说明地址：http://docs.oracle.com/javaee/5/api/javax/persistence/MappedSuperclass.html
用来申明一个超类，继承这个类的子类映射时要映射此类中的字段，可以当做是对entity抽取封装的类。如果子类想重写此类的映射信息，可以使用 AttributeOverride and AssociationOverride annotations
- Example: Concrete class as a mapped superclass
- @MappedSuperclass
- public class Employee {
- @Id
- protected Integer empId;
- @Version
- protected Integer version;
- @ManyToOne @JoinColumn(name="ADDR")
- protected Address address;
- public Integer getEmpId() { ... }
- public void setEmpId(Integer id) { ... }
- public Address getAddress() { ... }
- public void setAddress(Address addr) { ... }
- }
- // Default table is FTEMPLOYEE table
- @Entity
- public class FTEmployee extends Employee {
- // Inherited empId field mapped to FTEMPLOYEE.EMPID
- // Inherited version field mapped to FTEMPLOYEE.VERSION
- // Inherited address field mapped to FTEMPLOYEE.ADDR fk
- // Defaults to FTEMPLOYEE.SALARY
- protected Integer salary;
- public FTEmployee() {}
- public Integer getSalary() { ... }
- public void setSalary(Integer salary) { ... }
- }
- @Entity @Table(name="PT_EMP")
- @AssociationOverride(name="address",
- joincolumns=@JoinColumn(name="ADDR_ID"))
- public class PartTimeEmployee extends Employee {
- // Inherited empId field mapped to PT_EMP.EMPID
- // Inherited version field mapped to PT_EMP.VERSION
- // address field mapping overridden to PT_EMP.ADDR_ID fk
- @Column(name="WAGE")
- protected Float hourlyWage;
- public PartTimeEmployee() {}
- public Float getHourlyWage() { ... }
- public void setHourlyWage(Float wage) { ... }
- }
- Example: Non-entity superclass
- public class Cart {
- // This state is transient
- Integer operationCount;
- public Cart() { operationCount = 0; }
- public Integer getOperationCount() { return operationCount; }
- public void incrementOperationCount() { operationCount++; }
- }
- @Entity
- public class ShoppingCart extends Cart {
- Collection items = new Vector();
- public ShoppingCart() { super(); }
- ...
- @OneToMany
- public Collection getItems() { return items; }
- public void addItem(Item item) {
- items.add(item);
- incrementOperationCount();
- }
- }
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