Memory leak while merging parent with No Cascade annotation with children

import java.util.ArrayList;
import java.util.List;

import javax.persistence.CascadeType;
import javax.persistence.Entity;
import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Id;
import javax.persistence.ManyToMany;
import javax.persistence.Persistence;


public final class MyTestCase {

    public static void main(String[] args)  {

        EntityManagerFactory emf = Persistence.createEntityManagerFactory(
                "objectdb:$objectdb/db/test.tmp;drop");

        for (int n=0;n<100;++n) {

            memoryUsage();

            EntityManager em = emf.createEntityManager();

            System.out.println("n = " + n);
            for(int i=0;i<1000;++i) {

                MyEntity e = new MyEntity("parent" + String.valueOf(i));
                e.addChild(String.valueOf("child" + String.valueOf(i)));
                em.merge(e);

            }

            if (! em.getTransaction().isActive()) {
                em.getTransaction().begin();
            }

            em.getTransaction().commit();
            em.clear();
            em.close();
           
            System.gc();
        }

        emf.close();
    }

    @Entity
    public static class MyEntity {

        @Id
        private String name;

        @ManyToMany(cascade = {
                // No CascadeType.ALL //=> Huge memory leak while merging parent !
                // CascadeType.MERGE or CascadeType.ALL // => No memory leak
        })
        private List <MyEntityChild> lst = new ArrayList <MyEntityChild> ();

        MyEntity(String name) {
            this.name = name;
        }

        public void addChild(String name) {
            lst.add(new MyEntityChild(name));
        }
    }

    @Entity
    public static class MyEntityChild {

        @Id
        private String name;

        MyEntityChild(String name) {
            this.name = name;
        }
    }

    private static void memoryUsage() {

        int mb = 1024*1024;

        //Getting the runtime reference from system
        Runtime runtime = Runtime.getRuntime();

        System.out.println("##### Heap utilization statistics [MB] #####");

        //Print used memory
        System.out.println("Used Memory:"
            + (runtime.totalMemory() - runtime.freeMemory()) / mb);

        //Print free memory
        System.out.println("Free Memory:"
            + runtime.freeMemory() / mb);

        //Print total available memory
        System.out.println("Total Memory:" + runtime.totalMemory() / mb);

        //Print Maximum available memory
        System.out.println("Max Memory:" + runtime.maxMemory() / mb);
    }
}