为什么我的程序 运行 在我的服务器上出现了两次？

Question

我正在编写 tcp 客户端-服务器程序。当客户端键入“Hello”时，服务器 returns 当前目录的文件和目录列表。当客户端键入“FileDownload”时，它会从服务器下载选定的文件。
当我键入“Hello”时它工作正常，但是当我键入“FileDownload”时，它在服务器端运行两次 else if(received.contains("FileDownload")) 块。因此，服务器发送了两倍的数据，导致客户端出现其他问题。

这是服务器代码：

public static void main(String[] args) throws IOException {
        
            ServerSocket servSock = new ServerSocket(1333);
            
            String received="";
            String[] s = null;
            File[] f1;
            int i=0;
            
            File f=new File(System.getProperty("user.dir"));
            f1=f.listFiles();
            
            for(File f2:f1) {
                if(f2.isDirectory())
                    System.out.println(f2.getName() + "\t<DIR>\t" + i);
                if(f2.isFile()) 
                    System.out.println(f2.getName() + "\t<FILE>\t" + i);
                i++;
            }
            
                while (true) { 
                    
                    Socket client = servSock.accept();
                    
                    InputStream in = client.getInputStream();
                    OutputStream out = client.getOutputStream();
                    BufferedReader reader = new BufferedReader(new InputStreamReader(in));
                    PrintWriter pw = new PrintWriter(out, true);
                    s=f.list();
                    
                    while(true) {

                        received = reader.readLine();
                        if(received.equals("END")) break;
                        
                        if(received.equals("Hello")) {
                            System.out.println("Hello-Start");

                            int length=f1.length;
                            pw.println(length);
                            i=0;
                            for(File f2:f1) {
                                if(f2.isDirectory())
                                    pw.println(f2.getName() + "\t<DIR>\t" + i);
                                if(f2.isFile()) 
                                    pw.println(f2.getName() + "\t<FILE>\t" + i);
                                i++;
                            }
                            pw.println("Options: " + "\tFileDownload <FID>" + "\tFileUpload <name>" + "\tChangeFolder <name>");
                            System.out.println("Hello-End");

                        }
                        else if(received.contains("FileDownload")) {
                            System.out.println("FileDownload-Start");

                            int j=-1;
                            try {
                                j=Integer.parseInt(received.substring(13).trim());
                            }catch(NumberFormatException e) { 
                                System.err.println("error: " + e);
                            }
                            
                            if(j>0 && j<s.length) {
                                FileInputStream fi=new FileInputStream(s[j]);
                                byte[] b=new byte[1024];
                                System.out.println("file: "+s[j]);

                                pw.println(s[j]);
                                fi.read(b,0,b.length);
                                out.write(b,0,b.length);
                                System.out.println("FileDownload-End");
                            }
                        }

客户端代码如下：

public static void main(String[] args) throws IOException {
        if ((args.length != 2))
        throw new IllegalArgumentException("Parameter(s): <Server> <Port>");
        
        Socket socket = new Socket(args[0], Integer.parseInt(args[1]));
        Scanner sc = new Scanner(System.in);
        InputStream in = socket.getInputStream();
        OutputStream out = socket.getOutputStream();
        BufferedReader reader = new BufferedReader(new InputStreamReader(in));
        PrintWriter pw = new PrintWriter(out, true);

        
        while(true) {
            String msg="", received="";
            String length;
            
            msg = sc.nextLine();
            pw.println(msg);
            
            if(msg.equals("END")) break; 
            
                if(poraka.equals("Hello")) {
                    System.out.println();
                    length = reader.readLine();
                    for(int i=0;i<Integer.parseInt(length);i++) {
                        received = reader.readLine();
                        System.out.println(received);
                    }
                    System.out.println("\n"+reader.readLine());
                }
                else if(msg.contains("FileDownload")) {
                    System.out.println("FileDownload-Start");
                    pw.println(msg);
                    byte[] b=new byte[1024];
                    File file=new File(reader.readLine().trim());
                    System.out.println(file.getName().trim());
                    FileOutputStream fo=new FileOutputStream("D:\Eclipse WorkSpace\proekt\src\client\"+file.getName().trim());
                    System.out.println("file: "+file.getName().trim());

                    in.read(b,0,b.length);
                    fo.write(b,0,b.length);
                    System.out.println("FileDownload-End");
                }

我找不到导致此问题的原因，因此非常感谢任何可能的帮助！

Answer 1

这是因为你的客户端请求了两次数据：

        msg = sc.nextLine();
        pw.println(msg);  // <-- this is the first time

之后

            else if(msg.contains("FileDownload")) {
                System.out.println("FileDownload-Start");
                pw.println(msg);  // <-- this is the second time