1、自定义类只须要重写doGet(HttpServletRequest request, HttpServletResponse response) 和doPost(HttpServletRequest request, HttpServletResponse, response)的缘由：
一、在HttpServlet接口中重写了父类的service(ServletRequest request, ServletResponse response),在这里面主要完成了将父类的request和response对象转化成专门用于处理HTTP请求响应的HttpServletRequest和HttpServletResponse对象，而后再调用一个重载的service方法，该方法接收HttpServletRequest和HttpServletResponse对象：
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);
}
}
二、在重载的service方法中，调用了doGet(HttpServletRequest request, HttpServletResponse)和doPost(HttpServletRequest request, HttpServletResponse)，用于处理Get和Post请求和响应：
protected void service(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException
{
String method = req.getMethod();
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);
}
}
}
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);
}
}
由上可知，Servlet引擎在处理用户请求时所调用的service(ServletRequest req, ServletResponse rep)最终会执行doGet和doPost方法中的代码完成。这就是在自定义的Servlet类中，只要继承了HttpServlet就只须要重写doGet和doPost便可。 2、Servlet的生命周期： 一、加载和实例化Servlet对象：由Servlet窗口负责加载Servlet，当Servlet窗口启动或是检测到须要这个Servlet响应第一个请求时，将加载这个Servlet。加载的方式是经过反射的API去调用Servelet的无参构造方法完成，因此在自定义的Servlet中不该该出现带参数的构造方法，若是须要定义有参的构造方法，则必定要把无参的也写出来。 二、初始化Servlet对象：调用init()方法完成相应的初始化，如读取初始化参数、链接DB等。 三、处理请求：调用service()方法处理请求。以前一这是init()方法执行成功。 四、销毁Servlet对象：调用detroy()方法。