One To Many And Many To One Mapping
File: Department.java
import java.util.HashMap;
import java.util.Map;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.MapKey;
import javax.persistence.OneToMany;
@Entity
public class Department {
@Id @GeneratedValue(strategy=GenerationType.IDENTITY)
private int id;
private String name;
@OneToMany(mappedBy="department")
@MapKey(name="name")
private Map<String, Professor> employees;
public Department() {
employees = new HashMap<String, Professor>();
}
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 addProfessor(Professor employee) {
if (!getProfessors().containsKey(employee.getName())) {
getProfessors().put(employee.getName(), employee);
if (employee.getDepartment() != null) {
employee.getDepartment().getProfessors().remove(employee.getName());
}
employee.setDepartment(this);
}
}
public Map<String,Professor> getProfessors() {
return employees;
}
public String toString() {
return "Department id: " + getId() +
", name: " + getName();
}
}
File: Professor.java
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.ManyToOne;
@Entity
public class Professor {
@Id @GeneratedValue(strategy=GenerationType.IDENTITY)
private int id;
private String name;
private long salary;
@ManyToOne
private Department 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 long getSalary() {
return salary;
}
public void setSalary(long salary) {
this.salary = salary;
}
public Department getDepartment() {
return department;
}
public void setDepartment(Department department) {
this.department = department;
}
public String toString() {
return "Professor id: " + getId() + " name: " + getName() +
" with " + getDepartment();
}
}
File: ProfessorService.java
import java.util.Collection;
import javax.persistence.EntityManager;
import javax.persistence.Query;
public class ProfessorService {
protected EntityManager em;
public ProfessorService(EntityManager em) {
this.em = em;
}
public Department createDepartment(String name) {
Department dept = new Department();
dept.setName(name);
em.persist(dept);
return dept;
}
public Collection<Department> findAllDepartments() {
Query query = em.createQuery("SELECT d FROM Department d");
return (Collection<Department>) query.getResultList();
}
public Professor createProfessor(String name, long salary) {
Professor emp = new Professor();
emp.setName(name);
emp.setSalary(salary);
em.persist(emp);
return emp;
}
public Professor setProfessorDepartment(int empId, int deptId) {
Professor emp = em.find(Professor.class, empId);
Department dept = em.find(Department.class, deptId);
dept.addProfessor(emp);
emp.setDepartment(dept);
return emp;
}
public Collection<Professor> findAllProfessors() {
Query query = em.createQuery("SELECT e FROM Professor e");
return (Collection<Professor>) query.getResultList();
}
}
File: Main.java
import java.util.Collection;
import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;
public class Main {
public static void main(String[] a) throws Exception {
JPAUtil util = new JPAUtil();
EntityManagerFactory emf = Persistence.createEntityManagerFactory("ProfessorService");
EntityManager em = emf.createEntityManager();
ProfessorService service = new ProfessorService(em);
em.getTransaction().begin();
Professor emp = service.createProfessor("empName",100L);
Department dept = service.createDepartment("deptName");
emp = service.setProfessorDepartment(emp.getId(),dept.getId());
System.out.println(emp.getDepartment() + " with Professors:");
System.out.println(emp.getDepartment().getProfessors());
Collection<Professor> emps = service.findAllProfessors();
if (emps.isEmpty()) {
System.out.println("No Professors found ");
} else {
System.out.println("Found Professors:");
for (Professor emp1 : emps) {
System.out.println(emp1);
}
}
Collection<Department> depts = service.findAllDepartments();
if (depts.isEmpty()) {
System.out.println("No Departments found ");
} else {
System.out.println("Found Departments:");
for (Department dept1 : depts) {
System.out.println(dept1 + " with " + dept1.getProfessors().size() +
" employees " + dept1.getProfessors());
}
}
util.checkData("select * from Professor");
util.checkData("select * from Department");
em.getTransaction().commit();
em.close();
emf.close();
}
}
File: JPAUtil.java
import java.io.Reader;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.ResultSetMetaData;
import java.sql.Statement;
public class JPAUtil {
Statement st;
public JPAUtil() throws Exception{
Class.forName("org.hsqldb.jdbcDriver");
System.out.println("Driver Loaded.");
String url = "jdbc:hsqldb:data/tutorial";
Connection conn = DriverManager.getConnection(url, "sa", "");
System.out.println("Got Connection.");
st = conn.createStatement();
}
public void executeSQLCommand(String sql) throws Exception {
st.executeUpdate(sql);
}
public void checkData(String sql) throws Exception {
ResultSet rs = st.executeQuery(sql);
ResultSetMetaData metadata = rs.getMetaData();
for (int i = 0; i < metadata.getColumnCount(); i++) {
System.out.print("\t"+ metadata.getColumnLabel(i + 1));
}
System.out.println("\n----------------------------------");
while (rs.next()) {
for (int i = 0; i < metadata.getColumnCount(); i++) {
Object value = rs.getObject(i + 1);
if (value == null) {
System.out.print("\t ");
} else {
System.out.print("\t"+value.toString().trim());
}
}
System.out.println("");
}
}
}
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>
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