一个实体类必须遵循一下要求：

1.        类必须用javax.persistence.Entity注释

2.        类必须有一个public或protected的无参数构造器。可以有其他的构造器。

3.        类不准许声明为final。

4.        实体类可以继承实体类也可以继承非实体类。非实体类可以继承实体类。

JPA中可以持久化的字段类型:

• Java原始类型
• Java.lang.String
• 可序列化类型
• Wrappersof 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-definedserializable types
• byte[]
• Byte[]
• char[]
• Character[]
• 枚举类型Enumerated Types
• 其他实体或实体集合
• 内嵌类

基本实体映射

@Entity

@SecondaryTable(name = "SALARY")

public class Employee implements Serializable {

    @Id

    @Column(name = "EMP_ID")

    @GeneratedValue(strategy = GenerationType.SEQUENCE)

    private long id;

@Basic

    @Version

private long version;

    @Basic

    @Column(name = "F_NAME")

    private String firstName;

    @Basic

    @Column(name = "L_NAME")

    private String lastName;

    @Basic

    @Column(name = "GENDER")

    @Enumerated(EnumType.STRING)

    private Gender gender = Gender.Male;

    @Column(table = "SALARY")

    private double salary;

         ……

}

抽象的实体注释

@Entity

public abstract class Employee {

    @Id

    protectedInteger employeeId;

    ...

}

@Entity

public class FullTimeEmployee extends Employee {

    protectedInteger salary;

    ...

}

@Entity

public class PartTimeEmployee extends Employee {

    protected FloathourlyWage;

}

超类映射

@MappedSuperclass

public class Employee {

    @Id

    protected Integer employeeId;

    ...

}

@Entity

public class FullTimeEmployee extends Employee {

    protected Integer salary;

    ...

}

@Entity

public class PartTimeEmployee extends Employee {

    protected Float hourlyWage;

    ...

}

内嵌实体类映射

@Embeddable

//內嵌到其他實體，與其共享identity

public class EmploymentPeriod {

    //@Temporal必須為java.util.Date and java.util.Calendar類型的字段或屬性註釋，

    //而且只能為這些類型的字段或屬性註釋。

    @Temporal(TemporalType.DATE)

    private Calendar startDate;

    @Temporal(TemporalType.DATE)

private Calendar endDate;

… …

}

public classEmployee implements Serializable {

    … …

    @Embedded

    @AttributeOverrides( {

        @AttributeOverride(name = "startDate", column = @Column(name = "START_DATE")),

        @AttributeOverride(name = "endDate", column = @Column(name = "END_DATE")) })

    private EmploymentPeriod period;

    … …

}