下面我們要處理用戶連接，也就是FtpConnection類。Ftp連接本質上是一個狀態機，當FtpConnection接收到用戶命令后，根據當前狀態決定響應及下一個狀態。不過我們不需要考慮實現一個復雜的狀態機，只須監聽/接收/處理/響應即可：

package jftp;

import java.net.*;
import java.io.*;
import java.util.*;
import java.text.*;

public class FtpConnection extends Thread {
    /** 主目錄 */
    static public String root = null;
    private String currentDir = "/"; // 當前目錄
    private Socket socket;
    private BufferedReader reader = null;
    private BufferedWriter writer = null;
    private String clientIP = null;
    private Socket tempSocket = null; // tempSocket用于傳送文件
    private ServerSocket pasvSocket = null; // 用于被動模式
    private String host = null;
    private int port = (-1);

    public FtpConnection(Socket socket) {
        this.socket = socket;
        this.clientIP = socket.getInetAddress().getHostAddress();
    }

    public void run() {
        String command;
        try {
            System.out.println(clientIP + " connected.");
            socket.setSoTimeout(60000); // ftp超時設定
            reader = new BufferedReader(new InputStreamReader(socket.getInputStream()));
            writer = new BufferedWriter(new OutputStreamWriter(socket.getOutputStream()));
            response("220-歡迎消息......");
            response("220-歡迎消息......");
            response("220 注意最后一行歡迎消息沒有“-”");
            for(;;) {
                command = reader.readLine();
                if(command == null)
                    break;
                System.out.println("command from " + clientIP + " : " + command);
                parseCommand(command);
                if(command.equals("QUIT")) // 收到QUIT命令
                    break;
            }
        }
        catch(Exception e) { e.printStackTrace(); }
        finally {
            try {
                if(reader!=null) reader.close();
            }catch(Exception e) {}
            try {
                if(writer!=null) writer.close();
            }catch(Exception e) {}
            try {
                if(this.pasvSocket!=null) pasvSocket.close();
            }catch(Exception e) {}
            try {
                if(this.tempSocket!=null) tempSocket.close();
            }catch(Exception e) {}
            try {
                if(this.socket!=null) socket.close();
            }catch(Exception e) {}
        }
        System.out.println(clientIP + " disconnected.");
    }
}

FtpConnection在run()方法中僅僅是獲得用戶命令/處理命令，當收到QUIT時，關閉連接，結束Ftp會話。

先準備幾個輔助方法：

private void response(String s) throws Exception {
    // System.out.println("  [RESPONSE] "+s);
    writer.write(s);
    writer.newLine();
    writer.flush(); // 注意要flush否則響應仍在緩沖區
}

// 生成一個字符串
private static String pad(int length) {
    StringBuffer buf = new StringBuffer();
    for (int i = 0; i < length; i++)
        buf.append((char)' ');
    return buf.toString();
}

// 獲取參數
private String getParam(String cmd, String start) {
    String s = cmd.substring(start.length(), cmd.length());
    return s.trim();
}

// 獲取路徑
private String translatePath(String path) {
    if(path==null) return root;
    if(path.equals("")) return root;
    path = path.replace('/', '\\');
    return root + path;
}

// 獲取文件長度，注意是一個字符串
private String getFileLength(long length) {
    String s = Long.toString(length);
    int spaces = 12 - s.length();
    for(int i=0; i<spaces; i++)
        s = " " + s;
    return s;
}

