JPA Lob延迟加载示例

JPA教程 – JPA Lob延迟加载示例

我们可以通过仅提取频繁访问的数据来优化检索实体时的性能。如果需要，可以提取剩余的数据。

当使用@Lob注释将字节数组或字符数组字段映射到数据库时，我们可以设置是否对该字段执行延迟加载。

延迟加载使应用程序运行速度更快，因为JPA只在加载字节数组或字符数组时才使用它们。

基本映射的获取类型可以配置为通过在相应的@Basic注释中指定fetch元素来延迟或急切加载。

FetchType枚举类型定义此元素的值，可以是EAGER或LAZY。

  @Basic(fetch=LAZY)
  @Lob @Column(name="PIC")
  private byte[] picture;

例子

下面的代码来自PersonDaoImpl.java。

package cn.w3cschool.common;

import java.util.List;

import javax.persistence.EntityManager;
import javax.persistence.PersistenceContext;
import javax.persistence.TypedQuery;

import org.springframework.transaction.annotation.Transactional;

@Transactional
public class PersonDaoImpl {
  public void test() {

    Professor emp = new Professor();
    emp.setId(1);
    emp.setName("name");
    emp.setSalary(12345);
    em.persist(emp);

  }

  @PersistenceContext
  private EntityManager em;
}

以下代码来自Professor.java。

package cn.w3cschool.common;
import javax.persistence.Basic;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.Lob;
import static javax.persistence.FetchType.LAZY;
@Entity

public class Professor {
  @Id
  private int id;
  private String name;
  private long salary;

  @Basic(fetch=LAZY)
  @Lob @Column(name="PIC")
  private byte[] picture;

  public int getId() {
      return id;
  }

  public void setId(int id) {
      this.id = id;
  }

  public String getName() {
      return name;
  }

  public void setName(String name) {
      this.name = name;
  }

  public long getSalary() {
      return salary;
  }

  public void setSalary(long salary) {
      this.salary = salary;
  }

  public byte[] getPicture() {
      return picture;
  }

  public void setPicture(byte[] picture) {
      this.picture = picture;
  }
  public String toString() {
      return "Employee id: " + getId() + " name: " + getName() + 
      " salary: " + getSalary() + " pic: " + new String(getPicture());
  }
}

下载 Lab_Lazy_Load.zip

以下是数据库转储。

Table Name: PROFESSOR
 Row:
    Column Name: ID,
    Column Type: INTEGER:
    Column Value: 1

    Column Name: NAME,
    Column Type: VARCHAR:
    Column Value: name

    Column Name: PIC,
    Column Type: BLOB:
    Column Value: null

    Column Name: SALARY,
    Column Type: BIGINT:
    Column Value: 12345