JPA fetchType.Lazy无法正常工作

我正在尝试fetchType.Lazy的示例，但是在调试代码时，似乎fetchType.Lazy无法正常工作。

实体Bean:地址

在地区字段上添加了具有属性fetch = fetchType.Lazy的注释@Basic。

我的实体bean由以下代码定义:

    package model;

    import java.io.Serializable;
    import javax.persistence.*;
    import java.util.List;


    /**
     * The persistent class for the address database table.
     */
    @Entity
    @Table(name="address", schema="home")
    public class Address implements Serializable {
        private static final long serialVersionUID = 1L;

        @TableGenerator(name = "addr_gen", table = "table_generator", pkColumnName = "gen_name", valueColumnName = "gen_val", allocationSize=1)
        @Id
        @GeneratedValue(strategy=GenerationType.TABLE, generator="addr_gen")
        private String addressId;

        private String city;

        @Basic(fetch=FetchType.LAZY)
        @Column(name="district")
        private String district;

        private String houseNumber;

        private String pincode;

        private String state;

        private String street;

        //bi-directional many-to-one association to Employee
        @OneToMany(mappedBy="address")
        private List<Employee> employees;

        public Address() {
        }


        public String getAddressId() {
            return this.addressId;
        }

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

        public String getCity() {
            return this.city;
        }

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

        public String getDistrict() {
            return this.district;
        }

        public void setDistrict(String district) {
            this.district = district;
        }

        public String getHouseNumber() {
            return this.houseNumber;
        }

        public void setHouseNumber(String houseNumber) {
            this.houseNumber = houseNumber;
        }

        public String getPincode() {
            return this.pincode;
        }

        public void setPincode(String pincode) {
            this.pincode = pincode;
        }

        public String getState() {
            return this.state;
        }

        public void setState(String state) {
            this.state = state;
        }

        public String getStreet() {
            return this.street;
        }

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

        public List<Employee> getEmployees() {
            return this.employees;
        }

        public void setEmployees(List<Employee> employees) {
            this.employees = employees;
        }


        public Employee addEmployees(Employee employees) {
            getEmployees().add(employees);
            employees.setAddress(this);

            return employees;
        }

        public Employee removeEmployees(Employee employees) {
            getEmployees().remove(employees);
            employees.setAddress(null);

            return employees;
        }

        /*@Override
        public String toString() {
            return "Address [addressId=" + addressId + ", city=" + city
                    + ", district=" + district + ", houseNumber=" + houseNumber
                    + ", pincode=" + pincode + ", state=" + state + ", street="
                    + street + ", employees=" + employees + "]";
        }*/
    }

使用以上实体bean的方法:

public Address findAddress(EntityManagerFactory emf, UserTransaction tx) {
    EntityManager em = emf.createEntityManager();
    Address addr = null;
    try {
        tx.begin();
        addr = em.find(Address.class, new String("154"));
        tx.commit();
    } catch (Exception e) {
        e.printStackTrace();
    }

    return addr;
}

因此，在调用find方法之后，当我检查了地址对象时，它已经充满了地区字段。

请让我知道我是否缺少某些配置，或者代码本身是否存在问题。

最佳答案

问题在于，当在@Basic批注中使用时，应用服务器可以/可以自行决定何时获取数据更好。如所记录的here



根据documentation，类似于@OneToMany的注释具有相同的问题，但是在这种情况下，JPA提供程序将更可能考虑FetchType提示。

另一方面，您可以尝试使用大数据字段，例如

@Lob
@Basic(fetch=FetchType.LAZY)
private byte[] document;

看看是否有任何变化(尽管再次，您的应用服务器可以决定总是获取该字段)。

为了检查是否已加载收集字段，我将检查数据库查询日志或执行以下操作:

@PersistenceContext private EntityManager em;
.....
Person person = em.find(Person.class, 1L);//Person entity has a OneToMany relationship to entity Address
em.detach(person);
person.getAddresses().size();//if the Address are now not fetched, it the call should  throw an error

关于JPA fetchType.Lazy无法正常工作，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/18693849/