service（）详解

ゞ浴缸里的玫瑰 2022-04-01 03:24 191阅读 0赞

> \*从public<code>service</code>方法接收标准HTTP请求，并将它们分派到该类中定义的<code>do</code><i>.</i>方法。此方法是\{@link javax.servlet.Servlet\#service\}方法的HTTP特定版本。不需要重写此方法。

源码：

/**
         * Receives standard HTTP requests from the public
         * <code>service</code> method and dispatches
         * them to the <code>do</code><i>Method</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
                    //servlet不支持if-modified-since因为，没有理由进行更昂贵的逻辑
                    doGet(req, resp);
                } else {
                    long ifModifiedSince;
                    try {
                        ifModifiedSince = req.getDateHeader(HEADER_IFMODSINCE);
                    } catch (IllegalArgumentException iae) {
                        // Invalid date header - proceed as if none was set
                        ifModifiedSince = -1;
                    }
                    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
                        /*
                          如果尚未设置Last-Modified实体标头字段，
                          如果它还没有被设置并且这个值是有用的，则设置该字段。在doGet之前调用，
                          以确保在写入响应数据之前设置头。子类可能已经设置了这个标头，
                          所以我们进行检查。
                        */
                        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.
                //
    /*注意，这意味着在这个服务器上的任何地方，任何servlet都不支持请求的任何方法。*/
    
                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);
            }
        }

自己理解是，service方法负责根据客户端请求的方式，来调用不同的方法执行请求和响应。

service有个重载的方法，**public void service(ServletRequest req, ServletResponse res)**

它的主要作用是把请求和响应的类型从

**ServletRequest req   =>  HttpServletRequest  request;**

**ServletResponse res  => HttpServletResponse response;**

看源码：

作用就是强转，转不了报错。

/**
         * Dispatches client requests to the protected
         * <code>service</code> 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 res   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
         */
        @Override
        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);
        }