单个URLConnection使用BufferedReader仅获取Java中的第一行 [英] Single URLConnection to fetch only the first line in java using BufferedReader

问题描述

我正在尝试编写一个Java程序，当且仅当更新时，该程序才应自动从网站下载文本.我遇到的问题是仅使用一个HTTPURLConnection来执行此操作，因为如果不这样做，由于使用了while(true)循环，将有数十亿个HTTPURLConnections到达Web服务器.这是我正在进行的工作，getMsg()方法接收一个URL并打开一个HTTPURLConnection.目前，每次必须读一行时，我都会开始一个新的连接，这不是我确定的最有效的方法.如何使用相同的HTTPURLConnection继续读取同一行?

I am trying to write a Java program that should automatically download text from a website if and only if it gets updated. The problem I am running into is using only one HTTPURLConnection to do that because if i don't there will be billions of HTTPURLConnections to the web server since I am using a while(true) loop. Here is my work-in-progress, the getMsg() method receives a url and opens an HTTPURLConnection. Currently I am starting a new connection every time I have to read a line, which is not the most efficient way I am sure. How do I keep reading the same line with the same HTTPURLConnection?

// Starts a new URLConnection to "localhost/currentmsg.dat"
// Receives JSON string from the URLConnection
// Sets up a different variable for each object received from the URL for eg. if delete=1 means that the admin is requesting to delete a message from the screen.

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

import org.json.*;
import java.io.*;
import java.util.Scanner;

public class jListenerURL {

// Current arguments retrieved from the URL
private static int msgID = 0;
private static int aptID = 1; // Apartment ID of current device
private static int unitID = 3; // Unit ID of current device
static String message; // Message received from admin
static int delete; // Delete a message?
static int dmsgID; // What message to delete?

public static void jListener() {

    URL url;
    boolean keepGoing = true;
    String msg = "";

    try {
        url = new URL("http://www.lotussmoke.com/msgtest/currentmsg.dat");
        while (keepGoing) {
            msg = getMsg(url);

            JSONObject jObj = null;

            try {
                jObj = new JSONObject(msg);
            }

            catch (JSONException je) {
                System.out.println("JSON Exception for message, try restarting terminal.");
            }

            int current = jObj.getInt("msgID");
            int targetaptID = jObj.getInt("aptID");
            int targetunitID = jObj.getInt("unitID");

            // Keep listening, if the message changes meaning a different msgID then print that message
            if (current!=msgID && targetaptID == aptID && targetunitID == unitID) {

                msgID = jObj.getInt("msgID");
                message = jObj.getString("message");
                delete = jObj.getInt("delete");
                dmsgID = jObj.getInt("dmsgID");

                if (delete==1) {
                    // Delete a message
                    System.out.println("Delete msg ID? " + dmsgID);
                    continue;
                }

                System.out.println("Message ID: " + msgID);
                System.out.println("Apartment ID: " + aptID);
                System.out.println("Unit ID: " + unitID);
                System.out.println("Message: " + message);
            }
        }
    }

    catch (MalformedURLException e) {
        System.err.println();
    }
}

  public static void main(String args[]) throws Exception {
      jListener();
      }

  private static String getMsg(URL url) {
        HttpURLConnection con = null;
        try {
            con = (HttpURLConnection) url.openConnection();
        } catch (IOException e1) {
            e1.printStackTrace();
        }
        BufferedReader in = null;
        String msg = "";
        try {
            in = new BufferedReader(new InputStreamReader(con.getInputStream()));
            String received;
            while((received = in.readLine()) != null) {
                //System.out.println(received);
                msg = received;
                }
            in.close();
            }
        catch (IOException e) {
            e.printStackTrace();
            }
        return msg;
  }

}

推荐答案

HttpURLConnection无法重用，但是它可以通过设置标头Connection: keep-alive在内部重用与同一服务器的开放连接.显然，如果您连接到其他服务器，这是没有意义的.

HttpURLConnection cannot be reused, but it can reuse an open connection to the same server internally by setting the header Connection: keep-alive. It doesn't make sense if you connect to different servers, obviously.

一种有效测试是否存在更改的方法是使用If-Modified-Since标头或类似的If-Unmodified-Since，If-None-Match或If-Match(请参见

One way to efficiently test whether there are changes, is to use If-Modified-Since header or the like If-Unmodified-Since, If-None-Match or If-Match (see HTTP Headers). In this case the web-server decides to deliver a new document if there are changes or just sends the response code 304 Not Modified without a body.

关于成员(尤其是静态成员)的使用的最后一件事:我将重构此代码，而剩下的唯一静态元素是static void main().您可以将静态成员视为某种全局变量.观察到诸如连接返回同一条消息"之类的内容，可能是对异常处理和静态成员的使用不当造成的.

One last thing regarding the use of members (and especially static members): I'd refactor this code and the only item which would be left as static is static void main(). You can see the static members as some kind of global variables. Observing something like 'the connection is returning the same message' might be a effect of inappropriate exception handling and usage of static members.

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