本章介绍了JPQL以及它如何与持久性单元一起使用.在本章中，示例遵循相同的包层次结构，我们在前一章中使用如下:
JPQL是JPA规范中定义的Java持久性查询语言.它用于创建针对要存储在关系数据库中的实体的查询. JPQL是基于SQL语法开发的.但它不会直接影响数据库.
JPQL可以使用SELECT子句检索信息或数据，可以使用UPDATE子句和DELETE子句进行批量更新. EntityManager.createQuery()API将支持查询语言.
JPQL语法与SQL的语法非常相似.使用SQL语法是一个优点，因为SQL是一种简单的结构化查询语言，许多开发人员在应用程序中使用它. SQL直接用于关系数据库表，记录和字段，而JPQL用于Java类和实例.
例如，JPQL查询可以从数据库检索实体对象而不是字段结果集和SQL一样. JPQL查询结构如下.
SELECT ... FROM ... [WHERE ...] [GROUP BY ... [HAVING ...]] [ORDER BY ...]
JPQL DELETE和UPDATE查询的结构更简单如下.
DELETE FROM ... [WHERE ...] UPDATE ... SET ... [WHERE ...]
标量函数根据输入返回结果值值.聚合函数通过计算输入值返回结果值.
按照前面章节中使用的相同示例员工管理.这里我们将使用JPQL的标量和聚合函数来查看服务类.
让我们假设jpadb.employee表包含以下记录.
| Eid | Ename | 薪水 | Deg |
|---|---|---|---|
| 1201 | Gopal | 40000 | 技术经理 |
| 1202 | Manisha | 40000 | 校对读者 |
| 1203 | Masthanvali | 40000 | 技术作家 |
| 1204 | Satish | 30000 | 技术作家 |
| 1205 | Krishna | 30000 | Technical Writer |
| 1206 | Kiran | 35000 | 证明读者 |
在 com.it1352.eclipselink.service 包下创建一个名为 ScalarandAggregateFunctions.java 的类，如下所示.
package com.it1352.eclipselink.service;
import java.util.List;
import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;
import javax.persistence.Query;
public class ScalarandAggregateFunctions {
public static void main( String[ ] args ) {
EntityManagerFactory emfactory = Persistence.createEntityManagerFactory( "Eclipselink_JPA" );
EntityManager entitymanager = emfactory.createEntityManager();
//Scalar function
Query query = entitymanager.
createQuery("Select UPPER(e.ename) from Employee e");
List<String> list = query.getResultList();
for(String e:list) {
System.out.println("Employee NAME :"+e);
}
//Aggregate function
Query query1 = entitymanager.createQuery("Select MAX(e.salary) from Employee e");
Double result = (Double) query1.getSingleResult();
System.out.println("Max Employee Salary :" + result);
}
}编译和执行上述程序后，您将在Eclipse IDE的控制台面板中输出如下内容:
Employee NAME :GOPAL Employee NAME :MANISHA Employee NAME :MASTHANVALI Employee NAME :SATISH Employee NAME :KRISHNA Employee NAME :KIRAN ax Employee Salary :40000.0
'Between'，'And'和'Like'是JPQL的主要关键字.这些关键字在查询中的Where子句之后使用.
在 com.it1352.eclipselink.service 下创建名为 BetweenAndLikeFunctions.java 的类b>包装如下:
package com.it1352.eclipselink.service;
import java.util.List;
import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;
import javax.persistence.Query;
import com.IT屋.eclipselink.entity.Employee;
public class BetweenAndLikeFunctions {
public static void main( String[ ] args ) {
EntityManagerFactory emfactory = Persistence.createEntityManagerFactory( "Eclipselink_JPA" );
EntityManager entitymanager = emfactory.createEntityManager();
//Between
Query query = entitymanager.createQuery( "Select e " + "from Employee e " + "where e.salary " + "Between 30000 and 40000" );
List<Employee> list=(List<Employee>)query.getResultList( );
for( Employee e:list ){
System.out.print("Employee ID :" + e.getEid( ));
System.out.println("\t Employee salary :" + e.getSalary( ));
}
//Like
Query query1 = entitymanager.createQuery("Select e " + "from Employee e " + "where e.ename LIKE 'M%'");
List<Employee> list1=(List<Employee>)query1.getResultList( );
for( Employee e:list1 ) {
System.out.print("Employee ID :"+e.getEid( ));
System.out.println("\t Employee name :"+e.getEname( ));
}
}
}编译并执行上述程序后，您将在控制台面板中输出Eclipse IDE如下:
Employee ID :1201 Employee salary :40000.0 Employee ID :1202 Employee salary :40000.0 Employee ID :1203 Employee salary :40000.0 Employee ID :1204 Employee salary :30000.0 Employee ID :1205 Employee salary :30000.0 Employee ID :1206 Employee salary :35000.0 Employee ID :1202 Employee name :Manisha Employee ID :1203 Employee name :Masthanvali
要在JPQL中订购记录，我们使用ORDER BY子句.此子句的用法与SQL中的用法相同，但它处理实体.按照示例顺序.
