HttpServlet类源码的getLastModified()返回值为什么是-1?

ゝ一世哀愁。 2022-02-24 02:54 213阅读 0赞

# 关于 HttpServlet #

## 有关Last-Modified缓存机制 ##

*  在客户端第一次输入url是，服务器端会返回内容和状态码200，表示请求成功，同时会添加一个“Last-Modified”的响应头，表示此文件在服务器上的最后更新时间；
 *  客户端第二次请求此URL时，客户端会像服务器发送请求头“If-Modified-Since”，询问服务器该时间之后当前请求内容是否有被修改过，如果服务器的内容没有变化，则自动返回HTTP304状态码（只要响应头，内容为空，这样就节省了网络带宽）

今日复习servlet时读了HttpServlet的源码,发现了最后修改时间这个方法直接返回-1,那这样不是就无视lastModified,始终继续调用方法吗?NO,NO!可以在子类中进行覆写呀,这样就可以定义最后修改时间了呀!

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;
                    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
                        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);
            }
        }