@WebServlet("/trans")
public class TransferServlet extends HttpServlet{
  private static final long serialVersionUID = 1L;
  
  protected void service(HttpServletRequest req, HttpServletResponse resp)
      throws ServletException, IOException {
    req.setCharacterEncoding("UTF-8");
    resp.setContentType("text/html;charset=UTF-8");
    PrintWriter out = resp.getWriter();
    String money = req.getParameter("money");
    //通过睡眠,模拟网络延迟
    try {
      Thread.sleep(3000);
    } catch (InterruptedException e) {
      e.printStackTrace();
    }
    System.out.println("转出金额:"+money);
    out.print("转出金额:"+money);  
  }
}

<%
      //创建令牌
      String token = java.util.UUID.randomUUID().toString();
      //存在session中一份,以后做判断
      session.setAttribute("TOKEN_IN_SESSION", token);
    %>
    
    <h3>转账界面</h3>
    <form action="/trans" method="post">
      <input type="hidden" name="token" value="<%=token%>"/>
      转账金额:<input type="text" name="money" min="1" required /><br/>
      <input type="submit" value="转账" />
    </form>

@WebServlet("/trans")
public class TransferServlet extends HttpServlet{
  private static final long serialVersionUID = 1L;
  
  protected void service(HttpServletRequest req, HttpServletResponse resp)
      throws ServletException, IOException {
    req.setCharacterEncoding("UTF-8");
    resp.setContentType("text/html;charset=UTF-8");
    PrintWriter out = resp.getWriter();
    //获取表单中的token值
    String token = req.getParameter("token");
    //获取session中的token值
    String sessionToken = (String) req.getSession().getAttribute("TOKEN_IN_SESSION");
    //session中的token容易为空,因为session中的token是需要被销毁的
    
    if (token.equals(sessionToken)) {
      //说明令牌相同
      req.getSession().removeAttribute("TOKEN_IN_SESSION");
      String money = req.getParameter("money");
      System.out.println("转出金额:"+money);
      out.print("转出金额:"+money);
      //最后销毁session中的令牌
    }
    //如果令牌不同说明就是重复提交,不能提交
  }
}

@WebServlet("/transfer")
public class CopyOfTransferServlet extends HttpServlet{
  private static final long serialVersionUID = 1L;
  
  protected void service(HttpServletRequest req, HttpServletResponse resp)
      throws ServletException, IOException {
    //创建令牌,并跳转到submit.jsp
    String token = UUID.randomUUID().toString();
    System.out.println(token);
    req.getSession().setAttribute("TOKEN_IN_SESSION", token);
    req.setAttribute("token", token);
    req.getRequestDispatcher("/views/repeatsubmit/submit.jsp").forward(req, resp);
    
  }
}

<h3>转账界面</h3>
    <form action="/trans" method="post">
      <input type="hidden" name="token" value="${token }"/>
      转账金额:<input type="text" name="money" min="1" required /><br/>
      <input type="submit" value="转账" />
    </form>

//令牌的工具类
//创建令牌
//校验令牌
//销毁令牌
public class TokenUtil {
  private final static String TOKEN_IN_SESSION = "TOKEN_IN_SESSION";
  public static void savaToken(HttpServletRequest req) {
    String token = UUID.randomUUID().toString();
    System.out.println(token);
    req.getSession().setAttribute(TOKEN_IN_SESSION, token);
    req.setAttribute("token", token);
  }

  public static boolean validateToken(HttpServletRequest req,
      String tokenInrequest) {
    //获取session中的token值
    String sessionToken = (String) req.getSession().getAttribute(
        TOKEN_IN_SESSION);
    if (tokenInrequest.equals(sessionToken)) {
      req.getSession().removeAttribute(TOKEN_IN_SESSION);
      return true;
    }
    return false;
  }
}