使用JPA和JSF保留在联接表中

Question

我的模型包含“ 学生”和“ 课程 ”类（请参见下文）。

Student.java ：

@NamedQueries({ @NamedQuery(name = "SelectStudents", query = "SELECT s FROM Student s"), })
@Entity
public class Student implements Serializable {

    private static final long serialVersionUID = -8776005542073703016L;

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private int id;

    @Size(min = 3, max = 30)
    private String firstName;

    @Size(min = 3, max = 30)
    private String lastName;

    @Temporal(TemporalType.DATE)
    @Past
    private Date dateOfBirth;

    private String email;

    @Size(min = 5, max = 5)
    private String regNr;

    private String faculty;

    @ManyToMany
    @JoinTable(name = "StudentCourse", joinColumns = { 
            @JoinColumn(name = "student_id", referencedColumnName = "id") }, inverseJoinColumns = { 
            @JoinColumn(name = "course_id", referencedColumnName = "id") })
    private List<Course> courses;

    public Student() {
    }

    // getter and setter
}

Course.java ：

@NamedQueries({
        @NamedQuery(name = "SelectCourses", query = "SELECT c FROM Course c"),
        @NamedQuery(name = "SelectStudentCoursesByName", query = "SELECT c FROM Course c WHERE c.name = :name") })
@Entity
public class Course implements Serializable {

    private static final long serialVersionUID = -5955154651849644853L;

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private int id;

    private String name;

    private int cp;

    public Course() {
    }
    // getter and setter
}

控制器如下所示。

StudentController.java ：

@ManagedBean(name = "studentController")
@SessionScoped
public class StudentController {

    private DataModel<Student> students;

    private Student student = new Student();

    @Resource
    private UserTransaction utx;

    @PersistenceContext
    private EntityManager em;

    @PostConstruct
    public void init() {
        students = new ListDataModel<Student>();
        students.setWrappedData(em.createNamedQuery("SelectStudents")
                .getResultList());
    }

    public String newStudent() {
        this.student = new Student();
        return "newStudent?faces-redirect=true";
    }

    public String saveStudent() {
        try {
            utx.begin();
        } catch (NotSupportedException e) {
            e.printStackTrace();
        } catch (SystemException e) {
            e.printStackTrace();
        }
        student = em.merge(student);
        em.persist(student);
        students.setWrappedData(em.createNamedQuery("SelectStudents")
                .getResultList());
        try {
            utx.commit();
        } catch (SecurityException e) {
            e.printStackTrace();
        } catch (IllegalStateException e) {
            e.printStackTrace();
        } catch (RollbackException e) {
            e.printStackTrace();
        } catch (HeuristicMixedException e) {
            e.printStackTrace();
        } catch (HeuristicRollbackException e) {
            e.printStackTrace();
        } catch (SystemException e) {
            e.printStackTrace();
        }
        return "studentList";
    }

    public String deleteStudent() {
        try {
            utx.begin();
        } catch (NotSupportedException e) {
            e.printStackTrace();
        } catch (SystemException e) {
            e.printStackTrace();
        }
        student = students.getRowData();
        // Transaktionsbeginn
        student = em.merge(student);
        em.remove(student);
        students.setWrappedData(em.createNamedQuery("SelectStudents")
                .getResultList());
        try {
            utx.commit();
        } catch (SecurityException e) {
            e.printStackTrace();
        } catch (IllegalStateException e) {
            e.printStackTrace();
        } catch (RollbackException e) {
            e.printStackTrace();
        } catch (HeuristicMixedException e) {
            e.printStackTrace();
        } catch (HeuristicRollbackException e) {
            e.printStackTrace();
        } catch (SystemException e) {
            e.printStackTrace();
        }
        return "studentList?faces-redirect=true";
    }

    public String editStudent() {
        student = students.getRowData();
        return "newStudent";
    }
    // getter and setter
}

CourseController.java ：

@ManagedBean
@SessionScoped
public class CourseController {

    private DataModel<Course> courses;

    private Course course;

    @Resource
    private UserTransaction utx;

    @PersistenceContext
    private EntityManager em;

    @PostConstruct
    public void init() {
        courses = new ListDataModel<Course>();
        courses.setWrappedData(em.createNamedQuery("SelectCourses")
                .getResultList());
    }

    public String newCourse() {
        this.course = new Course();
        return "newCourse?faces-redirect=true";
    }

    public String saveCourse() {
        try {
            utx.begin();
        } catch (NotSupportedException e) {
            e.printStackTrace();
        } catch (SystemException e) {
            e.printStackTrace();
        }
        course = em.merge(course);
        em.persist(course);
        courses.setWrappedData(em.createNamedQuery("SelectCourses")
                .getResultList());
        try {
            utx.commit();
        } catch (SecurityException e) {
            e.printStackTrace();
        } catch (IllegalStateException e) {
            e.printStackTrace();
        } catch (RollbackException e) {
            e.printStackTrace();
        } catch (HeuristicMixedException e) {
            e.printStackTrace();
        } catch (HeuristicRollbackException e) {
            e.printStackTrace();
        } catch (SystemException e) {
            e.printStackTrace();
        }
        return "courseList?faces-redirect=true";
    }

    public String deleteCourse() {
        try {
            utx.begin();
        } catch (NotSupportedException e) {
            e.printStackTrace();
        } catch (SystemException e) {
            e.printStackTrace();
        }
        course = courses.getRowData();
        // Transaktionsbeginn
        course = em.merge(course);
        em.remove(course);
        courses.setWrappedData(em.createNamedQuery("SelectCourses")
                .getResultList());
        try {
            utx.commit();
        } catch (SecurityException e) {
            e.printStackTrace();
        } catch (IllegalStateException e) {
            e.printStackTrace();
        } catch (RollbackException e) {
            e.printStackTrace();
        } catch (HeuristicMixedException e) {
            e.printStackTrace();
        } catch (HeuristicRollbackException e) {
            e.printStackTrace();
        } catch (SystemException e) {
            e.printStackTrace();
        }
        return "courseList?faces-redirect=true";
    }

    public String editCourse() {
        course = courses.getRowData();
        return "newCourse?faces-redirect=true";
    }
    // getter and setter
}

我已经创建了JSF页面，并且能够在Java EE应用程序中添加，删除和编辑“ 学生和课程” 。

如您所见， 联接表StudentCourse由JPA自动创建。 现在，我希望能够将已经创建并保留的课程添加到已经创建并保留的学生中 ，以便可以在任意数量的课程中注册学生 。 引用将存储在MySQL数据库的StudentCourse表中。

创建联接表是一个好主意还是应该创建模型类StudentCourse和控制器StudentCourseController来实现此问题？

Answer 1

对于您的问题所描述的联接表已足够，但是，如果您以后想要添加属性（例如它们获得的等级），则将需要一个额外的实体类来对关系进行建模。