java - 休眠不从数据库中检索数据

Question

嗨，我正在尝试从 mysql 表中读取所有数据并将其显示在 jsp 上，但我似乎无法从表中检索任何数据，因为 jsp 加载了不包括表中数据的其他数据，从我尝试为了找到问题的根源，我发现我的 DAO 没有从表中获取数据，因此没有数据传递给 servlet 和 jsp，我所做的所有谷歌搜索我一直无法找到一个可以解决的问题我的问题，对此问题的任何帮助将不胜感激

这是我的 DAO 课程

public class StudentDAO {


    @SuppressWarnings("unchecked")
    public List<Student> getAllStudents() {
        Transaction transaction = null;
        List<Student> students = new ArrayList<>();
        String hql = "from model.Student";
        try {
            Session session = HibernateUtil.getSessionFactory().openSession();
            transaction = session.beginTransaction();
            Query query = session.createQuery(hql); //get all student objects
            students = query.list();
            System.out.println(students + "list of students"); // trying to query the problem
            System.out.println(students);
            transaction.commit();
        } catch (Exception e) {
            if (transaction != null) {
                transaction.rollback();
            }
        }
        return students;
    }

这是我的 HibernateUtil 类

public class HibernateUtil {
    private static SessionFactory sessionFactory;
    public static SessionFactory getSessionFactory() {
        if (sessionFactory == null) {
            try {
                Configuration configuration = new Configuration();
                Properties settings = new Properties();
                
                // Hibernate settings equivalent to hibernate.cfg.xml's properties
                settings.put(Environment.DRIVER, "com.mysql.cj.jdbc.Driver");
                settings.put(Environment.URL, "jdbc:mysql://localhost:3306/week05");
                settings.put(Environment.USER, "user1");
                settings.put(Environment.PASS, "user1");
                settings.put(Environment.DIALECT, "org.hibernate.dialect.MySQL5Dialect");
                settings.put(Environment.SHOW_SQL, "true");
                settings.put(Environment.CURRENT_SESSION_CONTEXT_CLASS, "thread");
                settings.put(Environment.HBM2DDL_AUTO, "create-drop");
                configuration.setProperties(settings);
                configuration.addAnnotatedClass(Student.class);//jpa entity mapping
                ServiceRegistry serviceRegistry = new StandardServiceRegistryBuilder()
                        .applySettings(configuration.getProperties()).build();
                
                sessionFactory = configuration.buildSessionFactory(serviceRegistry);
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
        return sessionFactory;
    }
}

这是我的 servlet 类，以防我配置错误：

@WebServlet({ "/", "/new", "/edit", "/update", "/delete", "/insert" })
public class StudentServlet extends HttpServlet {
    private static final long serialVersionUID = 2L;
    private StudentDAO studentDao;
    String home = "/Week05";

    public StudentServlet() {
        this.studentDao = new StudentDAO();
    }

    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }

    
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        String sPath = request.getServletPath();
        //switch statement to call appropriate method
        switch (sPath) {
            case "/new":
                try {
                    showNewForm(request, response);
                } catch (ServletException | IOException e) {
                    e.printStackTrace();
                }
                break;
            case "/insert":
                try {
                    insertStudent(request, response);
                } catch (SQLException | IOException e) {
                    e.printStackTrace();
                } 
                break;
            case "/delete":
                try {
                    deleteStudent(request, response);
                } catch (SQLException | IOException e) {
                    e.printStackTrace();
                }
                break;
            case "/update":
                try {
                    updateStudent(request, response);
                } catch (SQLException | IOException e) {
                    e.printStackTrace();
                }
                break;
            case "/edit":
                try {
                    editStudent(request, response);
                } catch (ServletException | IOException e) {
                    e.printStackTrace();
                }
                break;
            default:
                try {
                    listAllStudents(request, response); //home page 
                } catch (ServletException | IOException | SQLException e) {
                    e.printStackTrace();
                } 
                break; 
            } 
    }
 // functions to fetch data from studentDao and display data on appropriate jsp
    private void listAllStudents(HttpServletRequest request, HttpServletResponse response) 
            throws ServletException, IOException, SQLException {
        List<Student> allStudents = studentDao.getAllStudents();
        request.setAttribute("listStudents", allStudents);
        System.out.println(allStudents+"hello");
        RequestDispatcher dispatch = request.getRequestDispatcher("index.jsp"); //home page  | list all objects from table
        dispatch.forward(request, response);
    }

