JPA教程 - JPA按ID查找示例

一旦我们将实体保存到数据库中，我们可以通过使用EntityManager中的find方法来检索它们。

以下代码显示如何使用find方法与实体id。

Person emp =  em.find(Person.class, 1L);

例子

下面的代码来自Person.java。

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

@Entity
@Table(name="EMP")
public class Person {
  @Id
  @Column(name = "EMP_ID")
  private long id;
  
  @Basic
  private String name;
  private String surname;
  
  @Lob 
  @Basic(fetch=LAZY)
  private byte[] picture;
  
  public Person() {}

  public Person(String name, String surname) {
    this.name = name;
    this.surname = surname;
  }
  
  public Long getId() {
    return id;
  }
  public void setId(Long id) {
    this.id = id;
  }
  public String getName() {
    return name;
  }
  public void setName(String name) {
    this.name = name;
  }
  public String getSurname() {
    return surname;
  }
  public void setSurname(String surname) {
    this.surname = surname;
  }
  
  public byte[] getPicture() {
    return picture;
  }

  public void setPicture(byte[] picture) {
    this.picture = picture;
  }

  @Override
  public String toString() {
    return "Person [id=" + id + ", name=" + name + ", surname=" + surname + "]";
  }
}

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 org.springframework.transaction.annotation.Transactional;

@Transactional
public class PersonDaoImpl {
  public void test(){
    Person p1 = new Person("Tom", "Smith");
    p1.setId(1L);
    p1.setPicture("asdf".getBytes());
    Person p2 = new Person("Jack", "Kook");
    p2.setId(2L);
    p1.setPicture("www.w3cschool.cn".getBytes());
    
    save(p1);
    save(p2);

    listAll();
    
    Person emp =  em.find(Person.class, 1L);
    if (emp != null) {
      em.remove(emp);
    }
    listAll();
  }
  private void listAll(){
    List<Person> persons = getAll();
    for (Person person : persons) {
      System.out.println(person);
    }

  }
  @PersistenceContext
  private EntityManager em;
  
  
  public Long save(Person person) {
    em.persist(person);
    return person.getId();
  }
  
  public List<Person>getAll() {
    return em.createQuery("SELECT p FROM Person p", Person.class).getResultList();
  }
  
}

下载 Find_By_ID.zip

上面的代码生成以下结果。

以下是数据库转储。

Table Name: EMP
 Row:
    Column Name: EMP_ID,
    Column Type: BIGINT:
    Column Value: 2

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

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

    Column Name: SURNAME,
    Column Type: VARCHAR:
    Column Value: Kook