Introduction to the Java Persistence API
JPA是Java Persistence API的简称，中文名Java持久层API，是JDK 5.0注解或XML描述对象－关系表的映射关系，并将运行期的实体对象持久化到数据库中。 [1]
Sun引入新的JPA ORM规范出于两个原因：其一，简化现有Java EE和Java SE应用开发工作；其二，Sun希望整合ORM技术，实现天下归一。
Entities
An entity is a lightweight persistence domain object. Typically an entity represents a table in a relational database, and each entity instance corresponds to a row in that table.
The persistent state of an entity is represented either through persistent fields or persistent properties. These fields or properties use object/relational mapping annotations to map the entities and entity relationships to the relational data in the underlying data store.
Requirements for Entity Classes
An entity class must follow these requirements:
The class must be annotated with the javax.persistence.Entity annotation.
The class must have a public or protected, no-argument constructor. The class may have other constructors.
The class must not be declared final. No methods or persistent instance variables must be declared final.
If an entity instance be passed by value as a detached object, such as through a session bean’s remote business interface, the class must implement the Serializable interface.
Entities may extend both entity and non-entity classes, and non-entity classes may extend entity classes.
Persistent instance variables must be declared private, protected, or package-private, and can only be accessed directly by the entity class’s methods. Clients must access the entity’s state through accessor or business methods.
Persistent Fields and Properties in Entity Classes
成员变量和属性
The persistent state of an entity can be accessed either through the entity’s instance variables or through JavaBeans-style properties. The fields or properties must be of the following Java language types:
- Java primitive types原始类型
在Java中，变量有两种类型，一种是原始类型，一种是引用类型。
原始类型一共有8种，它们分别是char,boolean,byte,short,int,long,float,double。在Java API中，有它们对应的包装类，分别是(首字母大写)Character,Boolean,Byte,Short,Integer,Long,Float,Double(char,int的变化稍微大点)。 - java.lang.String
- Other serializable types including:
- Wrappers of Java primitive types
- java.math.BigInteger
- java.math.BigDecimal
- java.util.Date
- java.util.Calendar
- java.sql.Date
- java.sql.Time
- java.sql.TimeStamp
- User-defined serializable types
byte[]，Byte[]，char[]，Character[] - Enumerated types
- Other entities and/or collections of entities
- Embeddable classes v.美[ɪm’bed]英[ɪm’bed]可嵌入