A-->List<B> 有两个..How JPA do ?

Compare to the previous one(I mean 'A-->B 有两个..How JPA do ?'), I only add a constructor to Class B:

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

/**
 * Entity implementation class for Entity: B
 * 
 */
@Entity
public class B implements Serializable {

	public B() {
		super();
	}
	
	public B(String name){
		super();
		this.name = name;		
	}

	@GeneratedValue(strategy = GenerationType.AUTO)
	@Id
	private int id;
	private String name;

	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;
	}

}

This is A -> List<B> , so the Class A:

import static javax.persistence.CascadeType.PERSIST;
import static javax.persistence.CascadeType.REMOVE;

import java.io.Serializable;
import java.util.List;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.OneToMany;

/**
 * Entity implementation class for Entity: A
 * 
 */
@Entity
public class A implements Serializable {

	public A() {
		super();
	}

	@GeneratedValue(strategy = GenerationType.AUTO)
	@Id
	private int id;
	@OneToMany(cascade = { PERSIST, REMOVE })
	private List<B> b1;
	@OneToMany(cascade = { PERSIST, REMOVE })
	private List<B> b2;

	public List<B> getB1() {
		return b1;
	}

	public void setB1(List<B> b1) {
		this.b1 = b1;
	}

	public List<B> getB2() {
		return b2;
	}

	public void setB2(List<B> b2) {
		this.b2 = b2;
	}

	public int getId() {
		return id;
	}

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

}

Let's see the table A first:
There is only one field -- ID.
But, there is another table -- A_B, so we know, there is only one table exists between two relational entities.
A_B:
A_ID  B2_ID  B1_ID
Try it:

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

public class Test {

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		
		List<B> b1 = new ArrayList<B>();
		b1.add(new B("b1"));
		b1.add(new B("b1."));
		
		List<B> b2 = new ArrayList<B>();
		b2.add(new B("b2"));
		b2.add(new B("b2."));

		A a = new A();
		a.setB1(b1);
		a.setB2(b2);
		OperateData.create(a);
		
		a = (A) OperateData.find(a);
		
		for(B b : a.getB1()){
			System.out.println("b1:" + b.getName());
		}
		for(B b : a.getB2()){
			System.out.println("b2:" + b.getName());
		}
		
	}

}

There is an error message -- '列“B2_ID”无法接受空值。'
What should we do ? Maybe I can add a property as Class B \'s state.
But, why don't use two different B ?