重温模糊知识点之继承httpServlet为什么不重写service方法
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名字
servlet完整类名
1（如果初始化的servlet多的话，这里配置先后次序)
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
* service method and dispatches
* them to the doXXX 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。