Mostrar conta | Spring MVC: É hora de criar uma webapp com Spring MVC4

Controller

package br.com.caelum.contas.controller;

import java.util.List;

import org.springframework.stereotype.Controller;
import org.springframework.ui.Model;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.portlet.ModelAndView;

import br.com.caelum.contas.dao.ContaDAO;
import br.com.caelum.contas.modelo.Conta;

@Controller
public class ContaController {

    @RequestMapping("/form")
    public String form() {
        return "formulario";
    }

    @RequestMapping("/adicionaConta")
    public String adicionaConta(Conta c) {
        ContaDAO dao = new ContaDAO();
        dao.adiciona(c);
        return "sucesso";
    }

    @RequestMapping("/deletaConta")
    public String deleta(Conta conta) {
        System.out.println(conta.getId());
        new ContaDAO().remove(conta);
        return "redirect:listaContas";
    }

    @RequestMapping("/listaContas")
    public String lista(Model mv) {
        ContaDAO dao = new ContaDAO();
        List<Conta> contas = dao.lista();
        //ModelAndView mv = new ModelAndView("conta/lista");
        //mv.addObject("contas", contas);
        System.out.println(contas);
        mv.addAttribute("contas",contas);
        return "conta/lista";
    }

    @RequestMapping("/mostraConta")
    public String mostra(Long id, Model model) {
      ContaDAO dao = new ContaDAO();
      model.addAttribute("conta", dao.buscaPorId(id));
      return "conta/mostra";
    }

    @RequestMapping("/alteraConta")
    public String altera(Conta conta) {
        System.out.println(conta.getId());
        ContaDAO dao = new ContaDAO();
        dao.altera(conta);
        return "redirect:listaContas";
    }
}

JSP

<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
<%@taglib uri="http://java.sun.com/jsp/jstl/fmt" prefix="fmt"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Detalhe conta</title>
</head>
<body>
    <h3>Alterar Conta</h3>
    <form action="alteraConta" method="post">
        Descrição: <br/>
        <textarea name="descricao" rows="5" cols="100">${conta.descricao}</textarea>
        <br/>
        Valor: <br/>
        <input type="text" name="valor" />
        Tipo: <br/>
        <select name="tipo">
            <option value="ENTRADA" ${conta.tipo=='ENTRADA' ? 'selected':''}>Entrada</option>
            <option value="SAIDA" ${conta.tipo=='SAIDA' ? 'selected':''}>Saída</option>
        </select>
        <br/>
        Pago? <input type="checkbox" name="paga"     ${conta.paga?'checked':''} />
        <br/>
        Data de Pagamento: <input type="text" name="dataPagamento" value="<fmt:formatDate value="${conta.dataPagamento.time}" pattern="dd/MM/yyyy" />" />
        <br/>
        <input type="hidden" name="id" value="${conta.id}"/>
        <input type="submit" value="Alterar"/>
    </form>
</body>
</html>

Erro que aparece no navegador:


HTTP Status 400 -

type Status report

message

description The request sent by the client was syntactically incorrect.

Apache Tomcat/7.0.42