惠州JPA如何查询新对象？_北大青鸟IT学校

JPA如何查询新对象？这是大家在学习Java的过程都会遇到的问题，那么下面我们一起来看看惠州北大青鸟学术部老师是怎么讲解这道题的。

我们可以在JPQL中创建新对象。

以下代码调用JPQL中的EmployeeDetails构造函数。

executeAndPrintQuery("SELECT NEW cn.w3cschool.common.EmployeeDetails(e.name, e.salary, e.department.name) FROM Employee e");

实例

下面的代码来自PersonDaoImpl.java。

package cn.w3cschool.common;

import java.util.Arrays;
import java.util.Date;
import java.util.List;

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

import org.springframework.transaction.annotation.Transactional;

@Transactionalpublic class PersonDaoImpl {  public void test() {
   Employee emp = new Employee();
   emp.setName("Tom");
   emp.setSalary(123);
   emp.setStartDate(new Date());
   emp.setId(1);

   Project pro = new Project();
   pro.setName("Design");
   pro.getEmployees().add(emp);

   Department dept = new Department();
   dept.setName("Dept");
   dept.getEmployees().add(emp);

   emp.setDepartment(dept);
   emp.getProjects().add(pro);

   em.persist(dept);
   em.persist(pro);
   em.persist(emp);

   executeAndPrintQuery("SELECT NEW cn.w3cschool.common.EmployeeDetails(e.name, e.salary, e.department.name) FROM Employee e");
 }  private void executeAndPrintQuery(String queryString) {    try {
       Query query = em.createQuery(queryString);
       printQueryResult(queryString, query.getResultList());
   } finally {
       em.close();
   }
}  private void printQueryResult(String queryString, List result) {
   System.out.println("EJB QL: " + queryString+"\n");
   System.out.println("Result:\n");    if (result.isEmpty()) {
       System.out.println("No results Found\n");
   } else {        for (Object o : result) {
           System.out.println(resultAsString(o));
       }
   }
 }  private String resultAsString(Object o) {    if (o instanceof Object[]) {
       return Arrays.asList((Object[])o).toString();
   } else {
       return String.valueOf(o);
   }
}
 @PersistenceContext  private EntityManager em;
}class EmployeeDetails {  private String name;  private long salary;  private String deptName;  
 public EmployeeDetails(String name, long salary, String deptName) {
     this.name = name;
     this.salary = salary;
     this.deptName = deptName;
 }  
 public String toString() {
     return "EmployeeDetails name: " + name +             ", salary: " + salary +             ", deptName: " + deptName;
 }
}

下面的代码来自Department.java。

package cn.w3cschool.common;


import java.util.ArrayList;
import java.util.Collection;

import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.OneToMany;

@Entitypublic class Department {
   @Id    private int id;    private String name;
   @OneToMany(mappedBy="department")    private Collection<Employee> employees;    public Department() {
       employees = new ArrayList<Employee>();
   }    
   public void setId(int id) {
     this.id = id;
   }    public void setName(String name) {
     this.name = name;
   }    public void setEmployees(Collection<Employee> employees) {
     this.employees = employees;
   }    public int getId() {
       return id;
   }    
   public String getName() {
       return name;
   }    
   public Collection<Employee> getEmployees() {
       return employees;
   }    public String toString() {
       return "Department no: " + getId() +
              ", name: " + getName();
   }
}

以下代码来自Employee.java。

package cn.w3cschool.common;



import java.util.ArrayList;
import java.util.Collection;
import java.util.Date;

import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.ManyToMany;
import javax.persistence.ManyToOne;
import javax.persistence.NamedQueries;
import javax.persistence.NamedQuery;
import javax.persistence.OneToMany;
import javax.persistence.Temporal;
import javax.persistence.TemporalType;
@Entitypublic class Employee {
   @Id    private int id;    private String name;    private long salary;
   @Temporal(TemporalType.DATE)    private Date startDate;
   
   @ManyToOne    private Employee manager;
   
   @OneToMany(mappedBy="manager")    private Collection<Employee> directs;

   @ManyToOne    private Department department;
   
   @ManyToMany
   private Collection<Project> projects;    public Employee() {
       projects = new ArrayList<Project>();
       directs = new ArrayList<Employee>();
   }    public void setId(int id) {
     this.id = id;
   }    public void setName(String name) {
     this.name = name;
   }    public void setSalary(long salary) {
     this.salary = salary;
   }    public void setStartDate(Date startDate) {
     this.startDate = startDate;
   }    public void setManager(Employee manager) {
     this.manager = manager;
   }    public void setDirects(Collection<Employee> directs) {
     this.directs = directs;
   }    public void setDepartment(Department department) {
     this.department = department;
   }    public void setProjects(Collection<Project> projects) {
     this.projects = projects;
   }    public int getId() {
       return id;
   }    
   public String getName() {
       return name;
   }    public long getSalary() {
       return salary;
   }    public Date getStartDate() {
       return startDate;
   }    
   public Department getDepartment() {
       return department;
   }    
   public Collection<Employee> getDirects() {
       return directs;
   }    
   public Employee getManager() {
       return manager;
   }    public Collection<Project> getProjects() {
       return projects;
   }    
   public String toString() {
       return "Employee " + getId() +
              ": name: " + getName() +               ", salary: " + getSalary() +               ", dept: " + ((getDepartment() == null) ? null : getDepartment().getName());
   }
}

以下代码来自Project.java。

package cn.w3cschool.common;


import java.util.ArrayList;
import java.util.Collection;

import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.ManyToMany;

@Entitypublic class Project {
   @Id    protected int id;    protected String name;
   @ManyToMany(mappedBy="projects")    private Collection<Employee> employees;    public Project() {
       employees = new ArrayList<Employee>();
   }    public void setId(int id) {
     this.id = id;
   }    public void setName(String name) {
     this.name = name;
   }    public void setEmployees(Collection<Employee> employees) {
     this.employees = employees;
   }    public int getId() {
       return id;
   }    
   public String getName() {
       return name;
   }    
   public Collection<Employee> getEmployees() {
       return employees;
   }    
   public String toString() {
       return "Project id: " + getId() + ", name: " + getName();
   }
}

上面的代码生成以下结果。

   EJB QL: SELECT NEW cn.w3cschool.common.EmployeeDetails(e.name, e.salary, e.department.name) FROM Employee e
   
   Result:
   
   EmployeeDetails name: Tom, salary: 123, deptName: Dept

以下是数据库转储。

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

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





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: 123

   Column Name: STARTDATE,
   Column Type: DATE:
   Column Value: 2014-12-29

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

   Column Name: MANAGER_ID,
   Column Type: INTEGER:
   Column Value: null





Table Name: EMPLOYEE_PROJECT
Row:
   Column Name: EMPLOYEES_ID,
   Column Type: INTEGER:
   Column Value: 1

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





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

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

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