重温模糊知识点之继承httpServlet为什么不重写service方法

来源：互联网发布：ps4应用程序数据损坏编辑：程序博客网时间：2024/05/01 05:13

当写一般的servlet的时候，我们编写一个servlet 去继承 GenericServlet ，那么我们就得覆盖它的一个抽象方法： abstract service(ServletRequest,ServletResponse)这个抽象方法。

存在这个一个继承关系：
interface Servlet
|---- abstract GenericServlet
|---abstract HttpServlet

我们在说一说Servlet的生命周期:init-----》service----》destory
1.默认情况下，Servlet是在第一次被访问的时候调用它的init()方法进行初始化的，执行Service方法后，调用destory()方法消亡，生命周期完结。
2.可以进行配置，使得Servlet在容器（tomcat）加载的时候，进行初始化，在web.xml中进行下面的配置：

<servlet><servlet-name>Servlet名字</servlet-name><servlet-class>servlet完整类名</servlet-class><load-on-startup>1（如果初始化的servlet多的话，这里配置先后次序)</load-on-startup></servlet>

当写一般的servlet的时候，我们编写一个servlet 去继承 GenericServlet ，那么我们就得覆盖它的一个抽象方法： abstract service(ServletRequest,ServletResponse)这个抽象方法,执行业务，但是为什么在写有关Http协议的Servlet的时候，我们没有重写service方法？
下面是HttpServlet的原码：

  /**     * Receives standard HTTP requests from the public     * <code>service</code> method and dispatches     * them to the <code>do</code><i>XXX</i> methods defined in      * this class. This method is an HTTP-specific version of the      * {@link javax.servlet.Servlet#service} method. There's no     * need to override this method.     *     * @param req   the {@link HttpServletRequest} object that     *                  contains the request the client made of     *                  the servlet     *     * @param resp  the {@link HttpServletResponse} object that     *                  contains the response the servlet returns     *                  to the client                                     *     * @exception IOException   if an input or output error occurs     *                              while the servlet is handling the     *                              HTTP request     *     * @exception ServletException  if the HTTP request     *                                  cannot be handled     *      * @see javax.servlet.Servlet#service     */    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);                }            }        } else if (method.equals(METHOD_HEAD)) {            long lastModified = getLastModified(req);            maybeSetLastModified(resp, lastModified);            doHead(req, resp);        } else if (method.equals(METHOD_POST)) {            doPost(req, resp);                    } else if (method.equals(METHOD_PUT)) {            doPut(req, resp);                            } else if (method.equals(METHOD_DELETE)) {            doDelete(req, resp);                    } else if (method.equals(METHOD_OPTIONS)) {            doOptions(req,resp);                    } else if (method.equals(METHOD_TRACE)) {            doTrace(req,resp);                    } else {            //            // Note that this means NO servlet supports whatever            // method was requested, anywhere on this server.            //            String errMsg = lStrings.getString("http.method_not_implemented");            Object[] errArgs = new Object[1];            errArgs[0] = method;            errMsg = MessageFormat.format(errMsg, errArgs);                        resp.sendError(HttpServletResponse.SC_NOT_IMPLEMENTED, errMsg);        }    }

通过对原码的阅读我们发现，其实HttpServlet已经实现了GenericServlet类的service方法，并且对提交方式（get,post......）进行了一些列的判断,执行相应的do....()方法.我们看一下HttpServlet中doget dopost方法的实现：

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);        }    }  protected void doPost(HttpServletRequest req, HttpServletResponse resp)        throws ServletException, IOException {        String protocol = req.getProtocol();        String msg = lStrings.getString("http.method_post_not_supported");        if (protocol.endsWith("1.1")) {            resp.sendError(HttpServletResponse.SC_METHOD_NOT_ALLOWED, msg);        } else {            resp.sendError(HttpServletResponse.SC_BAD_REQUEST, msg);        }    }

通过阅读dopost和doget的原码，我们可以看到，doget和dopost都有默认的实现，但是这个实现是“通知实现类去重写该方法的提示信息”----也就是说，我们必须在HttpServlet的实现类里实现doget和dopost方法，并且，不能去重写service(httpServletRequest,HttpServletResponse)这个方法（实现后，就不能执行do....()方法了），所以不建议并且不能实现service方法。

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