ServletAPI详解

Servlet 技术是JavaWeb中最核心的组件Servlet主要功能：1,动态生成HTML文档2,转发请求给同Web中的另一个Servlet组件3,转发请求至其他Web应用中的Servlet组件4.读取以及写入Cookie5.访问其他服务器资源(数据库or 基于java的应用程序)Servlet APIServlet接口：public interface Servlet{public abstract void init(ServletConfig servletconfig) throws ServletException;//init方法负责初始化Servlet对象，容器在创建好Servlet对象就会调用该方法(个人认为这方法估计在构造函数中调用的) public abstract void service(ServletRequestservletrequest, ServletResponse servletresponse) throws ServletException, IOException;//该方法负责响应用户请求，为客户提供相应的服务public abstract void destroy();//该方法负责释放Servlet对象所占用的资源，Servlet结束生命周期时调用public abstract String getServletInfo();//返回字符串包含版本创建者以及版权信息public abstract ServletConfig getServletConfig();//返回ServletConfig对象，该对象包含Servlet初始化信息}在ServletAPI中 javax.servlet.GenericServlet抽象类实现Servlet接口。

javax.servlet.http.HttpServlet是GenericServlet 抽象类的子类。

我们开发的Servlet大多从继承HttpServlet开始。

下面介绍javax.servlet.GenericServlet抽象类public abstract class GenericServletimplements Servlet, ServletConfig,Serializable {private transient ServletConfig config;public GenericServlet() {}public void destroy() {}public String getInitParameter(String name) {return getServletConfig().getInitParameter(name);}public Enumeration getInitParameterNames() {return getServletConfig().getInitParameterNames();}public ServletConfig getServletConfig() {return config;}public ServletContext getServletContext() {return getServletConfig().getServletContext();}public String getServletInfo() {return"";}public void init(ServletConfig config) throwsServletException {this.config = config;init();}public void init() throws ServletException {}public void log(String msg) {getServletContext().log(getServletName() + ": " + msg);}public void log(String message, Throwable t) {getServletContext().log(getServletName() + ": " + message, t);}public abstract void service(ServletRequest servletrequest, ServletResponse servletresponse) throws ServletException, IOException;//该方法是GenericServlet类中唯一抽象方法，交由子类实现从而为特定客户提供特定的服务public String getServletName() {return config.getServletName();}}HttpServlet抽象类该类是GenericServlet的子类,提供了HTTP协议相关的通用实现，在HTTP协议的通信的Servlet中一般都继承HttpServlet类(部分代码)public abstract class HttpServlet extends GenericServlet implementsSerializable {private static final String METHOD_DELETE = "DELETE";private static final String METHOD_GET = "GET";private static final String METHOD_POST = "POST";private static final String METHOD_PUT = "PUT";public HttpServlet() {}//HttpServlet为所有请求方式都提供了doXXX();并提供了默认实现，若该方法没有被子类重写，则会向客户端返回一个错误：返回错误之前判断HTTP协议版本，若版本为HTTP1.1则（405）；否则400.此处以doGet()为例。

此种写法很像抽象类中的抽象方法，此处虽不强制子类实现，但若想调用且未重写，则返回一个错误。

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(405, msg);elseresp.sendError(400, msg);}protected long getLastModified(HttpServletRequest req) {return -1L;}private Method[] getAllDeclaredMethods(Class c) {if (c.getName().equals("javax.servlet.http.HttpServlet"))return null;int j = 0;Method parentMethods[] = getAllDeclaredMethods(c.getSuperclass());Method thisMethods[] = c.getDeclaredMethods();if (parentMethods != null) {Method allMethods[] = new Method[parentMethods.length+ thisMethods.length];for (int i = 0; i < parentMethods.length; i++) {allMethods[i] = parentMethods[i];j = i;}for (int i = ++j; i < thisMethods.length + j; i++)allMethods[i] = thisMethods[i - j];return allMethods;} else {return thisMethods;}}protected void service(HttpServletRequest req, HttpServletResponse resp)throws ServletException, IOException {String method = req.getMethod();if (method.equals("GET")) {long lastModified = getLastModified(req);if (lastModified == -1L) {doGet(req, resp);} else {long ifModifiedSince = req.getDateHeader("If-Modified-Since");if (ifModifiedSince < (lastModified / 1000L) * 1000L) {maybeSetLastModified(resp, lastModified);doGet(req, resp);} else {resp.setStatus(304);}}} else if (method.equals("HEAD")) {long lastModified = getLastModified(req);maybeSetLastModified(resp, lastModified);doHead(req, resp);} else if (method.equals("POST"))doPost(req, resp);else if (method.equals("PUT"))doPut(req, resp);else if (method.equals("DELETE"))doDelete(req, resp);else if (method.equals("OPTIONS"))doOptions(req, resp);else if (method.equals("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(501, errMsg);}}// 该方法其实是调用了重载过后的service(HttpServletRequest req, HttpServletResponse resp)方法；然后根据request对象获得用户的请求类型，决定调用不同方法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);}}ServletRequest接口ServletRequest类表示来自客户端的请求；当Servlet容器接收到客户端要求访问的特定的Servlet请求时，容器先解析客户端请求的数据，并包装成一个Servlet 对象。