我解析位于Web服务器XML文件，并存储分析数据数据库。我的应用程序，我使用的数据从数据库中。我需要解析只有当文件被修改，否则没必要解析xml文件。所以，我怎么能知道这个文件被修改？我知道我可以使用如果 - 修改 - 自头。但  我需要的如果 - 修改 - 自头一些例子 请帮我.......

I am parsing Xml file located in web server and storing parsed data in to database. for my app I am using data from database. I need to parse the xml file only if the file is modified otherwise no need to parse. So how can I know the file is modified? I know I can use "if-modified-since" header. But I need some examples of "if-modified-since" header please help me.......

推荐答案

由于您是从网络服务器检索您的.xml文件，这应该是无需做服务器端MD5校验和比较容易的。

Since you are retrieving your .xml file from a web server, this should be relatively easy without having to do a server side MD5 sum.

如果你正在做的XML文件，你可以简单地执行从Web服务器HEAD请求，这将返回如果该文件已经修改/修改，或者如果它不存在的HTTP请求。这也是轻量级的，最好的部分是服务器应该已经为你做这个。

If you are doing a HTTP request for the xml file you can simply perform a HEAD request from the web server and this will return if the file has changed/modified or if it doesn't exist. This is also lightweight and the best part is that the server should already do this for you.

修改：重读你的问题，看起来你有同样的想法。这里的code。

Edit: re-reading your question, looks like you had the same idea. Here's the code.

import java.net.*;
import java.io.*;

// Using HTTP_NOT_MODIFIED
public static boolean Changed(String url){
    try {
      HttpURLConnection.setFollowRedirects(false);
      HttpURLConnection con = (HttpURLConnection) new URL(url).openConnection();
      con.setRequestMethod("HEAD");
      return (con.getResponseCode() == HttpURLConnection.HTTP_NOT_MODIFIED);
    }
    catch (Exception e) {
       e.printStackTrace();
       return false;
    }
  }

// GET THE LAST MODIFIED TIME
public static long LastModified(String url)
{
  HttpURLConnection.setFollowRedirects(false);
  HttpURLConnection con = (HttpURLConnection) new URL(url).openConnection();
  long date = con.getLastModified();

  if (date == 0)
    System.out.println("No last-modified information.");
  else
    System.out.println("Last-Modified: " + new Date(date));

  return date;
}

请参阅：

HttpURLConnection HyperText_Transfer_Protocol的HTTPStatus 304（未修改） HttpURLConnection HyperText_Transfer_Protocol HttpStatus 304 (Not Modified)

另外，如果您的服务器支持他们，你可以使用的ETag来看看你的文件已被修改。

Alternatively if your server supports them you can use ETags to find out if your file has been modified.

http://www.xpertdeveloper.com/2011/03/last-modified-header-vs-expire-header-vs-etag/