File: Student.java
import java.util.ArrayList;
import java.util.Collection;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.ManyToMany;
@Entity
public class Student {
@Id @GeneratedValue(strategy=GenerationType.IDENTITY)
private int id;
private String name;
@ManyToMany
private Collection<Department> departments;
public Student() {
departments = new ArrayList<Department>();
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public void addDepartment(Department department) {
if (!getDepartments().contains(department)) {
getDepartments().add(department);
}
if (!department.getStudents().contains(this)) {
department.getStudents().add(this);
}
}
public Collection<Department> getDepartments() {
return departments;
}
public void setDepartment(Collection<Department> departments) {
this.departments = departments;
}
public String toString() {
return "\n\nID:" + id + "\nName:" + name ;
}
}
File: Department.java
import java.util.ArrayList;
import java.util.Collection;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.ManyToMany;
@Entity
public class Department {
@Id @GeneratedValue(strategy=GenerationType.IDENTITY)
private int id;
private String name;
@ManyToMany(mappedBy="departments")
private Collection<Student> students;
public Department(){
students = new ArrayList<Student>();
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String deptName) {
this.name = deptName;
}
public void addStudent(Student student) {
if (!getStudents().contains(student)) {
getStudents().add(student);
}
if (!student.getDepartments().contains(this)) {
student.getDepartments().add(this);
}
}
public Collection<Student> getStudents() {
return students;
}
public void setStudent(Collection<Student> students) {
this.students = students;
}
public String toString() {
return "Department id: " + getId() +
", name: " + getName();
}
}
File: Helper.java
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.ResultSetMetaData;
import java.sql.Statement;
public class Helper {
public static void checkData() throws Exception {
Class.forName("org.hsqldb.jdbcDriver");
Connection conn = DriverManager.getConnection("jdbc:hsqldb:data/tutorial", "sa", "");
Statement st = conn.createStatement();
ResultSet mrs = conn.getMetaData().getTables(null, null, null, new String[] { "TABLE" });
while (mrs.next()) {
String tableName = mrs.getString(3);
System.out.println("\n\n\n\nTable Name: "+ tableName);
ResultSet rs = st.executeQuery("select * from " + tableName);
ResultSetMetaData metadata = rs.getMetaData();
while (rs.next()) {
System.out.println(" Row:");
for (int i = 0; i < metadata.getColumnCount(); i++) {
System.out.println(" Column Name: "+ metadata.getColumnLabel(i + 1)+ ", ");
System.out.println(" Column Type: "+ metadata.getColumnTypeName(i + 1)+ ": ");
Object value = rs.getObject(i + 1);
System.out.println(" Column Value: "+value+"\n");
}
}
}
}
}
File: Main.java
import java.util.List;
import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;
import javax.persistence.Query;
public class Main {
static EntityManagerFactory emf = Persistence.createEntityManagerFactory("JPAService");
static EntityManager em = emf.createEntityManager();
public static void main(String[] a) throws Exception {
em.getTransaction().begin();
Student student = new Student();
student.setName("Joe");
em.persist(student);
Student student1 = new Student();
student1.setName("Joe");
em.persist(student1);
Department dept = new Department();
dept.setName("dept name");
dept.addStudent(student);
dept.addStudent(student1);
em.persist(dept);
em.flush();
Query query = em.createQuery("SELECT e FROM Student e");
List<Student> list = (List<Student>) query.getResultList();
System.out.println(list);
query = em.createQuery("SELECT d FROM Department d");
List<Department> dList = (List<Department>) query.getResultList();
System.out.println(dList);
em.getTransaction().commit();
em.close();
emf.close();
Helper.checkData();
}
}
File: persistence.xml
<persistence xmlns="http://java.sun.com/xml/ns/persistence"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/persistence http://java.sun.com/xml/ns/persistence/persistence" version="1.0">
<persistence-unit name="JPAService" transaction-type="RESOURCE_LOCAL">
<properties>
<property name="hibernate.dialect" value="org.hibernate.dialect.HSQLDialect"/>
<property name="hibernate.hbm2ddl.auto" value="update"/>
<property name="hibernate.connection.driver_class" value="org.hsqldb.jdbcDriver"/>
<property name="hibernate.connection.username" value="sa"/>
<property name="hibernate.connection.password" value=""/>
<property name="hibernate.connection.url" value="jdbc:hsqldb:data/tutorial"/>
</properties>
</persistence-unit>
</persistence>
Download: JPA-JPACreatesJoinTableForManyToManyMapping.zip( 5,291 k)