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 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。
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。
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 −
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 −
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与 HQL 直接一起使用，如前一个教程中所述。
Pagination Using Criteria
查询接口有两个方法用于分页。
Sr.No. |
Method & Description |
1 |
public Criteria setFirstResult(int firstResult) 此方法采用一个整数来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 检索一个 maxResults 固定数量的对象。 |
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 −
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——
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。
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——
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——
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——
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。
<?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——
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。
-
如配置章节中所述，创建 hibernate.cfg.xml 配置文件。
-
如上所示创建 Employee.hbm.xml 映射文件。
-
创建 Employee.java 源文件（如上所示），并进行编译。
-
按上方代码创建一个 ManageEmployee.java 源文件并进行编译。
-
执行 ManageEmployee 二进制文件来运行该程序。
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如果你检查你的 EMPLOYEE 表，它应当有以下记录——
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>