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().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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