在Hibernate参考文档中有一个例子与你的例子完全相似。在这个例子之前，你可以找到解释。下面是这个例子，它符合你的问题（User是表A，Customer是表B）：

@Entity
class Customer {
   @EmbeddedId CustomerId id;
   boolean preferredCustomer;

   @MapsId("userId")
   @JoinColumns({
      @JoinColumn(name="userfirstname_fk", referencedColumnName="firstName"),
      @JoinColumn(name="userlastname_fk", referencedColumnName="lastName")
   })
   @OneToOne User user;
}

@Embeddable
class CustomerId implements Serializable {
   UserId userId;
   String customerNumber;

   //implements equals and hashCode
}

@Entity 
class User {
   @EmbeddedId UserId id;
   Integer age;
}

@Embeddable
class UserId implements Serializable {
   String firstName;
   String lastName;

   //implements equals and hashCode
}

注意：如果你为这两个表指定了一个代理标识符，那就简单多了。除非你不得不处理一个遗留模式，否则请使用代理键。

使用@PrimaryKeyJoinColumn和@PrimaryKeyJoinColumns注解。来自Hibernate手册：
@PrimaryKeyJoinColumn注解确实指出实体的主键用作关联实体的外键值。

public class User implements Serializable {
    /**
     * 
     */
    private static final long serialVersionUID = 5478661842746845130L;
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private int id;
}
@Entity
public class Author {

    @Id
    @Column(name = "AUTHOR_ID", nullable = false)
    private int authorId;
    @Column(name = "ENABLED", nullable = false, length = 1)
    private boolean enabled;

    @OneToOne
    @MapsId
    @JoinColumn(name = "AUTHOR_ID", referencedColumnName = "ID", nullable = false, insertable = false, updatable = false)
    User user;

    public boolean isEnabled() {
        return enabled;
    }

    public void setEnabled(boolean enabled) {
        this.enabled = enabled;
    }

    public User getUser() {
        return user;
    }

    public void setUser(User user) {
        this.user = user;
    }

}

对于表A，您可以使用embeddedId

@Emebddable
class PrimaryKeyIdTableA{
 private Integer firstId,

 private Integer secondId
}

class A {
 @EmebddedId
 private PrimaryKeyIdTableA id
}

对于表B，也可以使用Idclass，如下所示

Class PrimaryKeyIdTableB {

private PrimaryKeyIdTableA id,

private Integer thirdId

 }

 @Idclass(PrimaryKeyIdTableB.class)
 Class TableB{
 @Id
 private Integer thirdId,
 
  @Id
  @onetoone
  @Joincolumns({
  @Joincolumn(name=firstid),
  @Joincolumn(name=secondId)})
 private PrimaryKeyIdTableA id
    }