接下來便是處理用戶命令，這個方法有點長，需要重構一下，我只是把LIST命令單獨挪了出來：

private void parseCommand(String s) throws Exception {
    if(s==null || s.equals(""))
        return;
    if(s.startsWith("USER ")) {
        response("331 need password");
    }
    else if(s.startsWith("PASS ")) {
        response("230 welcome to my ftp!");
    }
    else if(s.equals("QUIT")) {
        response("221 歡迎再來！");
    }
    else if(s.equals("TYPE A")) {
        response("200 TYPE set to A.");
    }
    else if(s.equals("TYPE I")) {
        response("200 TYPE set to I.");
    }
    else if(s.equals("NOOP")) {
        response("200 NOOP OK.");
    }
    else if(s.startsWith("CWD")) { // 設置當前目錄，注意沒有檢查目錄是否有效
        this.currentDir = getParam(s, "CWD ");
        response("250 CWD command successful.");
    }
    else if(s.equals("PWD")) { // 打印當前目錄
        response("257 \"" + this.currentDir + "\" is current directory.");
    }
    else if(s.startsWith("PORT ")) {
        // 記錄端口
        String[] params = getParam(s, "PORT ").split(",");
        if(params.length<=4 || params.length>=7)
            response("500 command param error.");
        else {
            this.host = params[0] + "." + params[1] + "." + params[2] + "." + params[3];
            String port1 = null;
            String port2 = null;
            if(params.length == 6) {
                port1 = params[4];
                port2 = params[5];
            }
            else {
                port1 = "0";
                port2 = params[4];
            }
            this.port = Integer.parseInt(port1) * 256 + Integer.parseInt(port2);
            response("200 command successful.");
        }
    }
    else if(s.equals("PASV")) { // 進入被動模式
        if(pasvSocket!=null)
            pasvSocket.close();
        try {
            pasvSocket = new ServerSocket(0);
            int pPort = pasvSocket.getLocalPort();
            String s_port;
            if(pPort<=255)
                s_port = "255";
            else {
                int p1 = pPort / 256;
                int p2 = pPort - p1*256;
                s_port = p1 + "," + p2;
            }
            pasvSocket.setSoTimeout(60000);
            response("227 Entering Passive Mode ("
                + InetAddress.getLocalHost().getHostAddress().replace('.', ',')
                + "," + s_port + ")");
        }
        catch(Exception e) {
            if(pasvSocket!=null) {
                pasvSocket.close();
                pasvSocket = null;
            }
        }
    }
    else if(s.startsWith("RETR")) { // 傳文件
        String file = currentDir + (currentDir.endsWith("/") ? "" : "/") + getParam(s, "RETR");
        System.out.println("download file: " + file);
        Socket dataSocket;
        // 根據上一次的PASV或PORT命令決定使用哪個socket
        if(pasvSocket!=null)
            dataSocket = pasvSocket.accept();
        else
            dataSocket = new Socket(this.host, this.port);
        OutputStream dos = null;
        InputStream fis = null;
        response("150 Opening ASCII mode data connection.");
        try {
            fis = new BufferedInputStream(new FileInputStream(translatePath(file)));
            dos = new DataOutputStream(new BufferedOutputStream(dataSocket.getOutputStream()));
            // 開始正式發送數據:
            byte[] buffer = new byte[20480]; // 發送緩沖 20k
            int num = 0; // 發送一次讀取的字節數
            do {
                num = fis.read(buffer);
                if(num!=(-1)) {
                    // 發送：
                    dos.write(buffer, 0, num);
                    dos.flush();
                }
            } while(num!=(-1));
            fis.close();
            fis = null;
            dos.close();
            dos = null;
            dataSocket.close();
            dataSocket = null;
            response("226 transfer complete."); // 響應一個成功標志
        }
        catch(Exception e) {
            response("550 ERROR: File not found or access denied.");
        }
        finally {
            try {
                if(fis!=null) fis.close();
                if(dos!=null) dos.close();
                if(dataSocket!=null) dataSocket.close();
            }
            catch(Exception e) {}
        }
    }
    else if(s.equals("LIST")) { // 列當前目錄文件
        Socket dataSocket;
        // 根據上一次的PASV或PORT命令決定使用哪個socket
        if(pasvSocket!=null)
            dataSocket = pasvSocket.accept();
        else
            dataSocket = new Socket(this.host, this.port);
        PrintWriter writer = new PrintWriter(new BufferedOutputStream(dataSocket.getOutputStream()));
        response("150 Opening ASCII mode data connection.");
        try {
            responseList(writer, this.currentDir);
            writer.close();
            dataSocket.close();
            response("226 transfer complete.");
        }
        catch(IOException e) {
            writer.close();
            dataSocket.close();
            response(e.getMessage());
        }
        dataSocket = null;
    }
    else {
        response("500 invalid command"); // 沒有匹配的命令，輸出錯誤信息
    }
}

// 響應LIST命令
private void responseList(PrintWriter writer, String path) throws IOException {
    File dir = new File(translatePath(path));
    if(!dir.isDirectory())
        throw new IOException("550 No such file or directory");
    File[] files = dir.listFiles();
    String dateStr;
    for(int i=0; i<files.length; i++) {
        dateStr = new SimpleDateFormat("MMM dd hh:mm").format(new Date(files[i].lastModified()));
        if(files[i].isDirectory()) {
            writer.println("drwxrwxrwx  1 ftp      System            0 "
            + dateStr + " " + files[i].getName());
        }
        else {
            writer.println("-rwxrwxrwx  1 ftp      System "
            + getFileLength(files[i].length()) + " " + dateStr + " " + files[i].getName());
        }
    }

    String file_header = "-rwxrwxrwx  1 ftp      System            0 Aug  5 19:59 ";
    String dir_header =  "drwxrwxrwx  1 ftp      System            0 Aug 15 19:59 ";
    writer.println("total " + files.length);
    writer.flush();
}

（待續）

基本上我們的Ftp已經可以運行了，注意到我們在FtpConnection中處理USER和PASS命令，直接返回200 OK，如果需要驗證用戶名和口令，還需要添加相應的代碼。

如何調試Ftp服務器？

有個最簡單的方法，便是使用現成的Ftp客戶端，推薦CuteFtp，因為它總是把客戶端發送的命令和服務器響應打印出來，我們可以非常方便的看到服務器的輸出結果。

另外一個小Bug，文件列表在CuteFtp中可以正常顯示，在其他Ftp客戶端不一定能正常顯示，這說明輸出響應的“兼容性”還不夠好，有空了看看Ftp的RFC再改進！:）