多平台统一管理软件接口,如何实现多平台统一管理软件接口
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2023-07-24
在Java的Hibernate框架中对数据库数据进行查询操作
Hibernate查询语言(HQL)是一种面向对象的查询语言,类似于SQL,但不是对表和列操作,HQL适用于持久对象和它们的属性。 HQL查询由Hibernate转换成传统的SQL查询,这在圈上的数据库执行操作。
虽然可以直接使用SQL语句和Hibernate使用原生SQL,但建议使用HQL尽可能避免数据库可移植性的麻烦,并采取Hibernate的SQL生成和缓存策略的优势。
都像SELECT,FROM和WHERE等关键字不区分大小写,但如表名和列名的属性是区分在HQL敏感。
FROM 语句
使用FROM子句,如果要加载一个完整的持久化对象到内存中。下面是一个使用FROM子句的简单的语法:
String hql = "FROM Employee";
Query query = session.createQuery(hql);
List results = query.list();
如果需要完全限定在HQL一个类名,只需指定如下的包和类名:
String hql = "FROM com.hibernatebook.criteria.Employee";
Query query = session.createQuery(hql);
List results = query.list();
AS 语句
AS子句可以用来别名分配给类中的HQL查询,特别是当有很长的查询。例如,我们前面简单的例子是以下几点:
String hql = "FROM Employee AS E";
Query query = session.createQuery(hql);
List results = query.list();
AS关键字是可选的,也可以直接在之后的类名指定别名,如下所示:
String hql = "FROM Employee E";
Query query = session.createQuery(hql);
List results = query.list();
SELECT 子句
SELECT子句提供了更多的控制权比from子句的结果集。如果想获得对象而不是整个对象的几个属性,使用SELECT子句。下面是一个使用SELECT语句来获取Employee对象只是FIRST_NAME字段的简单的语法:
String hql = "SELECT E.firstName FROM Employee E";
Query query = session.createQuery(hql);
List results = query.list();
值得注意的是在这里,Employee.firstName是Employee对象的一个属性,而不是EMPLOYEE表的一个字段。
WHERE 子句
如果想缩小了从存储返回的特定对象,可以使用WHERE子句。下面是一个使用WHERE子句的简单的语法:
String hql = "FROM Employee E WHERE E.id = 10";
Query query = session.createQuery(hql);
List results = query.list();
ORDER BY 子句
若要排序HQL查询的结果,将需要使用ORDER BY子句。您可以在结果集按升序(ASC)或降序(DESC)通过在对象的任何属性排序结果。下面是一个使用ORDER BY子句的简单的语法:
String hql = "FROM Employee E WHERE E.id > 10 ORDER BY E.salary DESC";
Query query = session.createQuery(hql);
List results = query.list();
如果想通过一个以上的属性进行排序,你会仅仅是额外的属性添加到由子句用逗号隔开,如下所示的命令的结尾:
String hql = "FROM Employee E WHERE E.id > 10 " +
"ORDER BY E.firstName DESC, E.salary DESC ";
Query query = session.createQuery(hql);
List results = query.list();
GROUP BY 子句
该子句允许从Hibernate的它基于属性的值的数据库和组提取信息,并且通常使用结果包括总值。下面是一个使用GROUP BY子句的语法很简单:
String hql = "SELECT SUM(E.salary), E.firtName FROM Employee E " +
"GROUP BY E.firstName";
Query query = session.createQuery(hql);
List results = query.list();
使用命名参数
Hibernate命名在其HQL查询参数支持。这使得编写接受来自用户的输入容易,不必对SQL注入攻击防御HQL查询。下面是一个使用命名参数的简单的语法:
String hql = "FROM Employee E WHERE E.id = :employee_id";
Query query = session.createQuery(hql);
query.setParameter("employee_id",10);
List results = query.list();
UPDATE 子句
批量更新是新的HQL与Hibernate3,以及不同的删除工作,在Hibernate 3和Hibernate2一样。 Query接口现在包含一个名为executeUpdate()方法用于执行HQL UPDATE或DELETE语句。
在UPDATE子句可以用于更新一个或多个对象中的一个或多个属性。下面是一个使用UPDATE子句的简单的语法:
String hql = "UPDATE Employee set salary = :salary " +
"WHERE id = :employee_id";
Query query = session.createQuery(hql);
query.setParameter("salary", 1000);
query.setParameter("employee_id", 10);
int result = query.executeUpdate();
System.out.println("Rows affected: " + result);
DELETE 子句
DELETE子句可以用来删除一个或多个对象。下面是一个使用DELETE子句的简单的语法:
String hql = "DELETE FROM Employee " +
"WHERE id = :employee_id";
Query query = session.createQuery(hql);
query.setParameter("employee_id", 10);
int result = query.executeUpdate();
System.out.println("Rows affected: " + result);
INSERT 子句
HQL支持INSERT INTO子句中只记录在那里可以插入从一个对象到另一个对象。以下是使用INSERT INTO子句的简单的语法:
String hql = "INSERT INTO Employee(firstName, lastName, salary)" +
"SELECT firstName, lastName, salary FROM old_employee";
Query query = session.createQuery(hql);
int result = query.executeUpdate();
System.out.println("Rows affected: " + result);
聚合方法
HQL支持多种聚合方法,类似于SQL。他们工作在HQL同样的方式在SQL和下面的可用功能列表:
