import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.Id;

@Entity
public class A {

	@Id
	@Column(name="AID", length=8)
	private String id;
	
	public String getId() {
		return id;
	}
}

import java.io.Serializable;

import javax.persistence.Column;
import javax.persistence.Embeddable;
import javax.persistence.EmbeddedId;
import javax.persistence.Entity;
import javax.persistence.JoinColumn;
import javax.persistence.ManyToOne;
import javax.persistence.MapsId;

@Entity
public class B {

	@Embeddable
	public static class B_Key implements Serializable {
		
		private static final long serialVersionUID = 1L;

		private String aId;
		
		public B_Key() {}
		
		@Column(name = "BB", length = 1)
		private String b;
		
		public String getAId() {
			return aId;
		}
		
		public void setAId(String value) {
			aId = value;
		}
		
		public String getB() {
			return b;
		}
		
		public void setB(String value) {
			b = value;
		}
		
		@Override
		public boolean equals(Object obj) {
			if (this == obj) return true;
			if (obj == null || !(obj.getClass().equals(this.getClass()))) return false;
			B_Key other = (B_Key) obj;
			return (aId == null && other.aId == null || aId != null && aId.equals(other.aId)) &&
				(b == null && other.b == null || b != null && b.equals(other.b));
		}
		
		@Override
		public int hashCode() {
			return (aId == null ? 0 : aId.hashCode()) ^ (b == null ? 0 : b.hashCode());
		}
	}
	
	@EmbeddedId
	private B_Key id;
	
	@MapsId("aId")
	@ManyToOne
	@JoinColumn(name = "FK_A")
	private A a;
	
	public A getA() {
		return a;
	}
	
	public void setA (A value) {
		a = value;
	}
	
	public String getB() {
		return id.getB();
	}
	
	public void setB(String value) {
		id.setB(value);
	}
}

    create table A (
        AID varchar(8) not null,
        primary key (AID)
    )

    create table B (
        FK_A varchar(255),
        BB varchar(1) not null,
        primary key (FK_A, BB)
    )

    alter table B 
        add constraint FK_8u5iivujidu0ix4s4f9cw76ns 
        foreign key (FK_A) 
        references A