Java/JPA/Many to Many Mapping

Содержание

1 Many To Many Bidirectional Mapping
2 Many To Many Joined Table
3 Many to Many Mapping Key Table Is Created Automatically
4 Many To Many Mapping Remove Action

Many To Many Bidirectional Mapping

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.ManyToMany;
@Entity
public class Professor {
    @Id @GeneratedValue(strategy=GenerationType.IDENTITY)
    private int id;
    private String name;
    private long salary;
    
    @ManyToMany 
    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 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();
}
    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();
}
}

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", 100L);
    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().rumit();
    em.close();
    emf.close();
  }
}



File: persistence.xml
<persistence xmlns="http://java.sun.ru/xml/ns/persistence"
             xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
             xsi:schemaLocation="http://java.sun.ru/xml/ns/persistence http://java.sun.ru/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>

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().rumit();
    em.close();
    emf.close();
  }
}


File: persistence.xml
<persistence xmlns="http://java.sun.ru/xml/ns/persistence"
             xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
             xsi:schemaLocation="http://java.sun.ru/xml/ns/persistence http://java.sun.ru/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>

Many to Many Mapping Key Table Is Created Automatically

File: Professor.java

import java.util.ArrayList;
import java.util.Collection;
import java.util.Date;
import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.ManyToMany;
import javax.persistence.Temporal;
import javax.persistence.TemporalType;
@Entity
public class Professor {
    @Id
    private int id;
    private String name;
    private long salary;
    @Temporal(TemporalType.DATE)
    private Date startDate;
    
    @ManyToMany 
    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 void setProjects(Collection<Project> projects) {
      this.projects = projects;
    }
    public void setSalary(long salary) {
      this.salary = salary;
    }
    public void setStartDate(Date startDate) {
      this.startDate = startDate;
    }
    public long getSalary() {
        return salary;
    }
    public Date getStartDate() {
        return startDate;
    }
    
    public Collection<Project> getProjects() {
        return projects;
    }
    
    public String toString() {
        return "Professor " + getId() + 
               ": name: " + getName() +
               ", salary: " + getSalary();
    }
}

File: ProfessorService.java
import java.util.Collection;
import javax.persistence.EntityManager;
public class ProfessorService {
  protected EntityManager em;
  public ProfessorService(EntityManager em) {
    this.em = em;
  }
  public Professor createProfessor(String name, long salary) {
    Professor emp = new Professor();
    emp.setName(name);
    emp.setSalary(salary);
    em.persist(emp);
    return emp;
  }

  public Project createProject(String name) {
    Project space = new Project();
    space.setName(name);
    em.persist(space);
    return space;
  }

  public Collection<Professor> findAllProfessors() {
    return (Collection<Professor>) em.createQuery("SELECT e FROM Professor e").getResultList();
  }
  public void assignProfessorToProject(int empId, int projectId) {
    Project project = em.find(Project.class, projectId);
    Professor employee = em.find(Professor.class, empId);
    project.getProfessors().add(employee);
    employee.getProjects().add(project);
  }
  public Collection<Project> findAllProjects() {
    return (Collection<Project>) em.createQuery("SELECT p FROM Project p").getResultList();
  }
}

File: Project.java

import java.util.ArrayList;
import java.util.Collection;
import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.ManyToMany;
@Entity
public class Project {
    @Id
    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 String getName() {
        return name;
    }
    
    public Collection<Professor> getProfessors() {
        return employees;
    }
    
    public void setProfessors(Collection<Professor> employees) {
      this.employees = employees;
    }
    public void setId(int id) {
      this.id = id;
    }
    public void setName(String name) {
      this.name = name;
    }
    public String toString() {
        return "Project id: " + getId() + ", name: " + getName();
    }
}

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("name",100);
    Project proj = service.createProject("proj name");
    
    service.assignProfessorToProject(emp.getId(),proj.getId());
for (Object o : service.findAllProfessors()) {
System.out.println(o );
}
for (Object o : service.findAllProjects()) {
  System.out.println(o);
}

    util.checkData("select * from Professor");
    util.checkData("select * from Project");
    
    util.checkData("select * from Professor_Project");
    em.getTransaction().rumit();
    em.close();
    emf.close();
  }}



File: persistence.xml
<persistence xmlns="http://java.sun.ru/xml/ns/persistence"
             xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
             xsi:schemaLocation="http://java.sun.ru/xml/ns/persistence http://java.sun.ru/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>

Many To Many Mapping Remove Action

File: Project.java

import java.util.ArrayList;
import java.util.Collection;
import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.ManyToMany;
@Entity
public class Project {
    @Id
    protected int id;
    protected String name;
    @ManyToMany(mappedBy="projects")
    private Collection<Student> students;
    public Project() {
        students = new ArrayList<Student>();
    }
    public void setId(int i){
      id = i;
    }
    public int getId() {
        return id;
    }
    
    public String getName() {
        return name;
    }
    
    public Collection<Student> getStudents() {
        return students;
    }
    
    public String toString() {
        return "Project id: " + getId() + ", name: " + getName();
    }
}

File: Student.java
import java.util.ArrayList;
import java.util.Collection;
import java.util.Date;
import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.ManyToMany;
import javax.persistence.Temporal;
import javax.persistence.TemporalType;
@Entity
public class Student {
    @Id
    private int id;
    private String name;
    
    @ManyToMany 
    private Collection<Project> projects;
    public Student() {
        projects = new ArrayList<Project>();
    }
    public int getId() {
        return id;
    }
    
    public void setId(int id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    
    public Collection<Project> getProjects() {
        return projects;
    }
    
    public String toString() {
        return "Student " + 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.setId(1);
    em.persist(student);
    
    Project dept = new Project();
    dept.setId(1);
    em.persist(dept);
    
    em.flush();


    em.remove(dept);
    
    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 Project d");
    List<Project> dList = (List<Project>) query.getResultList();
    System.out.println(dList);
    
    em.getTransaction().rumit();
    em.close();
    emf.close();
    
    Helper.checkData();
  }
}


File: persistence.xml
<persistence xmlns="http://java.sun.ru/xml/ns/persistence"
             xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
             xsi:schemaLocation="http://java.sun.ru/xml/ns/persistence http://java.sun.ru/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>

Java/JPA/Many to Many Mapping

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Many To Many Bidirectional Mapping

Many To Many Joined Table

Many to Many Mapping Key Table Is Created Automatically

Many To Many Mapping Remove Action

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