DISTINCT关键字只计算在该行设定的唯一值。下面的查询将只返回唯一的计数:
String hql = "SELECT count(distinct E.firstName) FROM Employee E";
Query query = session.createQuery(hql);
List results = query.list();
使用查询分页
有用于分页查询接口的两个方法。
Query setFirstResult(int startPosition)
Query setMaxResults(int maxResult)
采用上述两种方法一起,可以在网站或Swing应用程序构建一个分页组件。下面是例子,可以扩展来获取10行:
String hql = "FROM Employee";
Query query = session.createQuery(hql);
query.setFirstResult(1);
query.setMaxResults(10);
List results = query.list();
查询条件
Hibernate提供了操作对象,并依次数据在RDBMS表可用的备用方式。其中一个方法是标准的API,它允许你建立一个标准的查询对象编程,可以套用过滤规则和逻辑条件。
Hibernate的Session接口提供了可用于创建一个返回的持久化对象的类的实例时,应用程序执行一个条件查询一个Criteria对象createCriteria()方法。
以下是最简单的一个条件查询的例子是将简单地返回对应于Employee类的每个对象。
Criteria cr = session.createCriteria(Employee.class);
List results = cr.list();
限制与标准:
可以使用add()方法可用于Criteria对象添加限制条件查询。下面是例子增加一个限制与薪水返回的记录是等于2000:
Criteria cr = session.createCriteria(Employee.class);
cr.add(Restrictions.eq("salary", 2000));
List results = cr.list();
以下是几个例子覆盖不同的场景,并且可以根据要求使用:
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"));
可以创建AND或OR使用LogicalExpression限制如下条件:
Criteria cr = session.createCriteria(Employee.class);
Criterion salary = Restrictions.gt("salary", 2000);
Criterion name = Restrictions.ilike("firstNname","zara%");
// To get records matching with OR condistions
LogicalExpression orExp = Restrictions.or(salary, name);
cr.add( orExp );
// To get records matching with AND condistions
LogicalExpression andExp = Restrictions.and(salary, name);
cr.add( andExp );
List results = cr.list();
虽然上述所有条件,可以直接使用HQL在前面的教程中介绍。
分页使用标准:
还有的标准接口,用于分页的两种方法。
public Criteria setFirstResult(int firstResult)
public Criteria setMaxResults(int maxResults)
采用上述两种方法一起,我们可以在我们的网站或Swing应用程序构建一个分页组件。下面是例子,可以扩展来每次获取10行:
Criteria cr = session.createCriteria(Employee.class);
cr.setFirstResult(1);
cr.setMaxResults(10);
List results = cr.list();
排序的结果:
标准的API提供了org.hibernate.criterion.Order类排序按升序或降序排列你的结果集,根据对象的属性。这个例子演示了如何使用Order类的结果集进行排序:
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
crit.addOrder(Order.desc("salary"));
// To sort records in ascending order
crit.addOrder(Order.asc("salary"));
List results = cr.list();
预测与聚合:
该Criteria API提供了一个org.hibernate.criterion.Projections类可用于获取平均值,最大值或最小值的属性值。Projections类是类似于类限制,因为它提供了几个静态工厂方法用于获得Projection 实例。 provides the
以下是涉及不同的方案的一些例子,可按规定使用:
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 例子:
考虑下面的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;
}
}
让我们创建下面的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)
);
以下将被映射文件。
"-//Hibernate/Hibernate Mapping DTD//EN"
"http://hibernate.org/dtd/hibernate-mapping-3.0.dtd">
This class contains the employee detail.
最后,我们将创建应用程序类的main()方法来运行,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 Toatl 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:
下面是步骤来编译并运行上述应用程序。请确保您已在进行的编译和执行之前,适当地设置PATH和CLASSPATH。
创建hibernate.cfg.xml配置文件中配置章节解释。
创建Employee.hbm.xml映射文件,如上图所示。
创建Employee.java源文件,如上图所示,并编译它。
创建ManageEmployee.java源文件,如上图所示,并编译它。
执行ManageEmployee二进制运行程序.
会得到以下结果,并记录将创建在EMPLOYEE表中。
$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
如果检查EMPLOYEE表,它应该记录如下:
mysql> select * from EMPLOYEE;
+----+------------+-----------+--------+
| id | first_name | last_name | salary |
+---mbptegUT-+------------+-----------+--------+
| 14 | Zara | Ali | 2000 |
| 15 | Daisy | Das | 5000 |
| 16 | John | Paul | 5000 |
| 17 | Mohd | Yasee | 3000 |
+----+------------+-----------+--------+
4 rows in set (0.00 sec)
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