JPA教程 - JPA合并示例

在实体中合并后，我们可以更改其属性，EntityManager将自动更新数据库。

例子

下面的代码来自PersonDaoImpl.java。

package cn.w3cschool.common;

import java.util.Date;

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

import org.springframework.transaction.annotation.Transactional;

@Transactional
public class PersonDaoImpl {
  public void test() {
    Employee emp = new Employee();
    emp.setName("Tom");

    em.persist(emp);
    
    emp = em.find(Employee.class, emp.getId());
    
    Employee managedEmp = em.merge(emp);
    managedEmp.setLastAccessTime(new Date());

  }

  @PersistenceContext
  private EntityManager em;
}

以下代码来自Employee.java。

package cn.w3cschool.common;
import java.util.Date;

import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.Temporal;
import javax.persistence.TemporalType;

@Entity
public class Employee {
    @Id
    private int id;
    private String name;
    
    @Temporal(TemporalType.TIMESTAMP)
    private Date lastAccessTime;
    

    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 Date getLastAccessTime() {
        return lastAccessTime;
    }
    
    public void setLastAccessTime(Date lastAccessTime) {
        this.lastAccessTime = lastAccessTime;
    }
    
    public String toString() {
        return "Employee " + getId() + 
               ": name: " + getName() +
               " lastAccessTime: " + getLastAccessTime();
    }
}

以下是数据库转储。

Table Name: EMPLOYEE
 Row:
    Column Name: ID,
    Column Type: INTEGER:
    Column Value: 0

    Column Name: LASTACCESSTIME,
    Column Type: TIMESTAMP:
    Column Value: 2014-12-29 13:59:56.633

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