这是我的 index.jsp 以防万一

<%@ page language="java" contentType="text/html; charset=ISO-8859-1" 
pageEncoding="ISO-8859-1" import="java.util.*" import="model.Student"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width, intial-scale=1 shink-to-fit=yes">

    <link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/bootstrap@5.0.2/dist/css/bootstrap.min.css" 
    integrity="sha384...">
</head>
<body>
    <nav class="navbar navbar-dark bg-primary pd-8">
         <a class="navbar-brand">&nbsp;&nbsp;&nbsp;&nbsp;XYZ University</a>
    </nav>
    <div class="container-fluid">
    <div class="container">

            <div class="form container-fluid p-4">
               <a href="<%=request.getContextPath()%>/new" class="btn btn-success" >Add
                    Student</a>
            </div>
            <br>
            
            <!--Assigning ArrayList object containing student data to the local object -->
            <% ArrayList<Student> studentList = (ArrayList) request.getAttribute("listStudents"); %> 
            <table class="table table-bordered">
            
                <thead>
                    <tr>
                        <th>First Name</th>
                        <th>Last Name</th>
                        <th>Email</th>
                        <th>Actions</th>
                    </tr>
                </thead>
                <tbody>
                    <%
                     if(request.getAttribute("listStudents") != null)  {
                            Iterator<Student> iterator = studentList.iterator();
                            while(iterator.hasNext()) {
                                Student studentDetails = iterator.next();
                    %>
                        <tr><td><%=studentDetails.getFirstname()%></td>
                            <td><%=studentDetails.getLastname()%></td>
                            <td><%=studentDetails.getEmail()%></td>
                            <td><a href="<%=request.getContextPath()%>/edit?id=<%=studentDetails.getId()%>
                            &firstname=<%=studentDetails.getFirstname() %>&lastname=<%=studentDetails.getLastname()%>
                            &email=<%=studentDetails.getEmail()%>">Update</a>
                                &nbsp;&nbsp;&nbsp;&nbsp; <a href="<%=request.getContextPath()%>/delete?id=<%=studentDetails.getId()%>">Delete</a></td>
                        </tr>
                    <% 
                            }
                     }
                    %>
                </tbody>
                
            </table>
            </div>
        </div>
        
        <footer class="text-center text-lg-start p-4">
          <div class="bg-primary text-center p-3">
            © 2021 Copyright:
            <a class="text-dark" href=""> SAiS Botswana</a>
          </div>
        </footer> 
    </body>
</html>

PS tl/dr 我能够正确地将数据插入表但无法检索

这是我的 jpa 类对象：

@Entity
@Table(name="students")
public class Student {
    
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Column(name = "id")
    private int id;
    @Column(name = "firstname")
    private String firstname;
    @Column(name = "lastname")
    private String lastname;
    @Column(name = "email")
    private String email;
    
    
    
    public Student() {
    }

    public Student(int id, String firstname, String lastname, String email) {
        super();
        this.id = id;
        this.firstname = firstname;
        this.lastname = lastname;
        this.email = email;
    }

    public Student(String firstname, String lastname, String email) {
        super();
        this.firstname = firstname;
        this.lastname = lastname;
        this.email = email;
    }


    public int getId() {
        return id;
    }

    public String getFirstname() {
        return firstname;
    }

    public String getLastname() {
        return lastname;
    }


    public String getEmail() {
        return email;
    }


    public void setId(int id) {
        this.id = id;
    }


    public void setFirstname(String firstname) {
        this.firstname = firstname;
    }


    public void setLastname(String lastname) {
        this.lastname = lastname;
    }

    public void setEmail(String email) {
        this.email = email;
    }

}

这是我在服务器启动后收到的控制台日志：

INFO: Server startup in 3040 ms
nullhello
Aug 10, 2021 4:09:49 PM org.hibernate.Version logVersion
INFO: HHH000412: Hibernate ORM core version 5.5.6.Final
Aug 10, 2021 4:09:50 PM org.hibernate.annotations.common.reflection.java.JavaReflectionManager <clinit>
INFO: HCANN000001: Hibernate Commons Annotations {5.1.2.Final}
Aug 10, 2021 4:09:50 PM org.hibernate.engine.jdbc.connections.internal.DriverManagerConnectionProviderImpl configure
WARN: HHH10001002: Using Hibernate built-in connection pool (not for production use!)
Aug 10, 2021 4:09:51 PM org.hibernate.engine.jdbc.connections.internal.DriverManagerConnectionProviderImpl buildCreator
INFO: HHH10001005: using driver [com.mysql.cj.jdbc.Driver] at URL [jdbc:mysql://localhost:3306/week05]
Aug 10, 2021 4:09:51 PM org.hibernate.engine.jdbc.connections.internal.DriverManagerConnectionProviderImpl buildCreator
INFO: HHH10001001: Connection properties: {password=****, user=user1}
Aug 10, 2021 4:09:51 PM org.hibernate.engine.jdbc.connections.internal.DriverManagerConnectionProviderImpl buildCreator
INFO: HHH10001003: Autocommit mode: false
Aug 10, 2021 4:09:51 PM org.hibernate.engine.jdbc.connections.internal.DriverManagerConnectionProviderImpl$PooledConnections <init>
INFO: HHH000115: Hibernate connection pool size: 20 (min=1)
Aug 10, 2021 4:09:51 PM org.hibernate.dialect.Dialect <init>
INFO: HHH000400: Using dialect: org.hibernate.dialect.MySQL5Dialect
Hibernate: drop table if exists hibernate_sequence
Aug 10, 2021 4:09:54 PM org.hibernate.resource.transaction.backend.jdbc.internal.DdlTransactionIsolatorNonJtaImpl getIsolatedConnection
INFO: HHH10001501: Connection obtained from JdbcConnectionAccess [org.hibernate.engine.jdbc.env.internal.JdbcEnvironmentInitiator$ConnectionProviderJdbcConnectionAccess@31a0e986] for (non-JTA) DDL execution was not in auto-commit mode; the Connection 'local transaction' will be committed and the Connection will be set into auto-commit mode.
Hibernate: drop table if exists students
Hibernate: create table hibernate_sequence (next_val bigint) engine=MyISAM
Aug 10, 2021 4:09:54 PM org.hibernate.resource.transaction.backend.jdbc.internal.DdlTransactionIsolatorNonJtaImpl getIsolatedConnection
INFO: HHH10001501: Connection obtained from JdbcConnectionAccess [org.hibernate.engine.jdbc.env.internal.JdbcEnvironmentInitiator$ConnectionProviderJdbcConnectionAccess@356bba04] for (non-JTA) DDL execution was not in auto-commit mode; the Connection 'local transaction' will be committed and the Connection will be set into auto-commit mode.
Hibernate: insert into hibernate_sequence values ( 1 )
Hibernate: create table students (id integer not null, email varchar(255), firstname varchar(255), lastname varchar(255), primary key (id)) engine=MyISAM
Hibernate: select next_val as id_val from hibernate_sequence for update
Hibernate: update hibernate_sequence set next_val= ? where next_val=?
Hibernate: insert into students (email, firstname, lastname, id) values (?, ?, ?, ?)

标签： javahibernatejspservlets

Answer 1

**试试这种方式**

<%
    List<Student> List = listStudents.getlistStudents();
%>


        <%
            for (Student  st: List) {
                int id = st.getId();
            
        %>

               <tr><td><%=st.getFirstname()%></td>
                        <td><%=st.getLastname()%></td>
                        <td><%=st.getEmail()%></td>
                        <td><a href="<%=request.getContextPath()%>/edit?id= 
                        <%=st.getId()%>
                        &firstname=<%=st.getFirstname() %>&lastname= 
                        <%=st.getLastname()%>
                        &email=<%=st.getEmail()%>">Update</a>
                            &nbsp;&nbsp;&nbsp;&nbsp; <a href=" 
                          <%=request.getContextPath()%>/delete?id= 
                          <%=st.getId()%>">Delete</a></td>
                    </tr>
                <% 
                        }
                 }
                %>

参考ShowAllFlat.jsp