惠州什么是JPA ElementCollection映射关键实体？_北大青鸟IT学校

相信大家都很好奇什么是JPA ElementCollection映射关键实体？那么下面我们一起来看看惠州北大青鸟老师是怎么说的。

在JPA中，我们可以将通用类型的Map映射到数据库。

以下代码定义了一个通用映射，其键值为Employee，值类型为Integer。

@ElementCollection
@CollectionTable(name="EMP_SENIORITY")
@MapKeyJoinColumn(name="EMP_ID")
@Column(name="SENIORITY")private Map<Employee, Integer> seniorities;

实例

下面的代码来自Department.java。

package cn.w3cschool.common;

import java.util.HashMap;
import java.util.Map;

import javax.persistence.CollectionTable;
import javax.persistence.Column;
import javax.persistence.ElementCollection;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.MapKeyJoinColumn;

@Entitypublic class Department {
   @Id @GeneratedValue(strategy=GenerationType.IDENTITY)    private int id;    private String name;
   
   @ElementCollection
   @CollectionTable(name="EMP_SENIORITY")
   @MapKeyJoinColumn(name="EMP_ID")
   @Column(name="SENIORITY")    private Map<Employee, Integer> seniorities;    
   public Department() {
       seniorities = new HashMap<Employee, Integer>();
   }    
   public int getId() {
       return id;
   }    
   public void setId(int id) {
       this.id = id;
   }    
   public String getName() {
       return name;
   }    
   public void setName(String deptName) {
       this.name = deptName;
   }    
   public Map<Employee, Integer> getEmployees() {
       return seniorities;
   }    public void setEmployeeSeniority(Employee employee, int seniority) {
       seniorities.put(employee, seniority);
   }    public void removeEmployee(Employee employee) {
       seniorities.remove(employee);
   }    public String toString() {
       StringBuffer aBuffer = new StringBuffer("Department ");
       aBuffer.append(" id: ");
       aBuffer.append(id);
       aBuffer.append(" name: ");
       aBuffer.append(name);
       aBuffer.append(" employeeCount: ");
       aBuffer.append(seniorities.size());
       return aBuffer.toString();
   }
}

下面的代码来自PersonDaoImpl.java。

package cn.w3cschool.common;

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

import org.springframework.transaction.annotation.Transactional;

@Transactionalpublic class PersonDaoImpl {  public void test() {
   Employee e = new Employee();
   e.setName("Tom");
   Department d = new Department();
 
   d.setName("test");
   d.getEmployees().put(e, e.getId());
   
   
   em.persist(e);    
   em.persist(d);
 

 }

 @PersistenceContext  private EntityManager em;
}
下载 ElementCollection_MapKeyEntity.zip

这里是数据库表转储。

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

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

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

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

   Column Name: SALARY,
   Column Type: BIGINT:
   Column Value: 0





Table Name: EMP_SENIORITY
Row:
   Column Name: DEPARTMENT_ID,
   Column Type: INTEGER:
   Column Value: 1

   Column Name: SENIORITY,
   Column Type: INTEGER:
   Column Value: 0

   Column Name: EMP_ID,
   Column Type: INTEGER:
   Column Value: 1

想知道更多关于Java的资讯，联系在线客服，或者来惠州北大青鸟新方舟校区了解了解。

Java

标签：惠州计算机JAVA软件开发惠州计算机Java软件开发惠州计算机JAVA培训惠州计算机JAVA软件开发学校惠州计算机Java软件开发培训JAVAJava软件开发北大青鸟IT计算机学校北大青鸟IT软件学校北大青鸟IT学校