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📜  Hibernate-条件查询

📅  最后修改于: 2020-11-16 07:02:03             🧑  作者: Mango

Hibernate提供了处理对象以及RDBMS表中可用数据的替代方法。方法之一是Criteria API，它使您可以以编程方式建立条件查询对象，在其中可以应用过滤规则和逻辑条件。

Hibernate会话接口提供了createCriteria()方法，该方法可用于创建Criteria对象，当您的应用程序执行条件查询时，该Criteria对象返回持久性对象的类的实例。

以下是一个最简单的标准查询示例，该查询将简单地返回与Employee类相对应的每个对象。

Criteria cr = session.createCriteria(Employee.class);
List results = cr.list();

限制条件

您可以使用Criteria对象可用的add()方法为条件查询添加限制。以下是添加限制以返回薪水等于2000的记录的示例-

Criteria cr = session.createCriteria(Employee.class);
cr.add(Restrictions.eq("salary", 2000));
List results = cr.list();

以下是一些涵盖不同场景的示例，可以根据需要使用-

Criteria cr = session.createCriteria(Employee.class);

// To get records having salary more than 2000
cr.add(Restrictions.gt("salary", 2000));

// To get records having salary less than 2000
cr.add(Restrictions.lt("salary", 2000));

// To get records having fistName starting with zara
cr.add(Restrictions.like("firstName", "zara%"));

// Case sensitive form of the above restriction.
cr.add(Restrictions.ilike("firstName", "zara%"));

// To get records having salary in between 1000 and 2000
cr.add(Restrictions.between("salary", 1000, 2000));

// To check if the given property is null
cr.add(Restrictions.isNull("salary"));

// To check if the given property is not null
cr.add(Restrictions.isNotNull("salary"));

// To check if the given property is empty
cr.add(Restrictions.isEmpty("salary"));

// To check if the given property is not empty
cr.add(Restrictions.isNotEmpty("salary"));

您可以使用LogicalExpression限制创建AND或OR条件，如下所示-

Criteria cr = session.createCriteria(Employee.class);

Criterion salary = Restrictions.gt("salary", 2000);
Criterion name = Restrictions.ilike("firstNname","zara%");

// To get records matching with OR conditions
LogicalExpression orExp = Restrictions.or(salary, name);
cr.add( orExp );

// To get records matching with AND conditions
LogicalExpression andExp = Restrictions.and(salary, name);
cr.add( andExp );

List results = cr.list();

尽管以上所有条件均可直接用于HQL，如先前教程中所述。

使用条件分页

用于分页的Criteria接口有两种方法。

Sr.No. Method & Description
1

public Criteria setFirstResult(int firstResult)

This method takes an integer that represents the first row in your result set, starting with row 0.
2

public Criteria setMaxResults(int maxResults)

This method tells Hibernate to retrieve a fixed number maxResults of objects.

结合使用以上两种方法，我们可以在Web或Swing应用程序中构造一个分页组件。以下是示例，您可以扩展为一次获取10行-

Criteria cr = session.createCriteria(Employee.class);
cr.setFirstResult(1);
cr.setMaxResults(10);
List results = cr.list();

结果排序

Criteria API提供org.hibernate.criterion.Order类，以根据对象的属性之一对结果集进行升序或降序排序。这个例子演示了如何使用Order类对结果集进行排序-

Criteria cr = session.createCriteria(Employee.class);

// To get records having salary more than 2000
cr.add(Restrictions.gt("salary", 2000));

// To sort records in descening order
cr.addOrder(Order.desc("salary"));

// To sort records in ascending order
cr.addOrder(Order.asc("salary"));

List results = cr.list();

投影与汇总

Criteria API提供了org.hibernate.criterion.Projections类，该类可用于获取属性值的平均值，最大值或最小值。 Projections类与Restrictions类相似，因为它提供了几种用于获取Projection实例的静态工厂方法。

以下是一些涵盖不同场景的示例，可以根据需要使用-

Criteria cr = session.createCriteria(Employee.class);

// To get total row count.
cr.setProjection(Projections.rowCount());

// To get average of a property.
cr.setProjection(Projections.avg("salary"));

// To get distinct count of a property.
cr.setProjection(Projections.countDistinct("firstName"));

// To get maximum of a property.
cr.setProjection(Projections.max("salary"));

// To get minimum of a property.
cr.setProjection(Projections.min("salary"));

// To get sum of a property.
cr.setProjection(Projections.sum("salary"));

条件查询示例

考虑以下POJO类-

public class Employee {
   private int id;
   private String firstName; 
   private String lastName;   
   private int salary;  

   public Employee() {}
   
   public Employee(String fname, String lname, int salary) {
      this.firstName = fname;
      this.lastName = lname;
      this.salary = salary;
   }
   
   public int getId() {
      return id;
   }
   
   public void setId( int id ) {
      this.id = id;
   }
   
   public String getFirstName() {
      return firstName;
   }
   
   public void setFirstName( String first_name ) {
      this.firstName = first_name;
   }
   
   public String getLastName() {
      return lastName;
   }
   
   public void setLastName( String last_name ) {
      this.lastName = last_name;
   }
   
   public int getSalary() {
      return salary;
   }
   
   public void setSalary( int salary ) {
      this.salary = salary;
   }
}

让我们创建以下EMPLOYEE表来存储Employee对象-

create table EMPLOYEE (
   id INT NOT NULL auto_increment,
   first_name VARCHAR(20) default NULL,
   last_name  VARCHAR(20) default NULL,
   salary     INT  default NULL,
   PRIMARY KEY (id)
);

以下是映射文件。


 


   
      
      
         This class contains the employee detail.

