servlet中service() 和doGet() 、doPost() 学习笔记

Sevlet接口定义如下：

与Sevlet接口相关的结构图：

service() 方法是 Servlet 的核心。每当一个客户请求一个HttpServlet 对象，该对象的service() 方法就要被调用(由web容器调用)，在GenericServlet中定义了service抽象方法：

public abstract void service(ServletRequest req, ServletResponse res) throws ServletException, IOException;

在HttpSevlet中覆写了GenericServlet中的service方法，源码如下：

   /**
     * This method is an HTTP-specific version of the method. There's no need to override this method.
     * @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);//调用doGet(rq,rp)
            } else {
                long ifModifiedSince = req.getDateHeader(HEADER_IFMODSINCE);
                if (ifModifiedSince < lastModified) {
                    // 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);
        }
    }
    

 从上可知service是用来转向的，并且service() 方法由容器调用，所以我们一般写一个servlet，只需要重写doGet或者doPost就可以了，例如，当一个客户通过HTML 表单发出一个HTTP POST请求时，doPost()方法被调用，发出一个HTTP GET请求时doGet()方法被调用。当然还有doTrace()、doDelete()、doHead()类似。