Jpa, insert into join table

hi all,
im having problems in inserting data in a join table using jpa.
im using eclipselink and mysql.
i have 3 tables:
tab1(pk(id),k1)
tab2(pk(id),k2)
tab3(fk(x)->tab1(id),fk(y)->tab2(id))
sa far i have done this.

this is tab1

package pak1;

import java.io.Serializable;
import javax.persistence.*;
import java.util.List;


/**
 * The persistent class for the tab1 database table.
 * 
 */
@Entity
public class Tab1 implements Serializable {
	private static final long serialVersionUID = 1L;

	@Id
	private int id;

	private int k1;

	//bi-directional many-to-many association to Tab2
	@ManyToMany(mappedBy="tab1s")
	private List<Tab2> tab2s;

	public Tab1() {
	}

	public int getId() {
		return this.id;
	}

	public void setId(int id) {
		this.id = id;
	}

	public int getK1() {
		return this.k1;
	}

	public void setK1(int k1) {
		this.k1 = k1;
	}

	public List<Tab2> getTab2s() {
		return this.tab2s;
	}

	public void setTab2s(List<Tab2> tab2s) {
		this.tab2s = tab2s;
	}

}

this is tab2

package pak1;

import java.io.Serializable;
import javax.persistence.*;
import java.util.List;


/**
 * The persistent class for the tab2 database table.
 * 
 */
@Entity
public class Tab2 implements Serializable {
	private static final long serialVersionUID = 1L;

	@Id
	private int id;

	private int k2;

	//bi-directional many-to-many association to Tab1
	@ManyToMany
	@JoinTable(
		name="tab3"
		, joinColumns={
			@JoinColumn(name="y")
			}
		, inverseJoinColumns={
			@JoinColumn(name="x")
			}
		)
	private List<Tab1> tab1s;

	public Tab2() {
	}

	public int getId() {
		return this.id;
	}

	public void setId(int id) {
		this.id = id;
	}

	public int getK2() {
		return this.k2;
	}

	public void setK2(int k2) {
		this.k2 = k2;
	}

	public List<Tab1> getTab1s() {
		return this.tab1s;
	}

	public void setTab1s(List<Tab1> tab1s) {
		this.tab1s = tab1s;
	}

}

and this is the servlet

package pak1;

import java.io.IOException;
import java.io.PrintWriter;
import java.util.ArrayList;
import java.util.List;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
 * Servlet implementation class Srv1
 */
@WebServlet("/Srv1")
public class Srv1 extends HttpServlet {
	private static final long serialVersionUID = 1L;
	static EntityManagerFactory emf = Persistence.createEntityManagerFactory("aaa");
	static  EntityManager em = emf.createEntityManager();
	
	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException 
	{
		
		PrintWriter out=response.getWriter();
		List<Tab2> l2=new ArrayList<>();
		List<Tab1> l1=new ArrayList<>();
		em.getTransaction().begin();
		
		
		Tab1 t1=new Tab1();
		t1.setK1(12);
		
		Tab1 t11=new Tab1();
		t11.setK1(12);
		
		l1.add(t1);
		l1.add(t11);
		em.persist(t1);
		em.persist(t11);
		
		Tab2 t2=new Tab2();
		t2.setK2(12);
		
		
				
		
		
		
		Tab2 t22=new Tab2();
		t22.setK2(123);
		t22.setTab1s(l1);
		em.persist(t2);
		em.persist(t22);
		em.getTransaction().commit();
		em.close();
		emf.close();
		out.println("fund");
	}

}

i get this:-----
javax.persistence.RollbackException: Exception [EclipseLink-4002] (Eclipse Persistence Services - 2.4.1.v20121003-ad44345): org.eclipse.persistence.exceptions.DatabaseException
Internal Exception: com.mysql.jdbc.exceptions.jdbc4.MySQLIntegrityConstraintViolationException: Cannot add or update a child row: a foreign key constraint fails (`prim`.`tab3`, CONSTRAINT `tab3_ibfk_1` FOREIGN KEY (`x`) REFERENCES `tab1` (`id`) ON DELETE CASCADE ON UPDATE CASCADE)
Error Code: 1452
Call: INSERT INTO tab3 (x, y) VALUES (?, ?)
bind => [2 parameters bound]
Query: DataModifyQuery(name="tab1s" sql="INSERT INTO tab3 (x, y) VALUES (?, ?)")
org.eclipse.persistence.internal.jpa.transaction.EntityTransactionImpl.commitInternal(EntityTransactionImpl.java:102)
org.eclipse.persistence.internal.jpa.transaction.EntityTransactionImpl.commit(EntityTransactionImpl.java:63)
pak1.Srv1.doGet(Srv1.java:59)
javax.servlet.http.HttpServlet.service(HttpServlet.java:621)
javax.servlet.http.HttpServlet.service(HttpServlet.java:722)


thank you