HttpServlet

由于大多数Web应用都是通过HTTP协议和客户端进行交互，因此，在Servlet 接口中，提供了一个抽象类javax.servlet.http.HttpServlet，它是GenericServlet的子类，专门用于创建应用于HTTP协议的Servlet。为了大家可以更好地了解HttpServlet，接下来，我们看一下HttpServlet类的源代码片段，具体如下：

 1    public abstract class HttpServlet extends GenericServlet {
 2        protected void doGet(HttpServletRequest req, 
                             HttpServletResponse resp)
 3            throws ServletException, IOException
 4        {
 5            ......
 6        }
 7        protected void doPost(HttpServletRequest req, 
                              HttpServletResponse resp)
 8            throws ServletException, IOException {
 9            ......
 10        }
 11        protected void service(HttpServletRequest req, 
                               HttpServletResponse resp)
 12            throws ServletException, IOException {
 13           String method = req.getMethod();
 14            if (method.equals(METHOD_GET)) {
 15                long lastModified = getLastModified(req);
 16                if (lastModified == -1) {
 17              // servlet doesn't support if-modified-since, no reason
 18              // to go through further expensive logic
 19               doGet(req, resp);
 20                } else {
 21          long ifModifiedSince =req.getDateHeader(HEADER_IFMODSINCE);
 22               if (ifModifiedSince < (lastModified / 1000 * 1000)) {
 23              // If the servlet mod time is later, call doGet()
 24              // Round down to the nearest second for a proper compare
 25              // A ifModifiedSince of -1 will always be less
 26                        maybeSetLastModified(resp, lastModified);
 27                        doGet(req, resp);
 28                    } else {
 29          resp.setStatus(HttpServletResponse.SC_NOT_MODIFIED);
 30                    }
 31                }
 32            } else if (method.equals(METHOD_HEAD)) {
 33                long lastModified = getLastModified(req);
 34                maybeSetLastModified(resp, lastModified);
 35                doHead(req, resp);        
 36            } else if (method.equals(METHOD_POST)) {
 37                doPost(req, resp);   
 38            } else if (method.equals(METHOD_PUT)) {
 39                doPut(req, resp);        
 40            } else if (method.equals(METHOD_DELETE)) {
 41                doDelete(req, resp);
 42            } else if (method.equals(METHOD_OPTIONS)) {
 43                doOptions(req,resp);
 44            } else if (method.equals(METHOD_TRACE)) {
 45                doTrace(req,resp);
 46            } else {
 47                String errMsg = 
     lStrings.getString("http.method_not_implemented");
 48                Object[] errArgs = new Object[1];
 49                errArgs[0] = method;
 50                errMsg = MessageFormat.format(errMsg, errArgs);
 51       resp.sendError(HttpServletResponse.SC_NOT_IMPLEMENTED, errMsg);
 52            }
 53        }
 54        public void service(ServletRequest req, ServletResponse res)
 55                throws ServletException, IOException {
 56            HttpServletRequest request;
 57            HttpServletResponse response;
 58            try {
 59                request = (HttpServletRequest) req;
 60                response = (HttpServletResponse) res;
 61            } catch (ClassCastException e) {
 62        throw new ServletException("non-HTTP request or response");
 63            }
 64            service(request, response);
 65        }
 66    }

通过分析HttpServlet的源代码片段，发现HttpServlet主要有两大功能，第一是根据用户请求方式的不同，定义相应的doXxx()方法处理用户请求。例如，与GET请求方式对应的doGet()方法，与POST方式对应的doPost()方法。第二是通过service()方法将HTTP请求和响应分别转为HttpServletRequest和HttpServletResponse类型的对象。

需要注意的是，由于HttpServlet类在重写的service()方法中，为每一种HTTP请求方式都定义了对应的doXxx()方法，因此，当定义的类继承HttpServlet后，只需根据请求方式，重写对应的doXxx()方法即可，而不需要重写service()方法。