To map a java.util.Map with entity keys and values, we can choose to use either @OneToMany or @ManyToMany annotation on the Map field/property. The difference between OneToMany and ManyToMany is: in OneToMany relationship, the Map cannot repeat the same value for different keys, whereas ManyToMany relationship can repeat the values. In this example, we are going to show how to use @ManyToMany.
Example
@Entity
public class Employee {
@Id
@GeneratedValue
private long id;
private String name;
@ManyToMany
private Map<AssignInfo, Task> taskMap;
.............
}
@Entity
public class AssignInfo {
@Id
@GeneratedValue
private long id;
private Date startDate;
private Date endDate;
.............
}
@Entity
public class Task {
@Id
@GeneratedValue
private long id;
private String name;
private String description;
.............
}
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 EMPLOYEE");
nativeQuery(em, "SHOW COLUMNS from TASK");
nativeQuery(em, "SHOW COLUMNS from EMPLOYEE_TASK");
nativeQuery(em, "SHOW COLUMNS from ASSIGNINFO");
} 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'
[ASSIGNINFO, PUBLIC]
[EMPLOYEE, PUBLIC]
[EMPLOYEE_TASK, PUBLIC]
[TASK, PUBLIC]
'SHOW COLUMNS from EMPLOYEE'
[ID, BIGINT(19), NO, PRI, NULL]
[NAME, VARCHAR(255), YES, , NULL]
'SHOW COLUMNS from TASK'
[ID, BIGINT(19), NO, PRI, NULL]
[DESCRIPTION, VARCHAR(255), YES, , NULL]
[NAME, VARCHAR(255), YES, , NULL]
'SHOW COLUMNS from EMPLOYEE_TASK'
[EMPLOYEE_ID, BIGINT(19), NO, PRI, NULL]
[TASKMAP_ID, BIGINT(19), NO, , NULL]
[TASKMAP_KEY, BIGINT(19), NO, PRI, NULL]
'SHOW COLUMNS from ASSIGNINFO'
[ID, BIGINT(19), NO, PRI, NULL]
[ENDDATE, TIMESTAMP(23), YES, , NULL]
[STARTDATE, TIMESTAMP(23), YES, , NULL]
Persisting and loading data
public class ExampleMain2 {
public static void main(String[] args) throws Exception {
EntityManagerFactory emf =
Persistence.createEntityManagerFactory("example-unit");
try {
persistEntity(emf);
runNativeQueries(emf);
loadEntity(emf);
} finally {
emf.close();
}
}
private static void persistEntity(EntityManagerFactory emf) throws Exception {
System.out.println("-- Persisting entities --");
EntityManager em = emf.createEntityManager();
SimpleDateFormat sdf = new SimpleDateFormat("MM-dd-yy");
Employee e1 = new Employee();
e1.setName("employee name 1");
Task task1 = new Task();
task1.setName("task 1");
task1.setDescription("task 1 desc");
AssignInfo assignInfo1 = new AssignInfo();
assignInfo1.setStartDate(sdf.parse("15-01-17"));
assignInfo1.setEndDate(sdf.parse("20-01-17"));
e1.addTask(assignInfo1, task1);
Task task2 = new Task();
task2.setName("task 2");
task2.setDescription("task 2 desc");
AssignInfo assignInfo2 = new AssignInfo();
assignInfo2.setStartDate(sdf.parse("02-01-17"));
assignInfo2.setEndDate(sdf.parse("11-02-17"));
e1.addTask(assignInfo2, task2);
System.out.println(e1);
Employee e2 = new Employee();
e2.setName("employee name 2");
AssignInfo assignInfo3 = new AssignInfo();
assignInfo3.setStartDate(sdf.parse("9-02-17"));
assignInfo3.setEndDate(sdf.parse("19-03-17"));
//using the task2 again
//sharing between e1 and e2
//in oneToMany it's not allowed
e2.addTask(assignInfo3, task2);
System.out.println(e2);
em.getTransaction().begin();
em.persist(task1);
em.persist(task2);
em.persist(assignInfo1);
em.persist(assignInfo2);
em.persist(assignInfo3);
em.persist(e1);
em.persist(e2);
em.getTransaction().commit();
em.close();
}
private static void runNativeQueries(EntityManagerFactory emf) {
System.out.println("-- Native queries --");
EntityManager em = emf.createEntityManager();
ExampleMain.nativeQuery(em, "Select * from Employee");
ExampleMain.nativeQuery(em, "Select * from Task");
ExampleMain.nativeQuery(em, "Select * from Employee_Task");
ExampleMain.nativeQuery(em, "Select * from AssignInfo");
}
private static void loadEntity(EntityManagerFactory emf) {
System.out.println("-- Loading entities --");
EntityManager em = emf.createEntityManager();
List<Employee> entityAList = em.createQuery("Select t from Employee t")
.getResultList();
entityAList.forEach(System.out::println);
em.close();
}
}Output-- Persisting entities --
Employee{id=0, name='employee name 1', taskMap={AssignInfo{id=0, startDate=Thu Mar 01 00:00:00 CST 2018, endDate=Wed Aug 01 00:00:00 CDT 2018}=Task{id=0, name='task 2', description='task 2 desc'}}}
Employee{id=0, name='employee name 2', taskMap={AssignInfo{id=0, startDate=Sat Sep 02 00:00:00 CDT 2017, endDate=Tue Jul 03 00:00:00 CDT 2018}=Task{id=0, name='task 2', description='task 2 desc'}}}
-- Native queries --
'Select * from Employee'
[6, employee name 1]
[7, employee name 2]
'Select * from Task'
[1, task 1 desc, task 1]
[2, task 2 desc, task 2]
'Select * from Employee_Task'
[6, 2, 3]
[7, 2, 5]
'Select * from AssignInfo'
[3, 2018-08-01 00:00:00.0, 2018-03-01 00:00:00.0]
[4, 2017-11-02 00:00:00.0, 2017-02-01 00:00:00.0]
[5, 2018-07-03 00:00:00.0, 2017-09-02 00:00:00.0]
-- Loading entities --
Employee{id=6, name='employee name 1', taskMap={AssignInfo{id=3, startDate=2018-03-01 00:00:00.0, endDate=2018-08-01 00:00:00.0}=Task{id=2, name='task 2', description='task 2 desc'}}}
Employee{id=7, name='employee name 2', taskMap={AssignInfo{id=5, startDate=2017-09-02 00:00:00.0, endDate=2018-07-03 00:00:00.0}=Task{id=2, name='task 2', description='task 2 desc'}}}
Example ProjectDependencies and Technologies Used: - h2 1.4.195: 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
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