Servlet -doGet() doPost()原理

一、自定义类只需要重写doGet(HttpServletRequest request, HttpServletResponse response) 和doPost(HttpServletRequest request, HttpServletResponse, response)的原因：

1、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，然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);
    }
}

2、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就只需要重写doGet和doPost即可。

二、Servlet的生命周期：

1、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需要这个Servlet响应第一个请求时，将加载这个Servlet。加载的方式是通过反射的API去调用Servelet的无参构造方法完成，所以在自定义的Servlet中不应该出现带参数的构造方法，如果需要定义有参的构造方法，则一定要把无参的也写出来。

2、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、连接DB等。

3、处理请求：调用service()方法处理请求。之前一这是init()方法执行成功。

4、销毁Servlet对象：调用detroy()方法。