Servlet生命周期中的service方法分析

问题
ServletLifeCycle中的service方法内,有super.service(request, response); 会执行this.doGet(HttpServletRequest request, HttpServletResponse response);
没有super.service(request, response);,则不执行this.doGet(...). 是怎么实现的?

举一反三:
一个子类,覆写的方法内,如果调用了父类的该方法,会执行子类内的另一个方法;
覆写的方法内,如果没有调用父类的该方法,就不会执行子类内的另一个方法;

分析 ----->符号是关键注释

  1 public class ServletLifeCycle extends HttpServlet {
  2     private static final long serialVersionUID = 1L;
  3
  4     @Override
  5     public void service(ServletRequest request, ServletResponse response) throws ServletException, IOException {
  6         // TODO Auto-generated method stub
  7         //this.doGet((HttpServletRequest)request, (HttpServletResponse)response);
  8         super.service(request, response);//------------------->执行父类的service(ServletRequest request, ServletResponse response)方法
  9
 10         System.out.println("处理客户端请求");
 11     }
 12
 13     protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
 14         // TODO Auto-generated method stub
 15         System.out.println("处理过程");
 16         response.getWriter().append("Served at: ").append(request.getContextPath());
 17     }
 18
 19 }
 20
 21
 22
 23
 24 public abstract class HttpServlet extends GenericServlet {
 25 public void service(ServletRequest req, ServletResponse res)
 26     throws ServletException, IOException {
 27
 28     HttpServletRequest  request;
 29     HttpServletResponse response;
 30
 31     try {
 32         request = (HttpServletRequest) req;
 33         response = (HttpServletResponse) res;
 34     } catch (ClassCastException e) {
 35         throw new ServletException("non-HTTP request or response");
 36     }
 37     service(request, response);//------------------->父类的方法的重载执行父类的service(HttpServletRequest request, HttpServletResponse response)方法
 38     //------------------->我的理解是,如果没有重载,会出现死循环.   走到此处又执行子类ServletLifeCycle的service方法,子类又调用父类service方法,循环嵌套.
 39 }
 40
 41
 42 protected void service(HttpServletRequest req, HttpServletResponse resp)
 43         throws ServletException, IOException {
 44
 45         String method = req.getMethod();
 46
 47         if (method.equals(METHOD_GET)) {
 48             long lastModified = getLastModified(req);
 49             if (lastModified == -1) {
 50                 // servlet doesn‘t support if-modified-since, no reason
 51                 // to go through further expensive logic
 52                 doGet(req, resp);//------------------->调用子类ServletLifeCycle的doGet方法
 53             } else {
 54                 long ifModifiedSince;
 55                 try {
 56                     ifModifiedSince = req.getDateHeader(HEADER_IFMODSINCE);
 57                 } catch (IllegalArgumentException iae) {
 58                     // Invalid date header - proceed as if none was set
 59                     ifModifiedSince = -1;
 60                 }
 61                 if (ifModifiedSince < (lastModified / 1000 * 1000)) {
 62                     // If the servlet mod time is later, call doGet()
 63                     // Round down to the nearest second for a proper compare
 64                     // A ifModifiedSince of -1 will always be less
 65                     maybeSetLastModified(resp, lastModified);
 66                     doGet(req, resp);
 67                 } else {
 68                     resp.setStatus(HttpServletResponse.SC_NOT_MODIFIED);
 69                 }
 70             }
 71
 72         } else if (method.equals(METHOD_HEAD)) {
 73             long lastModified = getLastModified(req);
 74             maybeSetLastModified(resp, lastModified);
 75             doHead(req, resp);
 76
 77         } else if (method.equals(METHOD_POST)) {
 78             doPost(req, resp);
 79
 80         } else if (method.equals(METHOD_PUT)) {
 81             doPut(req, resp);
 82
 83         } else if (method.equals(METHOD_DELETE)) {
 84             doDelete(req, resp);
 85
 86         } else if (method.equals(METHOD_OPTIONS)) {
 87             doOptions(req,resp);
 88
 89         } else if (method.equals(METHOD_TRACE)) {
 90             doTrace(req,resp);
 91
 92         } else {
 93             //
 94             // Note that this means NO servlet supports whatever
 95             // method was requested, anywhere on this server.
 96             //
 97
 98             String errMsg = lStrings.getString("http.method_not_implemented");
 99             Object[] errArgs = new Object[1];
100             errArgs[0] = method;
101             errMsg = MessageFormat.format(errMsg, errArgs);
102
103             resp.sendError(HttpServletResponse.SC_NOT_IMPLEMENTED, errMsg);
104         }
105     }
106
107
108 protected void doGet(HttpServletRequest req, HttpServletResponse resp)
109     throws ServletException, IOException
110 {
111     String protocol = req.getProtocol();
112     String msg = lStrings.getString("http.method_get_not_supported");
113     if (protocol.endsWith("1.1")) {
114         resp.sendError(HttpServletResponse.SC_METHOD_NOT_ALLOWED, msg);
115     } else {
116         resp.sendError(HttpServletResponse.SC_BAD_REQUEST, msg);
117     }
118 }
119 }