MapKey (Java EE 5 SDK)

javax.persistence Annotation Type MapKey

@Target(value={METHOD,FIELD})
@Retention(value=RUNTIME)
public @interface MapKey

Is used to specify the map key for associations of type Map.

If a persistent field or property other than the primary key is used as a map key then it is expected to have a uniqueness constraint associated with it.

    Example 1:

    @Entity
    public class Department {
        ...
        @OneToMany(mappedBy="department")
        @MapKey(name="empId")
        public Map getEmployees() {... }
        ...
    }

    @Entity
    public class Employee {
        ...
        @Id Integer getEmpid() { ... }
        @ManyToOne
        @JoinColumn(name="dept_id")
        public Department getDepartment() { ... }
        ...
    }

    Example 2:

    @Entity
        public class Department {
        ...
        @OneToMany(mappedBy="department")
        @MapKey(name="empPK")
        public Map getEmployees() {... }
        ...
    }

    @Entity
        public class Employee {
        @EmbeddedId public EmployeePK getEmpPK() { ... }
        ...
        @ManyToOne
        @JoinColumn(name="dept_id")
        public Department getDepartment() { ... }
        ...
    }

    @Embeddable
    public class EmployeePK {
        String name;
        Date bday;
    }
 

从以下版本开始:

Java Persistence 1.0

name

public abstract String name

The name of the persistent field or property of the associated entity that is used as the map key. If the name element is not specified, the primary key of the associated entity is used as the map key. If the primary key is a composite primary key and is mapped as IdClass, an instance of the primary key class is used as the key.

默认值:

""

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