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jpa 分页查询 JPA 查询分页示例

2023-05-31 11:18 Java教程

jpa 分页查询 JPA 查询分页示例

jpa 分页查询

JPA 分页查询是一种常用的数据库查询方式，它可以有效地减少查询时间，提高系统性能。JPA 分页查询的基本原理是将大量的数据分割成多个小块，然后逐个进行处理。

JPA 分页查询可以使用 Query 接口中的 setFirstResult() 和 setMaxResults() 方法来实现。setFirstResult() 方法用于设置从哪一条记录开始，setMaxResults() 方法用于设置最多返回多少条记录。例如：

Query query = entityManager.createQuery("SELECT e FROM Employee e"); 
query.setFirstResult(0);  // 从第 0 条开始 
query.setMaxResults(10);  // 最多返回 10 条记录 
List<Employee> list = query.getResultList();   // 返回列表

此外，JPA 还提供了一个 Pageable 类来实现分页功能。Pageable 类包含了 pageNumber、pageSize、sort 等属性，其中 pageNumber 表示要显示的页数，pageSize 表示要显示的记录数量，sort 表示要对哪些字段进行升序或者降序排序。例如：

 
Pageable pageable = PageRequest.of(0, 10, Sort.by("name").ascending());   // pageNumber=0, pageSize=10, sort=name asc  
Page<Employee> page = employeeRepository.findAll(pageable);   // 返回分页对象

JPA 查询分页示例

JPA教程 - JPA查询分页示例


PersonDaoImpl在下面的部分显示如何逐页显示结果。

例子

下面的代码来自Department.java。

package cn..common;


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

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

@Entity
public 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();
    }
}

下面的代码来自PersonDaoImpl.java。

package cn..common;

import java.util.Collection;
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;

@Transactional
public 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);
  }
  private String reportQueryName;
  private int currentPage;
  private int maxResults;
  private int pageSize;
  
  public int getPageSize() {
      return pageSize;
  }
  
  public int getMaxPages() {
      return maxResults / pageSize;
  }

  public void init(int pageSize, String countQueryName, 
                   String reportQueryName) {
      this.pageSize = pageSize;
      this.reportQueryName = reportQueryName;
      maxResults = ((Long) em.createNamedQuery(countQueryName)
                                  .getSingleResult()).intValue();
      currentPage = 0;
  }
  
  public List getCurrentResults() {
      return em.createNamedQuery(reportQueryName)
               .setFirstResult(currentPage * pageSize)
               .setMaxResults(pageSize)
               .getResultList();
  }
  
  public void next() {
      currentPage++;
  }
  
  public void previous() {
      currentPage--;
      if (currentPage < 0) {
          currentPage = 0;
      }
  }
  
  public int getCurrentPage() {
      return currentPage;
  }
  
  public void setCurrentPage(int currentPage) {
      this.currentPage = currentPage;
  }
  
  @PersistenceContext
  private EntityManager em;
}

以下代码来自Project.java。

package cn..common;


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

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

@Entity
public 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();
    }
}

以下代码来自Employee.java。

package cn..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;
@Entity
@NamedQueries({
  @NamedQuery(name="findEmployeesAboveSal",
              query="SELECT e " +
                    "FROM Employee e " +
                    "WHERE e.department = :dept AND " +
                    "      e.salary > :sal"),
  @NamedQuery(name="findHighestPaidByDepartment",
              query="SELECT e " +
                    "FROM Employee e " +
                    "WHERE e.department = :dept AND " +
                    "      e.salary = (SELECT MAX(e2.salary) " +
                    "                  FROM Employee e2 " +
                    "                  WHERE e2.department = :dept)")
})
public 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());
    }
}
下载 Query_Paging.zip

上面的代码生成以下结果。

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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