给出一个示例的java代码

好的，下面是一个简单的Java代码示例，用于实现一个SMTP服务器程序，可以接收来自邮件客户端的邮件，然后转发到目标邮件服务器的用户邮箱：

import java.io.*;
import java.net.*;

public class SMTPServer {

    public static void main(String[] args) throws IOException {
        ServerSocket serverSocket = null;
        boolean listening = true;

        try {
            // 创建ServerSocket对象并绑定到SMTP默认端口25
            serverSocket = new ServerSocket(25);
            System.out.println("SMTPServer started on port 25...");
        } catch (IOException e) {
            System.err.println("Could not listen on port 25.");
            System.exit(-1);
        }

        while (listening) {
            new SMTPServerThread(serverSocket.accept()).start();
        }

        serverSocket.close();
    }
}

class SMTPServerThread extends Thread {
    private Socket socket = null;

    public SMTPServerThread(Socket socket) {
        super("SMTPServerThread");
        this.socket = socket;
    }

    public void run() {
        try {
            PrintWriter out = new PrintWriter(socket.getOutputStream(), true);
            BufferedReader in = new BufferedReader(new InputStreamReader(socket.getInputStream()));

            String inputLine;
            String outputLine;

            // 发送SMTP服务器欢迎消息
            outputLine = "220 SMTPServer ready";
            out.println(outputLine);

            // 处理客户端SMTP请求
            while ((inputLine = in.readLine()) != null) {
                System.out.println("Received: " + inputLine);

                if (inputLine.toUpperCase().startsWith("HELO")) {
                    // 处理HELO命令
                    outputLine = "250 Hello";
                    out.println(outputLine);
                } else if (inputLine.toUpperCase().startsWith("MAIL FROM")) {
                    // 处理MAIL FROM命令
                    outputLine = "250 OK";
                    out.println(outputLine);
                } else if (inputLine.toUpperCase().startsWith("RCPT TO")) {
                    // 处理RCPT TO命令
                    outputLine = "250 OK";
                    out.println(outputLine);
                } else if (inputLine.toUpperCase().startsWith("DATA")) {
                    // 处理DATA命令，接收邮件内容并转发到目标邮件服务器
                    outputLine = "354 End data with <CR><LF>.<CR><LF>";
                    out.println(outputLine);

                    StringBuilder sb = new StringBuilder();
                    while ((inputLine = in.readLine()) != null) {
                        if (inputLine.equals(".")) {
                            break;
                        }
                        sb.append(inputLine);
                    }

                    System.out.println("Received mail: " + sb.toString());

                    // 将邮件内容转发到目标邮件服务器
                    // TODO: 实现邮件转发功能

                    outputLine = "250 OK";
                    out.println(outputLine);
                } else if (inputLine.toUpperCase().startsWith("QUIT")) {
                    // 处理QUIT命令，关闭连接
                    outputLine = "221 Goodbye";
                    out.println(outputLine);
                    break;
                } else {
                    // 处理未知命令
                    outputLine = "500 Command unrecognized";
                    out.println(outputLine);
                }
            }

            out.close();
            in.close();
            socket.close();

        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

这个示例代码实现了一个最基础的SMTP服务器框架，可以处理客户端发送的HELO、MAIL FROM、RCPT TO、DATA和QUIT等SMTP命令，并接收和转发邮件内容。具体实现需要根据具体情况进行调整和完善。