Many To Many Joined Table
File: Professor.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.JoinColumn;
import javax.persistence.JoinTable;
import javax.persistence.ManyToMany;
@Entity
public class Professor {
@Id @GeneratedValue(strategy=GenerationType.IDENTITY)
private int id;
private String name;
private long salary;
@ManyToMany
@JoinTable(name="EMP_PROJ",
joinColumns=@JoinColumn(name="EMP_ID"),
inverseJoinColumns=@JoinColumn(name="PROJ_ID"))
private Collection<Project> projects;
public Professor() {
projects = new ArrayList<Project>();
}
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 void addProject(Project project) {
if (!getProjects().contains(project)) {
getProjects().add(project);
}
if (!project.getProfessors().contains(this)) {
project.getProfessors().add(this);
}
}
public Collection<Project> getProjects() {
return projects;
}
public String toString() {
return "Professor id: " + getId() + " name: " + getName() +
" with " + getProjects().size() + " projects";
}
}
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 Project createProject(String name) {
Project proj = new Project();
proj.setName(name);
em.persist(proj);
return proj;
}
public Collection<Project> findAllProjects() {
Query query = em.createQuery("SELECT p FROM Project p");
return (Collection<Project>) 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 addProfessorProject(int empId, int projId) {
Professor emp = em.find(Professor.class, empId);
Project proj = em.find(Project.class, projId);
proj.addProfessor(emp);
return emp;
}
public Collection<Professor> findAllProfessors() {
Query query = em.createQuery("SELECT e FROM Professor e");
return (Collection<Professor>) query.getResultList();
}
}
File: Project.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 Project {
@Id @GeneratedValue(strategy=GenerationType.IDENTITY)
protected int id;
protected String name;
@ManyToMany(mappedBy="projects")
private Collection<Professor> employees;
public Project() {
employees = new ArrayList<Professor>();
}
public int getId() {
return id;
}
public void setId(int projectNo) {
this.id = projectNo;
}
public String getName() {
return name;
}
public void setName(String projectName) {
this.name = projectName;
}
public Collection<Professor> getProfessors() {
return employees;
}
public void addProfessor(Professor employee) {
if (!getProfessors().contains(employee)) {
getProfessors().add(employee);
}
if (!employee.getProjects().contains(this)) {
employee.getProjects().add(this);
}
}
public String toString() {
return "Project id: " + getId() + ", name: " + getName() +
" with " + getProfessors().size() + " employees";
}
}
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: 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",100);
Project proj = service.createProject("projName");
emp = service.addProfessorProject(emp.getId(),proj.getId());
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<Project> projs = service.findAllProjects();
if (projs.isEmpty()) {
System.out.println("No Projects found ");
} else {
System.out.println("Found Projects:");
for (Project proj1 : projs) {
System.out.println(proj1);
}
}
util.checkData("select * from Professor");
util.checkData("select * from Project");
em.getTransaction().commit();
em.close();
emf.close();
}
}
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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