如何基于Java实现自定义SMTP服务器并集成至现有TCP服务，完成邮件客户端本地模拟测试

Alright, let's fix this up. Your current TCP server can accept connections and read incoming messages, but it's missing the SMTP-specific response logic to match the flow you outlined. Here's how to modify your code to handle each step of the SMTP interaction:

Key Changes Needed #

We need to:

• Send the initial 220 greeting when a client connects
• Parse each incoming client command and return the correct SMTP response code
• Handle the special DATA phase, where we accept message content until we receive a single . on a line

Modified TCPServer Code #

Here's the updated server code with full SMTP logic implemented:

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.net.InetAddress;
import java.net.ServerSocket;
import java.net.Socket;

public class TCPServer {
    private ServerSocket server;

    public TCPServer(String ipAddress, int port) throws Exception {
        if (ipAddress != null && !ipAddress.isEmpty())
            this.server = new ServerSocket(port, 1, InetAddress.getByName(ipAddress));
        else
            this.server = new ServerSocket(0, 1, InetAddress.getLocalHost());
    }

    private void listen() throws Exception {
        Socket client = this.server.accept();
        String clientAddress = client.getInetAddress().getHostAddress();
        System.out.println("\r\nNew client connection from " + clientAddress);

        // Setup input/output streams for SMTP communication
        BufferedReader in = new BufferedReader(new InputStreamReader(client.getInputStream()));
        PrintWriter out = new PrintWriter(client.getOutputStream(), true);
        String data;

        // Send initial SMTP greeting (required first step)
        out.println("220 " + server.getInetAddress().getHostAddress());

        while ((data = in.readLine()) != null) {
            System.out.println("\r\nMessage from " + clientAddress + ": " + data);
            String command = data.trim().toUpperCase();

            // Handle each SMTP command
            if (command.startsWith("HELLO")) {
                // Respond to HELO with 250 greeting
                out.println("250 Hello email1@email1.com, pleased to meet you");
            } else if (command.startsWith("MAIL FROM:")) {
                // Validate MAIL FROM (basic check here; you can add more logic)
                out.println("250 ok");
            } else if (command.startsWith("RCPT TO:")) {
                // Validate RCPT TO (basic check here)
                out.println("250 ok");
            } else if (command.startsWith("DATA")) {
                // Enter data mode, prompt client to end with <CR><LF>.<CR><LF>
                out.println("354 End data with <CR><LF>.<CR><LF>");
                // Read message lines until we get a single "."
                String line;
                while ((line = in.readLine()) != null) {
                    System.out.println("Message content: " + line);
                    if (line.trim().equals(".")) {
                        out.println("250 ok Message accepted for delivery");
                        break;
                    }
                }
            } else if (command.startsWith("QUIT")) {
                // Respond to QUIT and close connection
                out.println("221 " + server.getInetAddress().getHostAddress() + " closing connection");
                break;
            } else {
                // Handle unknown commands (optional, but good practice)
                out.println("500 Command unrecognized");
            }
        }

        // Cleanup resources
        in.close();
        out.close();
        client.close();
        System.out.println("\r\nConnection closed with " + clientAddress);
    }

    public InetAddress getSocketAddress() {
        return this.server.getInetAddress();
    }

    public int getPort() {
        return this.server.getLocalPort();
    }

    public static void main(String[] args) throws Exception {
        String serverIP = "192.168.1.235";
        int port = 8088;
        if (args.length > 0) {
            serverIP = args[0];
            port = Integer.parseInt(args[1]);
        }

        TCPServer server = new TCPServer(serverIP, port);
        System.out.println("\r\nRunning Server: " + "Host=" + server.getSocketAddress().getHostAddress() + " Port=" + server.getPort());
        server.listen();
    }
}

What's Changed? #

• Added PrintWriter to send formatted SMTP responses back to the client
• Implemented the initial 220 greeting on connection
• Added command parsing for each SMTP command (HELLO, MAIL FROM, RCPT TO, DATA, QUIT)
• Added special handling for the DATA phase, reading lines until a single . is received
• Added cleanup for streams and sockets after the connection closes

Testing the Setup #

1. Run the modified TCPServer first
2. Run your existing ClientTester
3. You should see the exact interaction flow you outlined:

   S: 220 192.168.1.235
   C: HELLO 192.168.1.235
   S: 250 Hello email1@email1.com, pleased to meet you
   ... (rest of the flow)
   

Your existing client code should work as-is since it's already following the SMTP command sequence.

内容的提问来源于stack exchange，提问作者Jimmy665566