重写getLastModified方法
HttpServlet新定义了service(HttpServletRequest req, HttpServletResponse resp)方法
protected void service(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException {
String method = req.getMethod();
long lastModified;
if (method.equals("GET")) {
lastModified = this.getLastModified(req);
if (lastModified == -1L) {
this.doGet(req, resp);
} else {
long ifModifiedSince;
try {
ifModifiedSince = req.getDateHeader("If-Modified-Since");
} catch (IllegalArgumentException var9) {
ifModifiedSince = -1L;
}
if (ifModifiedSince < lastModified / 1000L * 1000L) {
this.maybeSetLastModified(resp, lastModified);
this.doGet(req, resp);
} else {
resp.setStatus(304);
}
}
} else if (method.equals("HEAD")) {
lastModified = this.getLastModified(req);
this.maybeSetLastModified(resp, lastModified);
this.doHead(req, resp);
} else if (method.equals("POST")) {
this.doPost(req, resp);
} else if (method.equals("PUT")) {
this.doPut(req, resp);
}
// ...
}
}
其中,在处理get请求中有一个getLastModified
方法,默认如下
protected long getLastModified(HttpServletRequest req) {
return -1L;
}
这个函数返回的是要请求的资源最后一次更改的时间,返回-1也即不支持if-modified-since,这样的话每次请求都会进入doGet方法,可通过继承HttpServlet后重写getLastModified
方法达到没有修改时就不进入doGet以节省资源