在Java中實現郵箱驗證碼的步驟如下：

1. 生成驗證碼：使用隨機數生成器生成一個指定長度的隨機數作為驗證碼。

public String generateVerificationCode(int length) {
    StringBuilder sb = new StringBuilder();
    Random random = new Random();
    for (int i = 0; i < length; i++) {
        sb.append(random.nextInt(10));
    }
    return sb.toString();
}

2. 發送驗證碼郵件：使用JavaMail API發送包含驗證碼的郵件給用戶。

public void sendVerificationCodeEmail(String recipient, String code) {
    // 設置發件人郵箱和密碼
    String senderEmail = "your_email@example.com";
    String senderPassword = "your_password";
    
    // 設置SMTP服務器地址和端口號
    String smtpHost = "smtp.example.com";
    int smtpPort = 587;
    
    // 創建郵件會話
    Properties props = new Properties();
    props.put("mail.smtp.host", smtpHost);
    props.put("mail.smtp.port", smtpPort);
    props.put("mail.smtp.auth", "true");
    
    Session session = Session.getInstance(props, new Authenticator() {
        protected PasswordAuthentication getPasswordAuthentication() {
            return new PasswordAuthentication(senderEmail, senderPassword);
        }
    });
    
    try {
        // 創建郵件消息
        Message message = new MimeMessage(session);
        message.setFrom(new InternetAddress(senderEmail));
        message.setRecipients(Message.RecipientType.TO, InternetAddress.parse(recipient));
        message.setSubject("Verification Code");
        message.setText("Your verification code is: " + code);
        
        // 發送郵件
        Transport.send(message);
        System.out.println("Verification code email sent successfully");
    } catch (MessagingException e) {
        e.printStackTrace();
    }
}

3. 調用以上方法生成驗證碼并發送郵件給用戶。

String verificationCode = generateVerificationCode(6);
sendVerificationCodeEmail("recipient@example.com", verificationCode);

以上就是在Java中實現郵箱驗證碼的簡單示例。需要注意的是，發送郵件需要配置正確的發件人郵箱、密碼和SMTP服務器信息。