Question

以下是简单的UDP客户端 - 服务器类：

UDPServer：

public class Song {

private int id, size, totalTime, discnumber, disccount, trackNumber, trackCount, year;
private String name, artist, composer, album, genre, kind;
public Song(int id, int size, int totalTime, int discnumber, int disccount,
        int trackNumber, int trackCount, int year, String name,
        String artist, String composer, String album, String genre,
        String kind) {
    super();
    this.id = id;
    this.size = size;
    this.totalTime = totalTime;
    this.discnumber = discnumber;
    this.disccount = disccount;
    this.trackNumber = trackNumber;
    this.trackCount = trackCount;
    this.year = year;
    this.name = name;
    this.artist = artist;
    this.composer = composer;
    this.album = album;
    this.genre = genre;
    this.kind = kind;
}

UDPClient：

import java.net.*;
/**
 * Source:https://systembash.com/a-simple-java-udp-server-and-udp-client/
 * 
 */
class UDPServer {    
    public static void main(String args[]) throws Exception {            
        DatagramSocket serverSocket = new DatagramSocket(9876);            
        byte[] receiveData = new byte[1024];
        byte[] sendData = new byte[1024];            
        while (true) {            
            DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length);

            serverSocket.receive(receivePacket);                
            String sentence = new String(receivePacket.getData());                 

            // Sentence here is not equal "PING" ?
            if(sentence.equals("PING")) {                       
                System.out.println("It is PING: " + sentence);          
            } else {
                System.out.println(sentence.getClass());
                System.out.println("It is not equal PING. It is <" + sentence + ">");
            }                          
            System.out.println("RECEIVED: " + sentence);

            InetAddress IPAddress = receivePacket.getAddress();                
            int port = receivePacket.getPort();                
            String capitalizedSentence = "PONG"; 
            sendData = capitalizedSentence.getBytes();
            DatagramPacket sendPacket
                    = new DatagramPacket(sendData, sendData.length, IPAddress, port);
            serverSocket.send(sendPacket);
        }
    }
}

在UDPServer中检查响应（变量句子）。由于某种原因，等于必须时才检测到“PING”。而且它输出的句子字符串是“PING”，类字符串。这是为什么？

UDPServer的输出是else：

import java.net.*;
/**
 * Source:
 * https://systembash.com/a-simple-java-udp-server-and-udp-client/
 *
 * @author  
 */
class UDPClient {
    public static void main(String args[]) throws Exception {       
        DatagramSocket clientSocket = new DatagramSocket();        
        InetAddress IPAddress = InetAddress.getByName("localhost");        
        byte[] sendData = new byte[1024];
        byte[] receiveData = new byte[1024];

        String sentence =  "PING";
        sendData = sentence.getBytes();

        DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, IPAddress, 9876);
        clientSocket.send(sendPacket);        
        DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length);
        clientSocket.receive(receivePacket);        
        String modifiedSentence = new String(receivePacket.getData());
        System.out.println("FROM SERVER:" + modifiedSentence);        
        clientSocket.close();
    }
}

如果重要的话，java版本是“1.7.0_91”。

Answer 1

getData（）为您提供了一个缓冲区，您可以从中获取数据。

不要使用trim（），因为这可能会删除您实际想要保留的字符（除了价格昂贵）

相反，你应该使用消息的长度

String sentence = new String(receivePacket.getData(), 0, receivePacket.getLength());

这样，您只需使用所需数据提取缓冲区的一部分。

Answer 2

DatagramPacket的getData方法返回底层缓冲区，它是您之前创建的1024字节缓冲区。您可以从此缓冲区创建一个String。未填写的所有字节都将为0（默认初始化），因此您的String将包含许多额外的零。这就是equals方法返回false的原因。你可以用trim方法解决这个问题，如果你确定发送的消息不会有前导空格或尾随空格（或者至少对于程序的其余部分没有任何影响）。

Answer 3

调用equals时没有实现，因为响应是1024字节长。您可以修改UDPServer上的响应以解决此问题：

String sentence = new String(receivePacket.getData()).trim();

Java String Equals不适用于UDP服务器接收的数据包

3 个答案: