Java深入
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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
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);
- }
- }
- }
原文章地址:http://blog.csdn.net/initphp/article/details/38171219
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