Java Hibernate@ManyToMany关系如何工作？

我有3个实体和一个用于链接这三个实体的实体：

@Entity
public class A {

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

    @OneToMany(cascade = CascadeType.ALL, mappedBy = "a", fetch = FetchType.LAZY)
    private List<ABCXref> abcXref = new ArrayList<ABCXref>();

          // getters/setters
}

@Entity
public class B {

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

    @OneToMany(cascade = CascadeType.ALL, mappedBy = "b", fetch = FetchType.LAZY)
    private List<ABCXref> abcXref = new ArrayList<ABCXref>();

          // getters/setters
}

@Entity
public class C {

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

    @OneToMany(cascade = CascadeType.ALL, mappedBy = "c", fetch = FetchType.LAZY)
    private List<ABCXref> abcXref = new ArrayList<ABCXref>();

          // getters/setters
}


@Entity
public class ABCXref {

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

    @JoinColumn(name = "A_ID", nullable = true)
    @ManyToOne(optional = true, fetch = FetchType.LAZY)
    private A a;

    @JoinColumn(name = "B_ID")
    @ManyToOne(optional = false, fetch = FetchType.LAZY)  
    private B b;  

    @JoinColumn(name = "C_ID")
    @ManyToOne(optional = false, fetch = FetchType.LAZY)
    private C c;
}

println(b.getAbcXref().size()); //0
c.getAbcXref().add(new ABCXref(a, b, c)); 
println(b.getAbcXref().size() == 1) //is `true` or `false`?