JPA - Composite Primary Key with @IdClass

A composite primary key consists of multiple primary key fields. Each primary key field must be one of the supported types for the primary key.

JPA specification requires a separate 'composite primary key class' having the primary key fields. The @Entity class must be linked to that class via @IdClass annotation.

public class MyCompositeKeyClass ....{
  //primary key fields
  ......
}

@Entity
@IdClass(MyCompositeKeyClass.class)
public static class MyEntity{
   ......
}

The @Entity class still needs to define the id fields (annotated with @Id) having the same names and types as of the 'composite primary key class'.

The 'composite primary key class' must implement Serializable and must implement equals and hasCode methods.

Example

@Entity
@IdClass(CompositeTaskId.class)
public class Task {
    @Id
    private int employeeId;
    @Id
    private int taskId;
    private String taskName;
    private Date date;

    public Task() {
    }

    public Task(int employeeId, int taskId) {
        this.employeeId = employeeId;
        this.taskId = taskId;
    }
    .............
}

public class CompositeTaskId implements Serializable{
    private int employeeId;
    private int taskId;

    public CompositeTaskId() {
    }

    public CompositeTaskId(int employeeId, int taskId) {
        this.employeeId = employeeId;
        this.taskId = taskId;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        CompositeTaskId taskId1 = (CompositeTaskId) o;
        if (employeeId != taskId1.employeeId) return false;
        return taskId == taskId1.taskId;
    }

    @Override
    public int hashCode() {
        return Objects.hash(employeeId, taskId);
    }
}

Main class showing table mappings

public class ExampleMain {

    public static void main(String[] args) {
        EntityManagerFactory emf =
                Persistence.createEntityManagerFactory("example-unit");
        try {
            EntityManager em = emf.createEntityManager();
            nativeQuery(em, "SHOW TABLES");
            nativeQuery(em, "SHOW COLUMNS from TASK");
        } finally {
            emf.close();
        }
    }

    public static void nativeQuery(EntityManager em, String s) {
        System.out.printf("'%s'%n", s);
        Query query = em.createNativeQuery(s);
        List list = query.getResultList();
        for (Object o : list) {
            if (o instanceof Object[]) {
                System.out.println(Arrays.toString((Object[]) o));
            } else {
                System.out.println(o);
            }
        }
    }
}

Output

'SHOW TABLES'
[TASK, PUBLIC]
'SHOW COLUMNS from TASK'
[EMPLOYEEID, INTEGER(10), NO, PRI, NULL]
[TASKID, INTEGER(10), NO, PRI, NULL]
[DATE, TIMESTAMP(23), YES, , NULL]
[TASKNAME, VARCHAR(255), YES, , NULL]

Note that there are two primary key columns (EMPLOYEEID and TASKID) which form a composite key.

Persisting and loading data

public class ExampleMain2 {

    public static void main(String[] args) throws Exception {
        EntityManagerFactory emf =
                Persistence.createEntityManagerFactory("example-unit");
        try {
            persistEntity(emf);
            runNativeQuery(emf);
            findEntityById(emf);
        } finally {
            emf.close();
        }
    }

    private static void persistEntity(EntityManagerFactory emf) throws Exception {
        System.out.println("-- Persisting entity --");
        EntityManager em = emf.createEntityManager();

        Task task = new Task(5, 10);
        task.setTaskName("coding");
        task.setDate(new Date());

        em.getTransaction().begin();
        em.persist(task);
        em.getTransaction().commit();
        em.close();
    }

    private static void runNativeQuery(EntityManagerFactory emf) {
        System.out.println("-- Native query --");
        EntityManager em = emf.createEntityManager();
        ExampleMain.nativeQuery(em, "Select * from Task");
    }

    private static void findEntityById(EntityManagerFactory emf) {
        System.out.println("-- Finding entity --");
        EntityManager em = emf.createEntityManager();
        CompositeTaskId taskId = new CompositeTaskId(5, 10);
        Task task = em.find(Task.class, taskId);
        System.out.println(task);
        em.close();
    }
}

Output

-- Persisting entity --
-- Native query --
'Select * from Task'
[5, 10, 2025-10-29 13:19:23.42, coding]
-- Finding entity --
Task{employeeId=5, taskId=10, taskName='coding', date=2025-10-29 13:19:23.42}

Example Project

Dependencies and Technologies Used:

• h2 1.4.196: H2 Database Engine.
• hibernate-core 5.2.10.Final: The core O/RM functionality as provided by Hibernate.
  Implements javax.persistence:javax.persistence-api version 2.1
• JDK 1.8
• Maven 3.3.9