Criteria Queries（Criteria Queries）

Hibernate提供了另外的操作对象的方法，反过来又提供了RDBMS表中可用的数据。 其中一种方法是Criteria API，它允许您以编程方式构建条件查询对象，您可以在其中应用过滤规则和逻辑条件。

Hibernate Session接口提供了createCriteria()方法，该方法可用于创建Criteria对象，该对象在应用程序执行条件查询时返回持久对象类的实例。

以下是条件查询的最简单示例之一，它将简单地返回与Employee类对应的每个对象。

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

标准限制

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();

以下是几个涵盖不同场景的示例，可根据要求使用 -

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();

虽然上述所有条件都可以直接与HQL一起使用，如前面的教程中所述。

使用标准的分页

分页的Criteria接口有两种方法。

将上述两种方法结合使用，我们可以在Web或Swing应用程序中构建一个分页组件。 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();

对结果进行排序

Criteria API提供org.hibernate.criterion.Order类，根据您的某个对象属性，按升序或降序对结果集进行排序。 此示例演示了如何使用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();

预测和聚合

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

以下是涵盖不同场景的少数示例，可根据要求使用 -

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。

<?xml version = "1.0" encoding = "utf-8"?>
<!DOCTYPE hibernate-mapping PUBLIC 
"-//Hibernate/Hibernate Mapping DTD//EN"
"http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd"> 
<hibernate-mapping>
   <class name = "Employee" table = "EMPLOYEE">
      <meta attribute = "class-description">
         This class contains the employee detail. 
      </meta>
      <id name = "id" type = "int" column = "id">
         <generator/>
      </id>
      <property name = "firstName" column = "first_name" type = "string"/>
      <property name = "lastName" column = "last_name" type = "string"/>
      <property name = "salary" column = "salary" type = "int"/>
   </class>
</hibernate-mapping>

最后，我们将使用main（）方法创建应用程序类，以运行我们将使用Criteria查询的应用程序 -

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(); 
      }
   }
}

编译和执行 (Compilation and Execution)

以下是编译和运行上述应用程序的步骤。 确保在进行编译和执行之前已适当地设置了PATH和CLASSPATH。

• 按配置章节中的说明创建hibernate.cfg.xml配置文件。

• 创建Employee.hbm.xml映射文件，如上所示。

• 如上所示创建Employee.java源文件并进行编译。

• 创建ManageEmployee.java源文件，如上所示并编译它。

• 执行ManageEmployee二进制文件以运行该程序。

您将获得以下结果，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

如果您检查您的EMPLOYEE表，它应该有以下记录 -

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>