JPA 列長度示例

JPA教程 - JPA列長度示例

使用JPA，我們可以設置字段的列長度。例如，當將字符串字段映射到VARCHAR時，我們可以在@Column 注釋中設置VARCHAR長度。

@Column(length=40)
private String name;

例子

下面的代碼來自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;
  
  @Column(length=40)
  private String name;
  private String surname;
  
  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;
  }

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

下面的代碼來自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);

    Person p2 = new Person("Jack", "Kook");
    p2.setId(2L);
    
    save(p1);
    save(p2);

    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();
  }
  
}

下面的代碼是從App.java，主類。

package cn.w3cschool.common;

import org.springframework.context.support.ClassPathXmlApplicationContext;

public class App {

  public static void main(String[] args) {
    ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext(
        "applicationContext.xml");
    PersonDaoImpl dao = (PersonDaoImpl) context.getBean("personDao");

    dao.test();

    context.close();
    
    Helper.checkData();
  }
}

上面的代碼生成以下結果。

以下是數據庫轉儲。

Table Name: EMP
 Row:
    Column Name: EMP_ID,
    Column Type: BIGINT:
    Column Value: 1

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

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

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

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

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