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;@Basicprivate 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;}@Overridepublic 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);}}@PersistenceContextprivate 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: EMPRow:Column Name: EMP_ID,Column Type: BIGINT:Column Value: 2Column Name: NAME,Column Type: VARCHAR:Column Value: JackColumn Name: PICTURE,Column Type: BLOB:Column Value: nullColumn Name: SURNAME,Column Type: VARCHAR:Column Value: Kook