Hibernate 4 – 获取延迟加载的实体引用 - 《HowToDoInJava 中文系列教程》

原文： https://howtodoinjava.com/hibernate/hibernate-4-example-to-get-entity-reference-for-lazy-loading/

根据 Wikipedia 的定义， 延迟加载是一种设计模式，通常用于计算机编程中，以将对象的初始化推迟到需要的时间点。我们知道，在 Hiberate 映射中，可以通过在 Hiberate 映射注解中指定fetch = FetchType.LAZY来完成延迟加载。例如：

@ManyToOne ( fetch = FetchType.LAZY )
@JoinColumns( {
        @JoinColumn(name="fname", referencedColumnName = "firstname"),
        @JoinColumn(name="lname", referencedColumnName = "lastname")
        } )
public EmployeeEntity getEmployee() {
    return employee;
}

关键是仅在定义两个实体之间的映射时才应用它。如果已经在DepartmentEntity中定义了以上实体，则如果您获取DepartmentEntity，则EmployeeEntity将被延迟加载。

但是，如果您要延迟加载DepartmentEntity本身，即主实体本身应延迟加载，该怎么办。

可以通过在IdentifierLoadAccess类中使用getReference()方法解决此问题。

让我们通过此示例了解用法。

作为参考，最新的 Hiberate Maven 依赖项如下：

<dependency>
    <groupId>org.hibernate</groupId>
    <artifactId>hibernate-core</artifactId>
    <version>4.3.0.Beta3</version>
</dependency>

现在，我有一个主实体类EmployeeEntity，它可以具有多个属性以及与其他实体的映射。

EmployeeEntity.java

@Entity
@Table(name = "Employee", uniqueConstraints = {
        @UniqueConstraint(columnNames = "ID"),
        @UniqueConstraint(columnNames = "EMAIL") })
public class EmployeeEntity implements Serializable {
    private static final long serialVersionUID = -1798070786993154676L;
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "ID", unique = true, nullable = false)
    private Integer employeeId;
    //Use the natural id annotation here
    @NaturalId (mutable = false)
    @Column(name = "EMAIL", unique = true, nullable = false, length = 100)
    private String email;
    @Column(name = "FIRST_NAME", unique = false, nullable = false, length = 100)
    private String firstName;
    @Column(name = "LAST_NAME", unique = false, nullable = false, length = 100)
    private String lastName;
    //Setters and Getters
}

我想在我的代码中延迟加载的主实体之上延迟加载，即我可以在一个地方获取实体的引用，但实际上可能需要在另一个地方进行引用。仅在需要时，我才想初始化或加载其数据。到时候为止，我只想参考。

让我们在代码示例中这样做：

TestHibernate.java

public class TestHibernate
{
    public static void main(String[] args) 
    {
        Session session = HibernateUtil.getSessionFactory().openSession();
        session.beginTransaction();
        //Add new Employee object
        EmployeeEntity emp = new EmployeeEntity();
        emp.setEmail("demo-user@mail.com");
        emp.setFirstName("demo");
        emp.setLastName("user");
        //Save entity
        session.save(emp);
        session.getTransaction().commit();
        session.close();
        session = HibernateUtil.getSessionFactory().openSession();
        session.beginTransaction();
        //Get only the reference of EmployeeEntity for now
        EmployeeEntity empGet = (EmployeeEntity) session.byId( EmployeeEntity.class ).getReference( 1 );
        System.out.println("No data initialized till now; Lets fetch some data..");
        //Now EmployeeEntity will be loaded from database when we need it
        System.out.println(empGet.getFirstName());
        System.out.println(empGet.getLastName());
        session.getTransaction().commit();
        HibernateUtil.shutdown();
    }
}
Output in console:
Hibernate: insert into Employee (EMAIL, FIRST_NAME, LAST_NAME) values (?, ?, ?)
No data initialized till now; Lets fetch some data..
Hibernate: select employeeen0_.ID as ID1_0_0_, employeeen0_.EMAIL as EMAIL2_0_0_, employeeen0_.FIRST_NAME as FIRST3_0_0_, 
employeeen0_.LAST_NAME as LAST4_0_0_ from Employee employeeen0_ where employeeen0_.ID=?
demo
user

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