最后，我们将使用main()方法创建应用程序类以运行应用程序，并在其中使用Criteria查询-

import java.util.List; 
import java.util.Date;
import java.util.Iterator; 
 
import org.hibernate.HibernateException; 
import org.hibernate.Session; 
import org.hibernate.Transaction;
import org.hibernate.SessionFactory;
import org.hibernate.Criteria;
import org.hibernate.criterion.Restrictions;
import org.hibernate.criterion.Projections;
import org.hibernate.cfg.Configuration;

public class ManageEmployee {
   private static SessionFactory factory; 
   public static void main(String[] args) {
      
      try {
         factory = new Configuration().configure().buildSessionFactory();
      } catch (Throwable ex) { 
         System.err.println("Failed to create sessionFactory object." + ex);
         throw new ExceptionInInitializerError(ex); 
      }
      
      ManageEmployee ME = new ManageEmployee();

      /* Add few employee records in database */
      Integer empID1 = ME.addEmployee("Zara", "Ali", 2000);
      Integer empID2 = ME.addEmployee("Daisy", "Das", 5000);
      Integer empID3 = ME.addEmployee("John", "Paul", 5000);
      Integer empID4 = ME.addEmployee("Mohd", "Yasee", 3000);

      /* List down all the employees */
      ME.listEmployees();

      /* Print Total employee's count */
      ME.countEmployee();

      /* Print Total salary */
      ME.totalSalary();
   }
   
   /* Method to CREATE an employee in the database */
   public Integer addEmployee(String fname, String lname, int salary){
      Session session = factory.openSession();
      Transaction tx = null;
      Integer employeeID = null;
      
      try {
         tx = session.beginTransaction();
         Employee employee = new Employee(fname, lname, salary);
         employeeID = (Integer) session.save(employee); 
         tx.commit();
      } catch (HibernateException e) {
         if (tx!=null) tx.rollback();
         e.printStackTrace(); 
      } finally {
         session.close(); 
      }
      return employeeID;
   }

   /* Method to  READ all the employees having salary more than 2000 */
   public void listEmployees( ) {
      Session session = factory.openSession();
      Transaction tx = null;
      
      try {
         tx = session.beginTransaction();
         Criteria cr = session.createCriteria(Employee.class);
         // Add restriction.
         cr.add(Restrictions.gt("salary", 2000));
         List employees = cr.list();

         for (Iterator iterator = employees.iterator(); iterator.hasNext();){
            Employee employee = (Employee) iterator.next(); 
            System.out.print("First Name: " + employee.getFirstName()); 
            System.out.print("  Last Name: " + employee.getLastName()); 
            System.out.println("  Salary: " + employee.getSalary()); 
         }
         tx.commit();
      } catch (HibernateException e) {
         if (tx!=null) tx.rollback();
         e.printStackTrace(); 
      } finally {
         session.close(); 
      }
   }
   
   /* Method to print total number of records */
   public void countEmployee(){
      Session session = factory.openSession();
      Transaction tx = null;
      
      try {
         tx = session.beginTransaction();
         Criteria cr = session.createCriteria(Employee.class);

         // To get total row count.
         cr.setProjection(Projections.rowCount());
         List rowCount = cr.list();

         System.out.println("Total Coint: " + rowCount.get(0) );
         tx.commit();
      } catch (HibernateException e) {
         if (tx!=null) tx.rollback();
         e.printStackTrace(); 
      } finally {
         session.close(); 
      }
   }
  
   /* Method to print sum of salaries */
   public void totalSalary(){
      Session session = factory.openSession();
      Transaction tx = null;
      
      try {
         tx = session.beginTransaction();
         Criteria cr = session.createCriteria(Employee.class);

         // To get total salary.
         cr.setProjection(Projections.sum("salary"));
         List totalSalary = cr.list();

         System.out.println("Total Salary: " + totalSalary.get(0) );
         tx.commit();
      } catch (HibernateException e) {
         if (tx!=null) tx.rollback();
         e.printStackTrace(); 
      } finally {
         session.close(); 
      }
   }
}

编译与执行

这是编译和运行上述应用程序的步骤。在继续进行编译和执行之前，请确保已正确设置了PATH和CLASSPATH。

  • 按照配置章节中的说明创建hibernate.cfg.xml配置文件。

  • 如上所示，创建Employee.hbm.xml映射文件。

  • 如上所示创建Employee.java源文件并进行编译。

  • 如上所示创建ManageEmployee.java源文件并进行编译。

  • 执行ManageEmployee二进制文件以运行程序。

您将得到以下结果，并且将在EMPLOYEE表中创建记录。

$java ManageEmployee
.......VARIOUS LOG MESSAGES WILL DISPLAY HERE........

First Name: Daisy  Last Name: Das  Salary: 5000
First Name: John  Last Name: Paul  Salary: 5000
First Name: Mohd  Last Name: Yasee  Salary: 3000
Total Coint: 4
Total Salary: 15000

如果您检查EMPLOYEE表，它应该具有以下记录-

mysql> select * from EMPLOYEE;
+----+------------+-----------+--------+
| id | first_name | last_name | salary |
+----+------------+-----------+--------+
| 14 | Zara       | Ali       |   2000 |
| 15 | Daisy      | Das       |   5000 |
| 16 | John       | Paul      |   5000 |
| 17 | Mohd       | Yasee     |   3000 |
+----+------------+-----------+--------+
4 rows in set (0.00 sec)
mysql>