在 com.it1352.eclipselink.service 包下创建一个类Ordering.java，如下所示:
package com.it1352.eclipselink.service;
import java.util.List;
import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;
import javax.persistence.Query;
import com.IT屋.eclipselink.entity.Employee;
public class Ordering {
public static void main( String[ ] args ) {
EntityManagerFactory emfactory = Persistence.createEntityManagerFactory( "Eclipselink_JPA" );
EntityManager entitymanager = emfactory.createEntityManager();
//Between
Query query = entitymanager.createQuery( "Select e " + "from Employee e " + "ORDER BY e.ename ASC" );
List<Employee> list = (List<Employee>)query.getResultList( );
for( Employee e:list ) {
System.out.print("Employee ID :" + e.getEid( ));
System.out.println("\t Employee Name :" + e.getEname( ));
}
}
}编译并执行上述程序后，您将在控制台面板中输出Eclipse IDE如下:
Employee ID :1201 Employee Name :Gopal Employee ID :1206 Employee Name :Kiran Employee ID :1205 Employee Name :Krishna Employee ID :1202 Employee Name :Manisha Employee ID :1203 Employee Name :Masthanvali Employee ID :1204 Employee Name :Satish
@NamedQuery注释被定义为具有预定义的不可更改查询字符串的查询.使用命名查询可以通过将JPQL查询字符串与POJO分离来改进代码组织，而不是动态查询.它还传递查询参数，而不是将文字动态嵌入到查询字符串中，从而提高查询效率.
首先，将@NamedQuery注释添加到名为的Employee实体类中在 com.it1352.eclipselink.entity 包下的Employee.java 如下:
package com .it1352.eclipselink.entity;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.NamedQuery;
import javax.persistence.Table;
@Entity
@Table
@NamedQuery(query = "Select e from Employee e where e.eid = :id", name = "find employee by id")
public class Employee {
@Id
@GeneratedValue(strategy = GenerationType.AUTO)
private int eid;
private String ename;
private double salary;
private String deg;
public Employee(int eid, String ename, double salary, String deg) {
super( );
this.eid = eid;
this.ename = ename;
this.salary = salary;
this.deg = deg;
}
public Employee( ) {
super();
}
public int getEid( ) {
return eid;
}
public void setEid(int eid) {
this.eid = eid;
}
public String getEname( ) {
return ename;
}
public void setEname(String ename) {
this.ename = ename;
}
public double getSalary( ) {
return salary;
}
public void setSalary(double salary) {
this.salary = salary;
}
public String getDeg( ) {
return deg;
}
public void setDeg(String deg) {
this.deg = deg;
}
@Override
public String toString() {
return "Employee [eid=" + eid + ", ename=" + ename + ", salary=" + salary + ", deg=" + deg + "]";
}
}在 com.it1352下创建一个名为 NamedQueries.java 的类.eclipselink.service 包如下:
package com.it1352.eclipselink.service;
import java.util.List;
import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;
import javax.persistence.Query;
import com.it1352.eclipselink.entity.Employee;
public class NamedQueries {
public static void main( String[ ] args ) {
EntityManagerFactory emfactory = Persistence.createEntityManagerFactory( "Eclipselink_JPA" );
EntityManager entitymanager = emfactory.createEntityManager();
Query query = entitymanager.createNamedQuery("find employee by id");
query.setParameter("id", 1204);
List<Employee> list = query.getResultList( );
for( Employee e:list ){
System.out.print("Employee ID :" + e.getEid( ));
System.out.println("\t Employee Name :" + e.getEname( ));
}
}
}编译并执行上述程序后，您将在控制台面板中输出Eclipse IDE如下:
员工ID:1204员工姓名:Satish
添加上述所有类后，包层次结构如下所示:
JPA的主要概念是在缓存内存中创建数据库的副本.在与数据库进行交易时，首先它会对重复数据产生影响，并且只有在使用实体管理器提交时，才会对数据库进行更改.
有两种方法可以从中获取记录数据库 - 急切获取和懒惰提取.
使用主键查找记录时获取整个记录.
它检查是否有可用主键通知它(如果存在).然后，如果你调用该实体的任何getter方法，那么它将获取整个.
但是当你第一次尝试获取记录时，可以进行延迟提取.这样，整个记录的副本已经存储在高速缓冲存储器中.性能方面，懒惰的抓取是可取的.
