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我正在学习 JPA，并且正在尝试编写一个小应用程序来显示带有约会的表格。

现在，我只使用h:dataTable(JSF) 显示约会的描述，但由于某种原因，这会导致 JPA 为每个约会生成一个选择以检索相关Creator实体。因此，例如，如果我有 100 个约会，这会生成 101 个对数据库的查询（一个用于检索约会，一个用于每个创建者），即使我没有Creator以任何方式访问此实体。

这是我的Appointment实体。它通过不是主键的代码引用Creator实体，但我无法更改表的结构。

@Entity
public class Appointment implements Serializable {
    private static final long serialVersionUID = 1L;

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

    private String description;

    private Timestamp date;


    @ManyToOne
    @JoinColumn(name = "APPOINTMENT_CREATOR", referencedColumnName = "USER_CODE")
    private Creator creator;

    // Getters and setters
}

这是Creator实体

@Entity
public class Creator implements Serializable {
    private static final long serialVersionUID = 1L;

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

    private String name;

    private String surname;

    @Column(name="USER_CODE")
    private int userCode;


    @OneToMany(mappedBy = "creator")
    private List<Cita> appointments;

    public Appointment addAppointment(Appointment appointment) {
        getAppointments().add(appointment);
        appointment.setCreator(this);

        return appointment;
    }

    public Appointment removeAppointment(Appointment appointment) {
        getAppointments().remove(appointment);
        appointment.setCreator(null);

        return appointment;
    }

    // Getters and setters
}

这是我的 DAO

@Repository
public class AppointmentDAO {
    private EntityManager entityManager;

    @PersistenceContext
    public void setEntityManager(EntityManager em) {
        this.entityManager = em;
    }

    public List<Appointment> findAll() {
        List<Appointment> result = null;
        try {
            result = this.entityManager.createQuery("SELECT a from Appointment a").getResultList();
        } catch (Exception ex) {
        }
        return result;
    }
}

这是我的日志摘录

[EL Finest]: connection: 2013-10-11 16:22:35.406--ServerSession(33459456)--Connection(11062282)--Thread(Thread["http-bio-8080"-exec-3,5,main])--Connection released to connection pool [read].

[EL Finest]: query: 2013-10-11 16:22:35.421--ServerSession(33459456)--Thread(Thread["http-bio-8080"-exec-3,5,main])--Execute query ReadObjectQuery(name="creator" referenceClass=Creator sql="SELECT ID, SURNAME, USER_CODE, NAME FROM CREATOR WHERE (USER_CODE = ?)")

[EL Finest]: connection: 2013-10-11 16:22:35.421--ServerSession(33459456)--Connection(20248250)--Thread(Thread["http-bio-8080"-exec-3,5,main])--Connection acquired from connection pool [read].

[EL Finest]: connection: 2013-10-11 16:22:35.421--ServerSession(33459456)--Thread(Thread["http-bio-8080"-exec-3,5,main]) --reconnecting to external connection pool

[EL Fine]: sql: 2013-10-11 16:22:35.453--ServerSession(33459456)--Connection(3304058)--Thread(Thread["http-bio-8080"-exec-3,5,main])--SELECT ID, SURNAME, USER_CODE, NAME FROM CREATOR WHERE (USER_CODE = ?)
    bind => [7054382]

[EL Finest]: connection: 2013-10-11 16:22:35.515--ServerSession(33459456)--Connection(20248250)--Thread(Thread["http-bio-8080"-exec-3,5,main])--Connection released to connection pool [read].

[EL Finest]: transaction: 2013-10-11 16:22:35.531--UnitOfWork(12558692)--Thread(Thread["http-bio-8080"-exec-3,5,main])--Register the existing object net.turbo.model.entities.Creator@b2e368

[EL Finest]: query: 2013-10-11 16:22:35.531--ServerSession(33459456)--Thread(Thread["http-bio-8080"-exec-3,5,main])--Execute query ReadObjectQuery(name="creator" referenceClass=Creator sql="SELECT ID, SURNAME, USER_CODE, NAME FROM CREATOR WHERE (USER_CODE = ?)")

[EL Finest]: connection: 2013-10-11 16:22:35.531--ServerSession(33459456)--Connection(32578948)--Thread(Thread["http-bio-8080"-exec-3,5,main])--Connection acquired from connection pool [read].

[EL Finest]: connection: 2013-10-11 16:22:35.531--ServerSession(33459456)--Thread(Thread["http-bio-8080"-exec-3,5,main])--reconnecting to external connection pool
[EL Fine]: sql: 2013-10-11 16:22:35.562--ServerSession(33459456)--Connection(13173146)--Thread(Thread["http-bio-8080"-exec-3,5,main])--SELECT ID, SURNAME, USER_CODE, NAME FROM CREATOR WHERE (USER_CODE = ?)
    bind => [7042199]

[EL Finest]: connection: 2013-10-11 16:22:35.64--ServerSession(33459456)--Connection(32578948)--Thread(Thread["http-bio-8080"-exec-3,5,main])--Connection released to connection pool [read].

[EL Finest]: transaction: 2013-10-11 16:22:35.64--UnitOfWork(12558692)--Thread(Thread["http-bio-8080"-exec-3,5,main])--Register the existing object net.turbo.model.entities.Creator@1b0c903

[EL Finest]: query: 2013-10-11 16:22:35.64--ServerSession(33459456)--Thread(Thread["http-bio-8080"-exec-3,5,main])--Execute query ReadObjectQuery(name="creator" referenceClass=Creator sql="SELECT ID, SURNAME, USER_CODE, NAME FROM CREATOR WHERE (USER_CODE = ?)")

[EL Finest]: connection: 2013-10-11 16:22:35.64--ServerSession(33459456)--Connection(7112313)--Thread(Thread["http-bio-8080"-exec-3,5,main])--Connection acquired from connection pool [read].

[EL Finest]: connection: 2013-10-11 16:22:35.64--ServerSession(33459456)--Thread(Thread["http-bio-8080"-exec-3,5,main])--reconnecting to external connection pool
[EL Fine]: sql: 2013-10-11 16:22:35.671--ServerSession(33459456)--Connection(15764427)--Thread(Thread["http-bio-8080"-exec-3,5,main])--SELECT ID, SURNAME, USER_CODE, NAME FROM CREATOR WHERE (USER_CODE = ?)
    bind => [7076961]



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2 回答 2

1

关系的默认 fetchTypeManyToOne是EAGER. 将此设置为LAZY，这将导致Creator实体在访问之前不会被查询。

@ManyToOne(fetch=FetchType.LAZY)
@JoinColumn(name = "APPOINTMENT_CREATOR", referencedColumnName = "USER_CODE")
private Creator creator;
于 2013-10-11T14:51:29.580 回答
0

好的，我解决了。这是因为我使用的是 Tomcat，而 EclipseLink 无法使用编织来更改类，没有这个，EclipseLink 只能使用预加载。

我刚刚从 Tomcat 迁移到一个真正的应用程序服务器。

于 2013-10-15T07:44:03.923 回答