使用MVC模式在JSP页面中以HTML格式显示JDBC ResultSet - NullPointerException

Question

如果我将 UsuarioDAO 中的代码放入 UsuarioServlet ， JSP 效果很好。但我正在尝试使用 MVC ， OO ，我无法理解我的代码有什么问题，因为我从 UsuarioDAO <调用创建的对象< / strong>在 UsuarioServlet 中，理论上应该很好用，但事实并非如此。如果有人能告诉我我做错了什么，我很感激。

  

Usuario.java

package modelo;

public class Usuario {

    private String nome;
    private String login;
    private String senha;
    private int tipo;
    private String cpf;

    public void setNome(String nome) {
        this.nome = nome;
    }

    public void setLogin(String login) {
        this.login = login;
    }

    public void setSenha(String senha) {
        this.senha = senha;
    }

    public void setTipo(int tipo) {
        this.tipo = tipo;
    }

    public void setCpf(String cpf) {
        this.cpf = cpf;
    }

    public String getNome() {
        return nome;
    }

    public String getLogin() {
        return login;
    }

    public String getSenha() {
        return senha;
    }

    public int getTipo() {
        return tipo;
    }

    public String getCpf() {
        return cpf;
    }

}

  

UsuarioDAO.java

package controle;

import modelo.Usuario;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.util.ArrayList;
import java.util.List;
import java.util.logging.Level;
import java.util.logging.Logger;
import javax.servlet.ServletConfig;
import javax.servlet.ServletException;

public class UsuarioDAO {

    private Connection con;

    public void init(ServletConfig config) throws ServletException {
        final String url = "jdbc:oracle:thin:@oracle.inf.poa.ifrs.edu.br:1521:XE";
        final String us = "kar";
        try {
            Class.forName("oracle.jdbc.OracleDriver");
        } catch (ClassNotFoundException ex) {
            Logger.getLogger(ex1.class.getName()).log(Level.SEVERE, null, ex);
        }
        try {
            con = DriverManager.getConnection(url, us, us);
        } catch (SQLException ex) {
            throw new ServletException(ex);
        }
    }

    public List<Usuario> list() throws SQLException {
        List<Usuario> lista = new ArrayList<>();
        try (
                Connection connection = con;
                PreparedStatement statement = connection.prepareStatement("SELECT * FROM JDBC_USUARIO");
                ResultSet rs = statement.executeQuery();) {
            while (rs.next()) {
                Usuario usuario = new Usuario();
                usuario.setNome(rs.getString(1));
                usuario.setLogin(rs.getString(2));
                usuario.setSenha(rs.getString(3));
                usuario.setTipo(rs.getInt(4));
                usuario.setCpf(rs.getString(5));
                lista.add(usuario);
            }
        }
        return lista;
    }

}

  

UsuarioServlet.java

    package controle;

    import modelo.Usuario;
    import java.io.IOException;
    import java.sql.SQLException;
    import java.util.List;
    import javax.servlet.ServletException;
    import javax.servlet.annotation.WebServlet;
    import javax.servlet.http.HttpServlet;
    import javax.servlet.http.HttpServletRequest;
    import javax.servlet.http.HttpServletResponse;

    @WebServlet(name = "UsuarioServlet", urlPatterns = {"/UsuarioServlet"})
    public class UsuarioServlet extends HttpServlet {

        UsuarioDAO usrDAO = new UsuarioDAO();

        @Override
        protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
            try {
                List<Usuario> retorno = usrDAO.list();
                request.setAttribute("listaJSP", retorno);
                request.getRequestDispatcher("/WEB-INF/listarUsuarios.jsp").forward(request, response);
            } catch (SQLException e) {
                throw new ServletException("Erro ao obter os dados", e);
            }
        }
    }

  

listarUsuarios.jsp

<%@ taglib prefix="c" uri="http://java.sun.com/jstl/core_rt" %>  
<%@ taglib prefix="fn" uri="http://java.sun.com/jsp/jstl/functions" %>
<%@page contentType="text/html" pageEncoding="UTF-8"%>
<!DOCTYPE html>
<html>
    <head>
        <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
        <title>Servlet Exibe Resultado</title>
    </head>
    <body>
        <TABLE BORDER=“1”>       

            <th>ID</th>
            <th>Nome</th>
            <th>Login</th>
            <th>Senha</th>
            <th>Tipo</th>
            <th>CPF</th>

            <c:forEach items="${listaJSP}" var="show" >
                <c:set var="i" value="${i+1}"  />
                <tr>
                    <td><c:out value="${i}"             /></td>
                    <td><c:out value="${show.nome}"     /></td>
                    <td><c:out value="${show.login}"    /></td>
                    <td><c:out value="${show.senha}"    /></td>
                    <td><c:out value="${show.tipo}"     /></td>
                    <td><c:out value="${show.cpf}"      /></td>
                </tr>
            </c:forEach>
        </table>
    </body>
</html>

Answer 1

您收到此错误是因为init()上的UsuarioDAO方法从未被调用过，而且此行

List<Usuario> retorno = usrDAO.list();

返回null。 在调用列表之前尝试usrDAO.init()。

原因可能是UsuarioDAO不是httpServlet，因此init()方法未在创建时运行