How to map a composite key with JPA and Hibernate?

To map a composite key, you can use the EmbeddedId or the IdClass annotations. I know this question is not strictly about JPA but the rules defined by the specification also applies. So here they are:

2.1.4 Primary Keys and Entity Identity

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A composite primary key must correspond to either a single persistent field or property or to a set of such fields or properties as described below. A primary key class must be defined to represent a composite primary key. Composite primary keys typically arise when mapping from legacy databases when the database key is comprised of several columns. The EmbeddedId and IdClass annotations are used to denote composite primary keys. See sections 9.1.14 and 9.1.15.

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The following rules apply for composite primary keys:

• The primary key class must be public and must have a public no-arg constructor.
• If property-based access is used, the properties of the primary key class must be public or protected.
• The primary key class must be serializable.
• The primary key class must define equals and hashCode methods. The semantics of value equality for these methods must be consistent with the database equality for the database types to which the key is mapped.
• A composite primary key must either be represented and mapped as an embeddable class (see Section 9.1.14, “EmbeddedId Annotation”) or must be represented and mapped to multiple fields or properties of the entity class (see Section 9.1.15, “IdClass Annotation”).
• If the composite primary key class is mapped to multiple fields or properties of the entity class, the names of primary key fields or properties in the primary key class and those of the entity class must correspond and their types must be the same.

With an IdClass

The class for the composite primary key could look like (could be a static inner class):

public class TimePK implements Serializable {    protected Integer levelStation;    protected Integer confPathID;    public TimePK() {}    public TimePK(Integer levelStation, Integer confPathID) {        this.levelStation = levelStation;        this.confPathID = confPathID;    }    // equals, hashCode}

And the entity:

@Entity@IdClass(TimePK.class)class Time implements Serializable {    @Id    private Integer levelStation;    @Id    private Integer confPathID;    private String src;    private String dst;    private Integer distance;    private Integer price;    // getters, setters}

The IdClass annotation maps multiple fields to the table PK.

With EmbeddedId

The class for the composite primary key could look like (could be a static inner class):

@Embeddablepublic class TimePK implements Serializable {    protected Integer levelStation;    protected Integer confPathID;    public TimePK() {}    public TimePK(Integer levelStation, Integer confPathID) {        this.levelStation = levelStation;        this.confPathID = confPathID;    }    // equals, hashCode}

And the entity:

@Entityclass Time implements Serializable {    @EmbeddedId    private TimePK timePK;    private String src;    private String dst;    private Integer distance;    private Integer price;    //...}

The @EmbeddedId annotation maps a PK class to table PK.

Differences:

• From the physical model point of view, there are no differences
• @EmbeddedId somehow communicates more clearly that the key is a composite key and IMO makes sense when the combined pk is either a meaningful entity itself or it reused in your code.
• @IdClass is useful to specify that some combination of fields is unique but these do not have a special meaning.

They also affect the way you write queries (making them more or less verbose):

• with IdClass

  select t.levelStation from Time t

• with EmbeddedId

  select t.timePK.levelStation from Time t

References

• JPA 1.0 specification
  • Section 2.1.4 "Primary Keys and Entity Identity"
  • Section 9.1.14 "EmbeddedId Annotation"
  • Section 9.1.15 "IdClass Annotation"

You need to use @EmbeddedId:

@Entityclass Time {    @EmbeddedId    TimeId id;    String src;    String dst;    Integer distance;    Integer price;}@Embeddableclass TimeId implements Serializable {    Integer levelStation;    Integer confPathID;}

Assuming you have the following database tables:

First, you need to create the @Embeddable holding the composite identifier:

@Embeddablepublic class EmployeeId implements Serializable {     @Column(name = "company_id")    private Long companyId;     @Column(name = "employee_number")    private Long employeeNumber;     public EmployeeId() {    }     public EmployeeId(Long companyId, Long employeeId) {        this.companyId = companyId;        this.employeeNumber = employeeId;    }     public Long getCompanyId() {        return companyId;    }     public Long getEmployeeNumber() {        return employeeNumber;    }     @Override    public boolean equals(Object o) {        if (this == o) return true;        if (!(o instanceof EmployeeId)) return false;        EmployeeId that = (EmployeeId) o;        return Objects.equals(getCompanyId(), that.getCompanyId()) &&                Objects.equals(getEmployeeNumber(), that.getEmployeeNumber());    }     @Override    public int hashCode() {        return Objects.hash(getCompanyId(), getEmployeeNumber());    }}

With this in place, we can map the Employee entity which uses the composite identifier by annotating it with @EmbeddedId:

@Entity(name = "Employee")@Table(name = "employee")public class Employee {     @EmbeddedId    private EmployeeId id;     private String name;     public EmployeeId getId() {        return id;    }     public void setId(EmployeeId id) {        this.id = id;    }     public String getName() {        return name;    }     public void setName(String name) {        this.name = name;    }}

The Phone entity which has a @ManyToOne association to Employee, needs to reference the composite identifier from the parent class via two @JoinColumnmappings:

@Entity(name = "Phone")@Table(name = "phone")public class Phone {     @Id    @Column(name = "`number`")    private String number;     @ManyToOne    @JoinColumns({        @JoinColumn(            name = "company_id",            referencedColumnName = "company_id"),        @JoinColumn(            name = "employee_number",            referencedColumnName = "employee_number")    })    private Employee employee;     public Employee getEmployee() {        return employee;    }     public void setEmployee(Employee employee) {        this.employee = employee;    }     public String getNumber() {        return number;    }     public void setNumber(String number) {        this.number = number;    }}