MappedSuperclass (Java EE 5)

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SUMMARY: REQUIRED | OPTIONAL

DETAIL: ELEMENT

javax.persistence Annotation Type MappedSuperclass

@Target(value=TYPE) @Retention(value=RUNTIME) public @interface MappedSuperclass

Implements: Annotation

@Target(value=TYPE)
@Retention(value=RUNTIME)

指定一个类，该类的映射信息将应用于继承该类的实体。映射的超类没有专门为其定义的表格。

可以以与实体相同的方式映射 MappedSuperclass 注释指定的类，但是映射仅适用于其子类，因为映射的超类本身不存在表格。应用于子类时，继承的映射将在子类表格的上下文中应用。使用 AttributeOverride 和 AssociationOverride 注释或相应的 XML 元素可以重写这种子类中的映射信息。

示例：作为映射的超类的具体类
    @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++; }
    }
    
    @E;ntity
    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();
        }
    }

since

Java Persistence 1.0

英文文档：

Designates a class whose mapping information is applied to the entities that inherit from it. A mapped superclass has no separate table defined for it.

A class designated with the MappedSuperclass annotation can be mapped in the same way as an entity except that the mappings will apply only to its subclasses since no table exists for the mapped superclass itself. When applied to the subclasses the inherited mappings will apply in the context of the subclass tables. Mapping information may be overridden in such subclasses by using the AttributeOverride and AssociationOverride annotations or corresponding XML elements.

    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++; }
    }
    
    @E;ntity
    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();
        }
    }

Since:: Java Persistence 1.0