Hibernate提供了另外的操作对象的方法,并提供了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 会话 interface提供 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();
您可以使用 add()方法为 Criteria 对象添加条件查询的限制.以下是添加限制以返回记录的示例,其中薪水等于2000 :
Criteria cr = session.createCriteria(Employee.class); cr.add(Restrictions.eq("salary", 2000)); List results = cr.list();
下面是几个涵盖不同场景的例子,可以根据要求使用 :
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();
尽管上述所有条件都可以直接用于HQL,如上一个教程中所述.
分页的Criteria接口有两种方法.
Sr.No. | 方法&描述 |
---|---|
1 | public Criteria setFirstResult(int firstResult) 此方法采用一个整数,表示结果集中的第一行,从第0行开始. |
2 | public Criteria setMaxResults(int maxResults) 此方法告诉Hibernate检索固定数量 maxResults 的对象./p> |
将上述两种方法结合使用,我们可以在网络中构建一个分页组件或者Swing应用程序.下面是一个示例,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();
Criteria API提供 org.hibernate.criterion.Order class,根据对象的某个属性,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();
Criteria API提供 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,因为它提供了几种静态工厂方法来获取 Projection 实例.
以下是几个涵盖不同场景和可以根据要求使用 :
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"));
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 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,我们将使用main()方法创建我们的应用程序类来运行应用程序,我们将使用 Criteria 查询和减去;
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(); } } }
这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经适当地设置了PATH和CLASSPATH.
创建hibernate.cfg.xml配置文件,如配置章节中所述.
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.
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.
你会得到以下结果,并且会在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
如果你检查你的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>