WEB-INF/resin-web.xml	resin-web.xml configuration
WEB-INF/classes/META-INF/persistence.xml	persistence.xml configuration
WEB-INF/classes/example/Student.java	The student bean
WEB-INF/classes/example/Course.java	The course bean
WEB-INF/classes/example/Grade.java	The grade bean
WEB-INF/classes/example/ManyToManyServlet.java	The course servlet

Entity Beans

The many-to-many relation connects two tables with an association table. In the example, each Student takes several Courses. A grade_map table connects the two. Using the many-to-many relation, the application can return the student's courses or the students in a course.

SQL Schema

CREATE TABLE Course (
  course_id BIGINT PRIMARY KEY,

  name VARCHAR(255)
)

CREATE TABLE Student (
  student_id BIGINT PRIMARY KEY,

  name VARCHAR(255)
)

CREATE TABLE grade_map (
  id BIGINT PRIMARY KEY auto_increment,

  student_id BIGINT REFERENCES Student(student_id),
  course_id BIGINT REFERENCES Course(course_id)
)

The Course has an @Id and a data column for the name.

Course.java

@Entity
public class Course {
  @Id@Column(name="course_id")
  public long getId()

  @Basic
  public String getName()
}

The Student includes the many-to-many relation in its definition.

Student.java

@Entity
public class Student {
  @Id@Column(name="student_id")
  public long getId()

  @Basic
  public String getName()

  @ManyToMany(targetEntity="Course")@JoinTable(
      table=@Table(name="student_course_map"),
      joinColumns=@JoinColumn(name="student_id")",
      inverseJoinColumns=@JoinColumn(name="course_id")")
  public Collection getCourses()
}

@ManyToMany

The @ManyToMany annotation marks a collection-valued field as a many-to-many relation. The targetEntity value specifies the target of the relation.

Since the many-to-many relation is a three-table relation, it needs to specify the association table as well as the columns.

Client

ManyToManyServlet.java

  private void doService(PrintWriter out)
    throws java.io.IOException
  {
    PrintWriter out = res.getWriter();

    res.setContentType("text/html");

    Query allStudent = _entityManager.createQuery("SELECT o FROM Student o");
    
    List students = allStudent.listResults();

    for (int i = 0; i < students.size(); i++) {
      Student student = (Student) students.get(i);

      out.println("<h3>" + student.getName() + "</h3>");

      Collection courses = student.getCourses();

      out.println("<ul>");
      Iterator iter = courses.iterator();
      while (iter.hasNext()) {
	Course course = (Course) iter.next();

	out.println("<li>" + course.getName());
      }
      out.println("</ul>");
    }
  }
}

Demo

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