resp.sendError(HttpServletResponse.SC_METHOD_NOT_ALLOWED, msg); } else { resp.sendError(HttpServletResponse.SC_BAD_REQUEST, msg); } } private Method[] getAllDeclaredMethods(Class c) { if (c.equals(javax.servlet.http.HttpServlet.class)) { return null; } Method[] parentMethods = getAllDeclaredMethods(c.getSuperclass()); Method[] thisMethods = c.getDeclaredMethods(); if ((parentMethods != null) && (parentMethods.length > 0)) { Method[] allMethods = new Method[parentMethods.length + thisMethods.length]; System.arraycopy(parentMethods, 0, allMethods, 0, parentMethods.length); System.arraycopy(thisMethods, 0, allMethods, parentMethods.length, thisMethods.length); thisMethods = allMethods; } return thisMethods; } protected void doOptions(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { Method[] methods = getAllDeclaredMethods(this.getClass()); boolean ALLOW_GET = false; boolean ALLOW_HEAD = false; boolean ALLOW_POST = false; boolean ALLOW_PUT = false; boolean ALLOW_DELETE = false; boolean ALLOW_TRACE = true; boolean ALLOW_OPTIONS = true; for (int i=0; i<methods.length; i++) { Method m = methods[i]; if (m.getName().equals("doGet")) { ALLOW_GET = true; ALLOW_HEAD = true; } if (m.getName().equals("doPost")) ALLOW_POST = true; if (m.getName().equals("doPut")) ALLOW_PUT = true; if (m.getName().equals("doDelete")) ALLOW_DELETE = true; } String allow = null; if (ALLOW_GET) if (allow==null) allow=METHOD_GET; if (ALLOW_HEAD) if (allow==null) allow=METHOD_HEAD; else allow += ", " + METHOD_HEAD; if (ALLOW_POST) if (allow==null) allow=METHOD_POST; else allow += ", " + METHOD_POST; if (ALLOW_PUT) if (allow==null) allow=METHOD_PUT; else allow += ", " + METHOD_PUT; if (ALLOW_DELETE) if (allow==null) allow=METHOD_DELETE; else allow += ", " + METHOD_DELETE; if (ALLOW_TRACE) if (allow==null) allow=METHOD_TRACE; else allow += ", " + METHOD_TRACE; if (ALLOW_OPTIONS) if (allow==null) allow=METHOD_OPTIONS; else allow += ", " + METHOD_OPTIONS; resp.setHeader("Allow", allow); } protected void doTrace(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { int responseLength; String CRLF = "\r\n"; String responseString = "TRACE "+ req.getRequestURI()+ " " + req.getProtocol(); Enumeration reqHeaderEnum = req.getHeaderNames(); while( reqHeaderEnum.hasMoreElements() ) { String headerName = (String)reqHeaderEnum.nextElement(); responseString += CRLF + headerName + ": " + req.getHeader(headerName); } responseString += CRLF; responseLength = responseString.length(); resp.setContentType("message/http"); resp.setContentLength(responseLength); ServletOutputStream out = resp.getOutputStream(); out.print(responseString); out.close(); return; } //最终所有的具体实现都是需要实现Service方法.容器最终调用的就是该方法 protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { String method = req.getMethod(); //这边通过HTTP的方法来判断,具体的HTTP METHOD,然后决定需要执行的方法名称。 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); } } private void maybeSetLastModified(HttpServletResponse resp, long lastModified) { if (resp.containsHeader(HEADER_LASTMOD)) return; if (lastModified >= 0) resp.setDateHeader(HEADER_LASTMOD, lastModified); } 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); } }
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。
可以看下下面的Servlet接口:
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); } } }