如何使用JPA（或Hibernate）注释创建复合主数据库

Question

I got this problem which is a very common problem and I decided to share the resolution here.

The Problem: I have a composite primary key in one of my tables. I need to map this with JPA annotations.

My POJO1:

public class Alarm {

    @Id
    @Column(name="alm_id")
    private String id;

    @Column(name="alm_description")
    private String desc;

    @ManyToOne
    @JoinColumn(name = "alm_norm_id")
    private Alarm normAlarm; 

    //getters and setters
}

My POJO2:

public class Equipment {

    @Id
    @Column(name="equ_id")
    private String id;

    @Column(name="equ_fixed_asset")
    private String fixedAsset;

    @Column(name="equ_service_tag")
    private String serviceTag;

    //getters and setters

}

So I have a table which the primary key is an Equipment + Alarm and I need to map that.

Answer 1

I didn't want to create a separate class just for handle the primary key with @IdClass annotation. So I decided to use @EmbeddedId and @Embeddable annotations instead. The result was:

@Entity
@Table(name="Alarm_Counter")
public class Counter {

    @EmbeddedId
    private CounterId id;

    @Column(name="aco_counter")
    private int counter = 1;

    @Column(name="aco_last_reset")
    private Date resetDate;

    public Counter(){}

    public Counter(CounterId id) {
        super();
        this.id = id;
    }


    //Getters and Setters

    //...

    //Embeddable class for handling primary key
    @Embeddable
    public class CounterId implements Serializable{

        /**
         * 
         */
        private static final long serialVersionUID = 1L;

        @ManyToOne
        @JoinColumn(name="aco_alarm_id")
        private Alarm alarm;

        @ManyToOne
        @JoinColumn(name="aco_equipment_id")
        private Equipment equipment;

        //Default Constructor
        public CounterId(){}

        //Constructor with fields
        public CounterId(Alarm alarm, Equipment equipment) {
            super();
            this.alarm = alarm;
            this.equipment = equipment;
        }

        //Getters and Setters
    }
}

I still needed to handle this object for persist it with hibernate. So I made the following:

Alarm al = alarmService.get(alarm);                 //Retrieve an Alarm object
Equipment eq = equipmentService.get(equipment);     //Retrieve an Equipment object

Counter ct = new Counter();                         //Instantiates a Counter object
CounterId cId = ct.new CounterId(alarm, equipment); //Instantiates a CounterId object (composite primary key for Counter)

ct.setId(cId);                                      //Sets the primary key (CounterId)