A-->B 有两个..How JPA do ?

Class B is easy:

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

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

	public B() {
		super();
	}

	@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;
	}

}

Class A is easy too, but it contains two B, one is named 'b1', the other 'b2':

import java.io.Serializable;
import javax.persistence.*;
import static javax.persistence.CascadeType.PERSIST;
import static javax.persistence.CascadeType.REMOVE;

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

	public A() {
		super();
	}

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

	@OneToOne(cascade = { PERSIST, REMOVE })
	private B b1;
	@OneToOne(cascade = { PERSIST, REMOVE })
	private B b2;

	public int getId() {
		return id;
	}

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

	public B getB1() {
		return b1;
	}

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

	public B getB2() {
		return b2;
	}

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

}

It is nothing serious, A knows which is b1, and which is b2, in the database it is    saved like this(the table A):
ID  B1_ID  B2_ID

Try it:

public class Test {

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

		A a = new A();
		a.setB1(b1);
		a.setB2(b2);

		OperateData.create(a);

		a = (A) OperateData.find(a);
		System.out.println("b1:" + a.getB1().getName());
		System.out.println("b2:" + a.getB2().getName());
	}

}

You can see in table A:
ID  B1_ID  B2_ID
1   1      2

That's great, maybe we should try the 'OneToMany' or 'ManyToMany'....

----------------------

Don't set b2's value, what will happen ? If you have read the 'A-->List<B> 有两个..How JPA do ?', you may think there will be a error message printed.

But, there is nothing, yes, in the table A, you can save an 'A' that have the b1's value but don't have b2's. Think about it.