创建一个具有两个互斥字段的实体

Question

I would like to create an entity which has two fields that are mutually exclusive ie only one or the other of the two fields should contain a value. Is there an annotation I can use to achieve this or do I need to do this by some other means?

Answer 1

JPA doesn't provide mechanism to implement mutual exclusive fields, but you can implement this in fields' setters. The final implementation depends on what exact behavior you want to achieve.

To explicitly disallow having 2 fields set at the same time use something like

@Entity
public class MutuallyExclusive1 {
    @Id
    @GeneratedValue
    private int Id;
    private String strValue;
    @Enumerated(EnumType.STRING)
    private MyEnum enumValue;

    public MutuallyExclusive1() {
        // do nothing
    }

    public void setEnum(final MyEnum enumValue) {
        if (strValue != null) {
            throw new IllegalStateException("stgValue and enumValue cannot be populated at the same time!");
        }
        this.enumValue = enumValue;
    }

    public void setString(final String strValue) {
        if (enumValue != null) {
            throw new IllegalStateException("stgValue and enumValue cannot be populated at the same time!");
        }
        this.strValue = strValue;
    }
}

To implicitly erase one value while setting another use

@Entity
public class MutuallyExclusive2 {
    @Id
    @GeneratedValue
    private int Id;
    private String strValue;
    @Enumerated(EnumType.STRING)
    private MyEnum enumValue;

    public MutuallyExclusive2() {
        // do nothing
    }

    public void setEnum(final MyEnum enumValue) {
        this.strValue = null;
        this.enumValue = enumValue;
    }

    public void setString(final String strValue) {
        this.enumValue = null;
        this.strValue = strValue;
    }
}

Either way you should keep in mind that mutually exclusiveness is enforced only by your implementation. That means you should either use only the setters above for write access to these fields or implement the same logic in every method having write access to them.

Answer 2

class Exclusive{

  private String value1 = null;
  private Enum value2 = null;

  public Exclusive(){
   ....
  }

  public void setValue1(String s){
    value1 = s;
    value2 = null;
  }

 public void setValue2(Enum e){
    value2 = e;
    value1 = null;
 }

}