Question

我正在尝试编写一个Java程序，该程序应该自动从网站下载文本，当且仅当它被更新时。我遇到的问题是只使用一个HTTPURLConnection来执行此操作，因为如果我不会有数十亿的HTTPURLConnections到Web服务器，因为我正在使用while（true）循环。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，每次我必须阅读一行时，我都会开始新的连接，这不是我确定的最有效的方式。如何使用相同的HTTPURLConnection继续读取同一行？

// 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;
  }

}

Answer 1

为什么不简单地在while循环之外声明你的HttpURLConnection对象？然后它不会在循环内的每次调用时打开连接。

Answer 2

HttpURLConnection无法重复使用，但可以通过设置标头Connection: keep-alive在内部重用与同一服务器的开放连接。如果你连接到不同的服务器，显然没有意义。

有效测试是否有更改的一种方法是使用If-Modified-Since标题或类似内容If-Unmodified-Since，If-None-Match或If-Match（请参阅HTTP Headers）。在这种情况下，如果有更改，Web服务器会决定发送新文档，或者只发送没有正文的响应代码304 Not Modified。

关于使用成员（尤其是静态成员）的最后一件事：我重构了这段代码，唯一可以保留为静态的项目是static void main()。您可以将静态成员视为某种全局变量。观察类似于＆＃39;连接正在返回相同的消息＆＃39;可能是不适当的异常处理和静态成员使用的影响。

单个URLConnection使用BufferedReader仅获取java中的第一行

2 个答案: