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我必须上课： Student 和 Address ，具有 OneToMany 关系，所以每个学生都可以有一个或多个地址，

现在我尝试在 JSP 页面中显示给定的学生详细信息（其学生 + 其地址详细信息），但仅显示学生信息而不显示其地址。

这是 sevlet 页面：

       @WebServlet(name = "TestServlet", urlPatterns = {"/TestServlet"})
          public class TestServlet extends HttpServlet {
        @EJB
        private StudentDaoLocal studentDao;

        protected void processRequest(HttpServletRequest request, HttpServletResponse response)
                throws ServletException, IOException {

            String studentIdStr = request.getParameter("studentId");

            if (studentIdStr != null && !studentIdStr.equals("")) {
                int studentId = Integer.parseInt(studentIdStr);

                Student student = studentActions(studentId);

                request.setAttribute("student", student);
                request.getRequestDispatcher("info.jsp").forward(request, response);
            }

        }

        private Student studentActions(int studentId)
                throws ServletException, IOException {
            Student student;
            student = studentDao.getStudent(studentId);

            return student;
        }

JSP 页面：

       <%@page contentType="text/html" pageEncoding="UTF-8"%>
    <!DOCTYPE html>
    <html>
    <head>
    <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
    <title>JSP Page</title>
    </head>
        <body>
            <h1> Student Details</h1>

                   Student Details
                <br>
                <table border="1">
                    <tr>
                    <th>ID</th>
                    <th>First Name</th>
                    <th>Last Name</th>
                    <th>Year Level</th> 
                    </tr>

                     <c:forEach>
                        <tr>  
                        <td>${student.studentId}</td>
                        <td>${student.firstname}</td>
                        <td>${student.lastname}</td>
                        <td>${student.yearLevel}</td>
                        </tr>
                </c:forEach>

            </table>  

    </br>
            Student'Adresse
            <table border="1">
                <tr>
                <th>Street</th>
                <th>City</th>
                <th>Country</th>
                </tr>


                <h1>${student.addresses}</h1>  


                     <c:forEach items="${student.addresses}" var="address"> 
                         <tr> 
                             <td>${address.street}</td> 
                             <td>${address.city}</td> 
                             <td>${address.country}</td> 
                         </tr> </c:forEach>
            </table> 

    <br>
</body>
</html>

学生班：

     @Entity
     @Table
     @NamedQueries({@NamedQuery(name="Student.getAll",query="SELECT e FROM Student e") })

public class Student implements Serializable{
    @Id
    @GeneratedValue(strategy=GenerationType.AUTO)
    @Column
    private int studentId;
    @Column
    private String firstname;
    @Column
    private String lastname;
    @Column
    private int yearLevel;
    @OneToMany(mappedBy="student", cascade=CascadeType.ALL)
    private List<Address> addresses;

public int getStudentId() {
    return studentId;
}

public void setStudentId(int studentId) {
    this.studentId = studentId;
}

public String getFirstname() {
    return firstname;
}

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

public String getLastname() {
    return lastname;
}

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

public int getYearLevel() {
    return yearLevel;
}

public void setYearLevel(int yearLevel) {
    this.yearLevel = yearLevel;
}

public List<Address> getAddresses() {
    return addresses;
}

public void setAddresses(List<Address> addresses) {
    this.addresses = addresses;
}

public Student(int studentId, String firstname, String lastname, int yearLevel) {
    this.studentId = studentId;
    this.firstname = firstname;
    this.lastname = lastname;
    this.yearLevel = yearLevel;
}

public Student(int studentId, String firstname, String lastname, int yearLevel, List<Address> addresses) {
    this.studentId = studentId;
    this.firstname = firstname;
    this.lastname = lastname;
    this.yearLevel = yearLevel;
    this.addresses = addresses;
}

public Student(){}

public void addAddress(Address address){
    this.addresses.add(address);
    if(address.getStudent() != this){
        address.setStudent(this);
    }
   }
  }

和地址类：

        @Entity
        @Table
        public class Address implements Serializable{
        @Id
        @GeneratedValue(strategy=GenerationType.AUTO)
        @Column
        private int addressId;
        @ManyToOne(fetch=FetchType.LAZY)
        @JoinColumn(name="studentId")
        private Student student;
        @Column
        private String street;
        @Column
        private String city;
        @Column
        private String country;

public Address(){}
public Address(int addressId, String street, String city, String country) {
    this.addressId = addressId;
    this.street = street;
    this.city = city;
    this.country = country;
}

public Address(int addressId, Student student, String street, String city, String country) {
    this.addressId = addressId;
    this.student = student;
    this.street = street;
    this.city = city;
    this.country = country;
}

public Student getStudent() {
    return student;
}

public void setStudent(Student student) {
    this.student = student;
}

public int getAddressId() {
    return addressId;
}

public void setAddressId(int addressId) {
    this.addressId = addressId;
}

public String getStreet() {
    return street;
}

public void setStreet(String street) {
    this.street = street;
}

public String getCity() {
    return city;
}

public void setCity(String city) {
    this.city = city;
}

public String getCountry() {
    return country;
}

public void setCountry(String country) {
    this.country = country;
}

@Override
public String toString() {
    return street + ", " + city + ", " + country;
}
public boolean isEmptyAddress(){
    return this.getStreet().isEmpty() && this.getCity().isEmpty() && this.getCountry().isEmpty();
}
public void updateAddress(Address address){
    this.street = address.street;
    this.city = address.city;
    this.country = address.country;
}

这就是问题所在；地址部分未加载。

1: - 那么现在我如何打印 servlet 中的地址

那么现在如何在将学生对象传递给jsp之前首先检查地址是否正在加载？一位朋友告诉我，我可以在 servlet 中打印地址，但我不知道该怎么做。

2： - JSP 代码可以显示学生的地址吗？

（两者之间，我是 Java EE 世界的新手，所以......）

JSP 页面

4

1 回答 1

0

要查看是否正在填充地址，您可以在 servlet 中执行此操作：

Student student = studentActions(studentId);
if(student.getAddresses() != null) {
    for(Address address : student.getAddresses()) {
        System.out.println(address);
    }
}

jsp代码在打印学生信息的部分有一个额外的<c:forEach>标签，应该删除。外部<c:forEach>没有指定collectionorvar属性，如果这是您的实际代码的方式，它不会打印任何内容。

<c:forEach>
    <tr> 
        <c:forEach> <!-- REMOVE THIS -->
            <td>${student.studentId}</td>
            <td>${student.firstname}</td>
            <td>${student.lastname}</td>
            <td>${student.yearLevel}</td>
    </tr>
</c:forEach>
于 2013-03-27T16:06:28.893 回答