java persist java persistence clomn

【IT168 资讯】Linda DeMichiel 说道：Java Persistence 2.0 发布草稿计划本周发布，我认为现在是时候浏览新版本的新功能。如果你熟悉之前发布的JPA 2.0第一个规格草稿，你应该了解它的重点是O/R mapping 改进。公开的草稿基于这些改进，改进的方式主要通过扩展Java 持久查询语言(Java Persistence Query Language:JPQL)和通过新的API标准。
因为草稿包含了许多部分，所以分为两个部分，一个部分是新的JPQL功能，另一个部分是API标准。
之前的草稿支持元素收集element collections，嵌套类nested embeddable classes，内嵌支持实体关系，生成详图maps，以及JPQL语法必需的次序化修改，让通过映射类型查询更加容易编写。更多的是，为JPQL添加了一些其他的非常需要的功能。
Navigation
扩展了点“.”导航语法，来处理内嵌关系。比如我们有一个Employee entity，内嵌 ContactInfo 类用来存储联系信息，拥有通过ContactInfo 内嵌提供的电话号码：
@Entity public class Employee {
@Id int empId;
String name;
@ManyToOne Department dept;
ContactInfo contactInfo;
...
}
@Embeddable public class ContactInfo {
Address address;
@OneToMany Set phones;
...
}
@Embeddable public class Address {
String street;
String city;
String zipcode;
...
}
@Entity public class Phone {
@Id int phoneId;
String areaCode;
String localNumber;
String internalExtension;
PhoneType phoneType;
@ManyToOne Account billedTo;
...
}
@Entity public class Employee {
@Id int empId;
String name;
@ManyToOne Department dept;
ContactInfo contactInfo;
...
}
@Embeddable public class ContactInfo {
Address address;
@OneToMany Set phones;
...
}
@Embeddable public class Address {
String street;
String city;
String zipcode;
...
}
@Entity public class Phone {
@Id int phoneId;
String areaCode;
String localNumber;
String internalExtension;
PhoneType phoneType;
@ManyToOne Account billedTo;
...
}
如果你需要获得employees 的电话号码，可以这样使用：
SELECT DISTINCT p.billedTo
FROM Employee e JOIN e.contactInfo.phones p
WHERE e.contactInfo.address.zipcode = '95054' AND p.phonetype = PhoneType.OFFICE
SELECT DISTINCT p.billedTo
FROM Employee e JOIN e.contactInfo.phones p
WHERE e.contactInfo.address.zipcode = '95054' AND p.phonetype = PhoneType.OFFICE
Maps
支持生成map功能，我们可以添加KEY和VALUE操作符，来提取map key 和mao value。缺省的，相同的value指向一个联合Map类型表示map value，所以，严格说起来，VALUE 操作符 主要用于文档目标。
比如，我们有一个map 图片名称和文件名称对应：
@Entity public class PictureCategory {
@Id String name;
@ElementCollection Map photos;
...
}
@Entity public class PictureCategory {
@Id String name;
@ElementCollection Map photos;
...
}
为了搜索我的某个egret图片，我这样写：
SELECT VALUE(p)
FROM PictureCategory c JOIN c.photos p
WHERE c.name = 'birds' AND KEY(p) LIKE '%egret%'
SELECT VALUE(p)
FROM PictureCategory c JOIN c.photos p
WHERE c.name = 'birds' AND KEY(p) LIKE '%egret%'
这样写也可以：
SELECT p
FROM PictureCategory c JOIN c.photos p
WHERE c.name = 'birds' AND KEY(p) LIKE '%egret%'
SELECT p
FROM PictureCategory c JOIN c.photos p
WHERE c.name = 'birds' AND KEY(p) LIKE '%egret%'
下面是另一个例子：
Sql代码
@Entity public class VideoStore {
@Id int id;
String name;
Address location;
@ElementCollection Map videoInventory;
...
}
@Entity public class Movie {
@Id int id;
String title;
String director;
@ManyToMany Set stars;
...
}
@Entity public class VideoStore {
@Id int id;
String name;
Address location;
@ElementCollection Map videoInventory;
...
}
@Entity public class Movie {
@Id int id;
String title;
String director;
@ManyToMany Set stars;
...
}
在新的规格语言中，KEY(p) and VALUE(p)是 通用确认变量 ：这意味着它们能用于更多的导航，因此，要搜索在Sun's Santa Clara campus附近的 Vertigo 电影院，可以这样写：
SELECT v.location.street
FROM VideoStore v JOIN v.videoInventory i
WHERE v.location.zipcode = '95054' AND KEY(i).title = 'Vertigo' AND VALUE(i) > 0
SELECT v.location.street
FROM VideoStore v JOIN v.videoInventory i
WHERE v.location.zipcode = '95054' AND KEY(i).title = 'Vertigo' AND VALUE(i) > 0
Map entries 不支持浏览，但是能够被选择，返回一个 java.util.Map.Entry 类型 作为结果。
SELECT v.location.street, ENTRY(i)
FROM VideoStore v JOIN v.videoInventory i
WHERE v.location.zipcode = '95054' AND KEY(i).director = 'Hitchcock' AND VALUE(i) > 0
SELECT v.location.street, ENTRY(i)
FROM VideoStore v JOIN v.videoInventory i
WHERE v.location.zipcode = '95054' AND KEY(i).director = 'Hitchcock' AND VALUE(i) > 0
Ordered Lists
Ordered lists 是另外一个经常需要的功能。JPA 1.0支持list作为集合类型，但是不包括支持维护一个持久排序。在JPA 2.0之前草稿，我们添加了记载OrderColumn annotation功能，这意味着当你处理列表排序的时候，持久化提供者需要使用分离的列来维护列表排序。JPQL INDEX操作符允许你在排序中查询。
比如，一个部门管理者需要维护职员的等级，进行加薪操作：
@Entity public class Employee {
@Id int empId;
String name;
@ManyToOne Department dept;
...
}
@Entity
public class Department {
@Id int deptId;
String name;
@OneToMany(mappedBy="dept") @OrderColumn List members;
...
}
@Entity public class Employee {
@Id int empId;
String name;
@ManyToOne Department dept;
...
}
@Entity
public class Department {
@Id int deptId;
String name;
@OneToMany(mappedBy="dept") @OrderColumn List members;
...
}
下面查询返回市场部门的最高级别职员：
SELECT e
FROM Employee e JOIN e.dept d
WHERE d.name='Marketing' AND INDEX(e) < 5
SELECT e
FROM Employee e JOIN e.dept d
WHERE d.name='Marketing' AND INDEX(e) < 5
Non-polymorphic Queries
另外一个添加到JPQL的是支持 Non-polymorphic Queries非多态查询。
TYPE操作符允许你选择entity的类型和限制查询为一个或多个entity类型。
如果我们有一个职员Employee 继承FullTimeEmployee，PartTimeEmployee, and Contractor 字类，我们能这样写查询：
SELECT e
FROM Employee e JOIN e.dept d
WHERE d.name = 'Marketing' AND TYPE(e) IN (PartTimeEmployee, Contractor)
SELECT e
FROM Employee e JOIN e.dept d
WHERE d.name = 'Marketing' AND TYPE(e) IN (PartTimeEmployee, Contractor)
PartTimeEmployee 和 Contractor 是 entity 名字. (重调用entity名字，缺省的，相当于entity类的唯一名字。)
IN操作符扩展接收集合数值参数，可以这样写：
SELECT e
FROM Employee e
WHERE TYPE(e) IN :empTypes
SELECT e
FROM Employee e
WHERE TYPE(e) IN :empTypes
参数传递给查询作为class对象集合。
entity type 也能返回作为查询结果：
SELECT TYPE(e), e.name
FROM Employee e JOIN e.dept d
WHERE d.name='Marketing'
SELECT TYPE(e), e.name
FROM Employee e JOIN e.dept d
WHERE d.name='Marketing'
当查询运行，employee type 作为class对象返回。
Case Expressions
另外查询语言添加了对case expressions支持，现在也支持NULLIF 和 COALESCE 操作符。
下面是例子：
SELECT c, CASE WHEN c.annualSpending > 10000 THEN 'Premier'
WHEN c.annualSpending > 5000 THEN 'Gold'
WHEN c.annualSpending > 2000 THEN 'Silver'
ELSE 'Bronze'
END
FROM Customer c
SELECT c, CASE WHEN c.annualSpending > 10000 THEN 'Premier'
WHEN c.annualSpending > 5000 THEN 'Gold'
WHEN c.annualSpending > 2000 THEN 'Silver'
ELSE 'Bronze'
END
FROM Customer c
update操作：
UPDATE EMPLOYEE e
SET e.salary = CASE e.rating WHEN 1 THEN e.salary * 1.1
WHEN 2 THEN e.salary * 1.05
ELSE e.salary * 1.001
END
UPDATE EMPLOYEE e
SET e.salary = CASE e.rating WHEN 1 THEN e.salary * 1.1
WHEN 2 THEN e.salary * 1.05
ELSE e.salary * 1.001
END
NULLIF 操作符非常有用：
SELECT AVG(NULLIF(e.salary, -99999))
FROM Employee e
SELECT AVG(NULLIF(e.salary, -99999))
FROM Employee e
Scalar Expressions in the SELECT clause
JPA 1.0支持在WHERE语句中的分级表达式，它不在SELECT语言中支持，在之前的查询演示中，这个经常性需求功能已经被加入了。