# Hibernate 4 – 获取延迟加载的实体引用 > 原文： [https://howtodoinjava.com/hibernate/hibernate-4-example-to-get-entity-reference-for-lazy-loading/](https://howtodoinjava.com/hibernate/hibernate-4-example-to-get-entity-reference-for-lazy-loading/) 根据 Wikipedia 的定义， [**延迟加载**](https://en.wikipedia.org/wiki/Lazy_loading "lazy loading")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： ```java @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()`**](http://docs.jboss.org/hibernate/orm/4.1/javadocs/org/hibernate/IdentifierLoadAccess.html#getReference%28java.io.Serializable%29 "IdentifierLoadAccess")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： ```java <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`** ```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`** ```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 ``` 要下载以上教程的源代码，请单击下面的下载链接。 **祝您学习愉快！**