Hibernate 简明教程

Hibernate - Criteria Queries

Hibernate 提供了处理对象以及反过来提供 RDBMS 表中数据的替代方法。其中一种方法是 Criteria API，它允许您以编程方式构建一个 criteria 查询对象，您可以在其中应用过滤规则和逻辑条件。

Hibernate provides alternate ways of manipulating objects and in turn data available in RDBMS tables. One of the methods is Criteria API, which allows you to build up a criteria query object programmatically where you can apply filtration rules and logical conditions.

Hibernate Session 接口提供 createCriteria() 方法，它可用于创建 Criteria 对象，当您的应用程序执行 criteria 查询时，该对象会返回持久对象类的实例。

The Hibernate Session interface provides createCriteria() method, which can be used to create a Criteria object that returns instances of the persistence object’s class when your application executes a criteria query.

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

Following is the simplest example of a criteria query is one, which will simply return every object that corresponds to the Employee class.

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

Restrictions with Criteria

您可以使用 Criteria 对象提供的 add() 方法为 criteria 查询添加限制。以下是添加一个限制的示例，以返回工资等于 2000 的记录 −

You can use add() method available for Criteria object to add restriction for a criteria query. Following is the example to add a restriction to return the records with salary is equal to 2000 −

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

以下是涵盖不同场景的更多示例，可根据要求使用 −

Following are the few more examples covering different scenarios and can be used as per the requirement −

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 −

You can create AND or OR conditions using LogicalExpression restrictions as follows −

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();

尽管上述所有条件都可以与 HQL 直接一起使用，如前一个教程中所述。

Though all the above conditions can be used directly with HQL as explained in previous tutorial.

Pagination Using Criteria

查询接口有两个方法用于分页。

There are two methods of the Criteria interface for pagination.

Sr.No.

Method & Description

public Criteria setFirstResult(int firstResult) This method takes an integer that represents the first row in your result set, starting with row 0.

public Criteria setMaxResults(int maxResults) This method tells Hibernate to retrieve a fixed number maxResults of objects.

将以上两个方法结合使用，我们可以在 Web 或 Swing 应用程序中构建一个分页组件。以下就是其示例，您可以对其进行扩展以一次获取 10 行 −

Using above two methods together, we can construct a paging component in our web or Swing application. Following is the example, which you can extend to fetch 10 rows at a time −

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

Sorting the Results

Criteria API 会提供 org.hibernate.criterion.Order 类按照对象的某个属性对结果集进行升序或降序排列。示例展示了如何使用 Order 类对结果集进行排序——

The Criteria API provides the org.hibernate.criterion.Order class to sort your result set in either ascending or descending order, according to one of your object’s properties. This example demonstrates how you would use the Order class to sort the result set −

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();

Projections & Aggregations

Criteria API 会提供 org.hibernate.criterion.Projections 类，该类可以用于获取某个属性值的平均值、最大值或最小值。Projections 类类似于 Restrictions 类，因为它会提供几个用于获取 Projection 实例的静态工厂方法。

The Criteria API provides the org.hibernate.criterion.Projections class, which can be used to get average, maximum, or minimum of the property values. The Projections class is similar to the Restrictions class, in that it provides several static factory methods for obtaining Projection instances.

接下来是几个涉及不同场景的示例，并且能根据需求来使用——

Following are the few examples covering different scenarios and can be used as per requirement −

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"));

Criteria Queries Example

考虑以下 POJO 类——

Consider the following POJO class −

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;
   }
}

我们创建下面的 EMPLOYEE 表以存储 Employee 对象——

Let us create the following EMPLOYEE table to store Employee objects −

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)
);

下面是映射文件。

Following will be the mapping file.

<?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 class="native"/>
      </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，我们通过 main() 方法创建我们的应用程序类以运行应用程序，在其中我们将使用 Criteria 查询——

Finally, we will create our application class with the main() method to run the application where we will use Criteria queries −

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();
      }
   }
}

Compilation and Execution

以下是编译和运行上述应用程序的步骤。请确保在继续编译和执行之前，已适当地设置 PATH 和 CLASSPATH。

Here are the steps to compile and run the above mentioned application. Make sure, you have set PATH and CLASSPATH appropriately before proceeding for the compilation and execution.

Create hibernate.cfg.xml configuration file as explained in configuration chapter.
Create Employee.hbm.xml mapping file as shown above.
Create Employee.java source file as shown above and compile it.
Create ManageEmployee.java source file as shown above and compile it.
Execute ManageEmployee binary to run the program.

您将获得以下结果，并且将在 EMPLOYEE 表中创建记录。

You would get the following result, and records would be created in the EMPLOYEE table.

$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

如果你检查你的 EMPLOYEE 表，它应当有以下记录——

If you check your EMPLOYEE table, it should have the following records−

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>